18-Bit, 2 MSPS, PulSAR 15 mW ADC in LFCSP AD7986 Data Sheet

18-Bit, 2 MSPS, PulSAR 15 mW ADC in LFCSP AD7986 Data Sheet
18-Bit, 2 MSPS, PulSAR
15 mW ADC in LFCSP
AD7986
Data Sheet
FEATURES
TYPICAL APPLICATION CIRCUIT
APPLICATIONS
Battery-powered equipment
Data acquisition systems
Medical instruments
Seismic data acquisition systems
V+
5V
2.5V
1.8V
TO
2.7V
10Ω
0V
TO
VIO
BVDD AVDD,
DVDD TURBO
1.5nF
VREF
VIO
IN+
V–
AD7986
V+
1.5nF
SCK
SDO
IN–
REF
10Ω
VREF
TO
0V
SDI
GND
CNV
3- OR 4-WIRE
INTERFACE:
SPI, CS
DAISY CHAIN
(TURBO = LOW)
10µF
V–
07956-001
18-bit resolution with no missing codes
Throughput: 2 MSPS (TURBO = high), 1.5 MSPS (TURBO = low)
Low power dissipation
15 mW at 2 MSPS, with external reference
26 mW at 2 MSPS with internal reference
INL: ±1 LSB typical, ±2.5 LSB maximum
SNR
95.5 dB, with on-chip reference
97.0 dB, with external reference
4.096 V internal reference: typical drift of 10 ppm/°C
True differential analog input voltage range: ±VREF
0 V to VREF with VREF up to 5.0 V
Allows use of any input range
No pipeline delay
Logic interface: 1.8 V/2.5 V/2.7 V
Proprietary serial interface: SPI-/QSPI™-/MICROWIRE™-/DSPcompatible
Ability to daisy-chain multiple ADCs with busy indicator
20-lead 4 mm × 4 mm LFCSP
NOTES
1. GND REFERS TO REFGND, AGND, AND DGND.
Figure 1.
GENERAL DESCRIPTION
The AD79861 is an 18-bit, 2 MSPS successive approximation,
analog-to-digital converter (ADC). It contains a low power,
high speed, 18-bit sampling ADC, an internal conversion clock,
an internal reference (and buffer), error correction circuits, and
a versatile serial interface port. On the rising edge of CNV, the
AD7986 samples the voltage difference between the IN+ and
IN− pins. The voltages on these pins usually swing in opposite
phases between 0 V and VREF. It features a very high sampling
rate turbo mode (TURBO = high) and a reduced power normal
mode (TURBO = low) for low power applications where the
power is scaled with the throughput.
In normal mode (TURBO = low), the SPI-compatible serial
interface also features the ability, using the SDI input, to daisychain several ADCs on a single 3-wire bus and provide an optional
busy indicator. It is compatible with 1.8 V, 2.5 V, and 2.7 V using the
separate VIO supply.
The AD7986 is available in a 20-lead LFCSP with operation
specified from −40°C to +85°C.
1
Protected by U.S. Patent 6,703,961.
Table 1. MSOP, LFCSP 14-/16-/18-Bit PulSAR® ADCs
Type
14-Bit
16-Bit
100 kSPS
AD7940
AD7680
AD7683
AD7684
18-Bit
1
250 kSPS
AD79421
AD76851
AD76871
AD7694
AD76911
400 kSPS to 500 kSPS
AD79461
AD76861
AD76881
AD76931
AD76901
≥1000 kSPS
ADC Driver
AD79801
AD79831
ADA4941-1
ADA4841-1
AD79821
AD79841
AD7986
ADA4941-1
ADA4841-1
AD8021
Pin for pin compatible.
Rev. D
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AD7986
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1 Analog Inputs ............................................................................. 15 Applications ....................................................................................... 1 Driver Amplifier Choice ........................................................... 15 Typical Application Circuit ............................................................. 1 Voltage Reference Input ............................................................ 16 General Description ......................................................................... 1 Power Supply............................................................................... 16 Revision History ............................................................................... 2 Digital Interface .............................................................................. 17 Specifications..................................................................................... 3 Data Reading Options ............................................................... 18 Timing Specifications .................................................................. 5 CS Mode, 3-Wire Without Busy Indicator ............................. 19 Absolute Maximum Ratings............................................................ 6 CS Mode, 3-Wire with Busy Indicator .................................... 20 ESD Caution .................................................................................. 6 CS Mode, 4-Wire Without Busy Indicator ............................. 21 Pin Configuration and Function Descriptions ............................. 7 CS Mode, 4-Wire with Busy Indicator .................................... 22 Typical Performance Characteristics ............................................. 9 Chain Mode Without Busy Indicator ...................................... 23 Terminology .................................................................................... 12 Chain Mode with Busy Indicator ............................................. 24 Theory of Operation ...................................................................... 13 Application Hints ........................................................................... 25 Circuit Information .................................................................... 13 Layout .......................................................................................... 25 Converter Operation .................................................................. 13 Evaluating the Performance of the AD7986............................... 25 Conversion Modes of Operation .............................................. 13 Outline Dimensions ....................................................................... 27 Typical Connection Diagram ................................................... 14 Ordering Guide .......................................................................... 27 REVISION HISTORY
3/16—Rev. C to Rev. D
Changes to Figure 1 and Table 1 ..................................................... 1
Change to Transition Noise Parameter, Table 2 ........................... 3
Deleted Endnote 4, Table 2 .............................................................. 3
Changes to Figure 4 .......................................................................... 8
Changes to Figure 23 ...................................................................... 14
Changes to Driver Amplifier Choice Section ............................. 15
Changes to Reference Decoupling Section ................................. 16
Changes to Reading During Conversion, Fast Host (Turbo or
Normal Mode) Section and Split-Reading, Any Speed Host
(Turbo or Normal Mode) Section ................................................ 18
Changes to Figure 30 ...................................................................... 21
Changes to Figure 34 ...................................................................... 23
Changes to Evaluating the Performance of the AD7986 Section .. 25
Updated Outline Dimensions ....................................................... 27
Changes to Ordering Guide .......................................................... 27
8/14—Rev. B to Rev. C
Replaced QFN with LFCSP .......................................... Throughout
Changed Application Diagram Section to Typical Application
Circuit Section ...................................................................................1
Change to Features Section ..............................................................1
Added Patent Note, Note 2 ...............................................................1
Changed 5 V to 4.096 V, Analog Input Heading, Table 7 ......... 14
Changes to Evaluating the Performance of the AD7986
Section.............................................................................................. 25
Updated Outline Dimensions ....................................................... 27
Changes to Ordering Guide .......................................................... 27
3/11—Rev. A to Rev. B
Added Common-Mode Input Range Parameter, Table 2 ............3
8/10—Rev. 0 to Rev. A
Changes to Conversion Time: CNV Rising Edge to Data
Available (Turbo Mode/Normal Mode) Parameter, Table 4 ........5
Changes to Figure 32...................................................................... 22
4/09—Revision 0: Initial Version
Rev. D | Page 2 of 28
Data Sheet
AD7986
SPECIFICATIONS
AVDD = DVDD = 2.5 V, BVDD = 5 V, VIO = 1.8 V to 2.7 V, VREF = 4.096 V, TA = −40°C to +85°C, unless otherwise noted.
Table 2.
Parameter
RESOLUTION
ANALOG INPUT
Voltage Range
Absolute Input Voltage
Common-Mode Input Range
Analog Input CMRR
Leakage Current at 25°C
Input Impedance
ACCURACY
No Missing Codes
Differential Linearity Error
Integral Linearity Error
Transition Noise
Gain Error, TMIN to TMAX 3
Gain Error Temperature Drift
Zero Error, TMIN to TMAX3
Zero Temperature Drift
Power Supply Sensitivity
THROUGHPUT
Conversion Rate
Transient Response
AC ACCURACY
Dynamic Range
Signal-to-Noise Ratio, SNR
Spurious-Free Dynamic Range, SFDR
Total Harmonic Distortion, THD
Signal-to-(Noise + Distortion), SINAD
SAMPLING DYNAMICS
−3 dB Input Bandwidth
Aperture Delay
Test Conditions/Comments
Min
18
Typ
(IN+) − (IN−)
IN+, IN−
IN+, IN−
fIN = 500 kHz
Acquisition phase
−VREF
−0.1
VREF × 0.475
Unit
Bits
+VREF
VREF + 0.1
VREF × 0.525
V
V
V
dB 1
nA
VREF × 0.5
100
250
See the Analog Inputs section
18
−0.95
−2.50
−20
±0.60
±1.00
1.4
±2.4
±0.5
−0.8
+1.50
+2.50
+20
+0.8
±0.3
±4
AVDD = 2.5 V ± 5%
0
2.00
100
Full-scale step
VREF = 4.096 V, internal reference
VREF = 5.0 V, external reference
fIN = 20 kHz, VREF = 4.096 V, internal
reference
fIN = 20 kHz, VREF = 5.0 V, external
reference
fIN = 20 kHz
fIN = 20 kHz, VREF = 4.096 V, internal
reference
fIN = 20 kHz, VREF = 5.0 V, external
reference
fIN = 20 kHz, VREF = 4.096 V
Max
Bits
LSB 2
LSB2
LSB2
LSB2
ppm/°C
mV
ppm/°C
LSB2
MSPS
ns
95.5
97
94.5
96.5
98
95.5
dB1
96.5
97.0
dB1
−115
−113
dB1
dB1
−114
dB1
95.5
dB1
19
0.7
MHz
ns
94.5
dB1
All specifications expressed in decibels are referred to a full-scale input FSR and tested with an input signal at 0.5 dB below full scale, unless otherwise specified.
LSB means least significant bit. With the 4.096 V input range, one LSB is 31.25 µV.
3
See the Terminology section. These specifications include full temperature range variation but not the error contribution from the external reference.
1
2
Rev. D | Page 3 of 28
AD7986
Data Sheet
AVDD = DVDD = 2.5 V, BVDD = 5 V, VIO = 1.8 V to 2.7 V, VREF = 4.096 V, TA = −40°C to +85°C, unless otherwise noted.
Table 3.
Parameter
INTERNAL REFERENCE
Output Voltage
Temperature Drift
Line Regulation
Turn-On Settling Time
REFIN Output Voltage
REFIN Output Resistance
EXTERNAL REFERENCE
Voltage Range
Current Drain
REFERENCE BUFFER
REFIN Input Voltage Range
REFIN Input Current
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
DIGITAL OUTPUTS
Data Format
Pipeline Delay
Test Conditions/Comments
PDREF = low
TA = 25°C
−40°C to +85°C
AVDD = 2.5 V ± 5%
CREF = 10 μF, CREFBUFIN = 0.1 μF
REFIN at 25°C
Min
Typ
Max
Unit
4.081
4.096
±10
±50
220
1.2
7.5
4.111
V
ppm/°C
ppm/V
ms
V
kΩ
5.1
500
V
µA
1.2
160
V
µA
VOL
VOH
POWER SUPPLIES
AVDD, DVDD
BVDD
VIO
VIO Range
Standby Current 1, 2
Power Dissipation
With Internal Reference
Without Internal Reference
With Internal Reference
Without Internal Reference
TEMPERATURE RANGE 3
Specified Performance
ISINK = +500 µA
ISOURCE = −500 µA
Serial, 18 bits, twos complement
Conversion results available immediately
after completed conversion
0.4
VIO − 0.3
Specified performance
2.375
4.75
1.8
PDREF = high, REFIN = low
2.4
2 MSPS, VREF = 5.0 V
−0.3
+0.9 × VIO
−1
−1
+0.1 × VIO
VIO + 0.3
+1
+1
2.5
5.0
2.5
AVDD = DVDD = VIO = 2.5 V, BVDD = 5.0 V
2.25
2 MSPS throughput
2 MSPS throughput
1.5 MSPS throughput
1.5 MSPS throughput
29
15
26
11.5
TMIN to TMAX
−40
With all digital inputs forced to VIO or GND as required.
During acquisition phase.
3
Contact an Analog Devices, Inc., sales representative for the extended temperature range.
1
2
Rev. D | Page 4 of 28
V
V
µA
µA
V
V
2.625
5.25
2.7
V
34
16.5
30
13
mW
mW
mW
mW
+85
°C
V
V
µA
Data Sheet
AD7986
TIMING SPECIFICATIONS
AVDD = DVDD = 2.5 V, BVDD = 5 V, VIO = 1.8 V to 2.7 V, VREF = 4.096 V, TA = −40°C to +85°C, unless otherwise noted. 1
Table 4.
Parameter
Conversion Time: CNV Rising Edge to Data Available (Turbo Mode/Normal Mode)
Acquisition Time
Time Between Conversions (Turbo Mode/Normal Mode)
CNV Pulse Width (CS Mode)
Data Read During Conversion (Turbo Mode/Normal Mode)
Quiet Time During Acquisition from Last SCK Falling Edge to CNV Rising Edge
SCK Period (CS Mode)
SCK Period (Chain Mode)
SCK Low Time
SCK High Time
SCK Falling Edge to Data Remains Valid
SCK Falling Edge to Data Valid Delay
CNV or SDI Low to SDO D17 MSB Valid (CS Mode)
CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
SDI Valid Setup Time from CNV Rising Edge
SDI Valid Hold Time from CNV Rising Edge (CS Mode)
SDI Valid Hold Time from CNV Rising Edge (Chain Mode)
SCK Valid Setup Time from CNV Rising Edge (Chain Mode)
SCK Valid Hold Time from CNV Rising Edge (Chain Mode)
SDI Valid Setup Time from SCK Falling Edge (Chain Mode)
SDI Valid Hold Time from SCK Falling Edge (Chain Mode)
SDI High to SDO High (Chain Mode with Busy Indicator)
See Figure 2 and Figure 3 for load conditions.
500µA
IOL
1.4V
TO SDO
CL
20pF
500µA
07956-002
IOH
Figure 2. Load Circuit for Digital Interface Timing
10% VIO
90% VIO
tDELAY
tDELAY
VIH1
VIL1
VIH1
VIL1
1MINIMUM
VIH AND MAXIMUM VIL USED. SEE DIGITAL INPUTS
SPECIFICATIONS IN TABLE 3.
Figure 3. Voltage Levels for Timing
Rev. D | Page 5 of 28
07956-003
1
Symbol
tCONV
tACQ
tCYC
tCNVH
tDATA
tQUIET
tSCK
tSCK
tSCKL
tSCKH
tHSDO
tDSDO
tEN
tDIS
tSSDICNV
tHSDICNV
tHSDICNV
tSSCKCNV
tHSCKCNV
tSSDISCK
tHSDISCK
tDSDOSDI
Min
Typ
Max
400/500
100
500/660
10
200/300
20
9
11
3.5
3.5
2
6
10
8
4
0
0
5
5
2
3
5
Unit
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
AD7986
Data Sheet
ABSOLUTE MAXIMUM RATINGS
Table 5.
Parameter
Analog Inputs
IN+, IN− to GND 1
Supply Voltage
REF, BVDD to GND, REFGND
AVDD, DVDD, VIO to GND
AVDD and DVDD to VIO
Digital Inputs to GND
Digital Outputs to GND
Storage Temperature Range
Junction Temperature
θJA Thermal Impedance
20-Lead LFCSP
Lead Temperatures
Vapor Phase (60 sec)
Infrared (15 sec)
1
Rating
−0.3 V to VREF + 0.3 V
or ±130 mA
−0.3 V to +6.0 V
−0.3 V to +2.7 V
+3 V to −6 V
−0.3 V to VIO + 0.3 V
−0.3 V to VIO + 0.3 V
−65°C to +150°C
150°C
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
ESD CAUTION
30.4°C/W
215°C
220°C
See the Analog Inputs section for an explanation of IN+ and IN−.
Rev. D | Page 6 of 28
Data Sheet
AD7986
20
19
18
17
16
REFIN
BVDD
AGND
AGND
AVDD
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
2
3
4
5
AD7986
TOP VIEW
(Not to Scale)
15
14
13
12
11
TURBO
SDI
CNV
SCK
DVDD
NOTES
1. THE EXPOSED PAD IS NOT CONNECTED INTERNALLY.
FOR INCREASED RELIABILITY OF THE SOLDER JOINTS,
IT IS RECOMMENDED THAT THE PAD BE SOLDERED TO
THE SYSTEM GROUND PLANE.
07956-004
IN+ 6
PDREF 7
VIO 8
SDO 9
DGND 10
REF
REF
REFGND
REFGND
IN–
Figure 4. Pin Configuration
Table 6. Pin Function Descriptions
Pin No.
1, 2
Mnemonic
REF
Type 1
AI
3, 4
5
6
7
REFGND
IN−
IN+
PDREF
AI
AI
AI
DI
8
VIO
P
9
10
11
12
13
SDO
DGND
DVDD
SCK
CNV
DO
P
P
DI
DI
14
SDI
DI
15
TURBO
DI
16
17,18
AVDD
AGND
P
P
Description
Reference Output/Input Voltage.
When PDREF = low, the internal reference and buffer are enabled, producing 4.096 V on this pin.
When PDREF = high, the internal reference and buffer are disabled, allowing an externally supplied
voltage reference up to 5.0 V.
Decoupling is required with or without the internal reference and buffer. This pin is referred to the
REFGND pins and must be decoupled closely to the REFGND pins with a 10 µF capacitor.
Reference Input Analog Ground.
Differential Negative Analog Input.
Differential Positive Analog Input.
Internal Reference Power-Down Input.
When low, the internal reference is enabled.
When high, the internal reference is powered down and an external reference must be used.
Input/Output Interface Digital Power. Nominally at the same supply as the host interface
(1.8 V, 2.5 V, or 2.7 V).
Serial Data Output. The conversion result is output on this pin. It is synchronized to SCK.
Digital Power Ground.
Digital Power. Nominally at 2.5 V.
Serial Data Clock Input. When the device is selected, the conversion result is shifted out by this clock.
Convert Input. This input has multiple functions. On the leading edge, it initiates the conversions
and selects the interface mode of the device: chain mode or CS mode. In CS mode, the SDO pin is
enabled when CNV is low. In chain mode, the data must be read when CNV is high.
Serial Data Input. This input provides multiple features. It selects the interface mode of the ADC as follows:
Chain mode is selected if SDI is low during the CNV rising edge. In this mode, SDI is used as a data
input to daisy-chain the conversion results of two or more ADCs onto a single SDO line. The digital
data level on SDI is output on SDO with a delay of 18 SCK cycles.
CS mode is selected if SDI is high during the CNV rising edge. In this mode, either SDI or CNV can
enable the serial output signals when low. If SDI or CNV is low when the conversion is complete, the
busy indicator feature is enabled.
Conversion Mode Selection.
When TURBO = high, the maximum throughput (2 MSPS) is achieved. The ADC does not power down
between conversions.
When TURBO = low, the maximum throughput is lower (1.5 MSPS). The ADC powers down between
conversions.
Input Analog Power. Nominally at 2.5 V.
Analog Power Ground.
Rev. D | Page 7 of 28
AD7986
Data Sheet
Pin No.
19
Mnemonic
BVDD
Type 1
P
20
REFIN
AI/O
21 (EPAD)
Exposed Pad
EP
1
Description
Reference buffer power. Nominally 5.0 V.
If an external reference buffer is used to achieve the maximum SNR performance with 5 V reference,
the reference buffer must be powered down by connecting the REFIN pin to ground. The external
reference buffer must be connected to the BVDD pin.
Internal Reference Output/Reference Buffer Input.
When PDREF = low, the internal band gap reference produces a 1.2 V (typical) voltage on this pin,
which needs external decoupling (0.1 µF typical).
When PDREF = high, use an external reference to provide a 1.2 V (typical) to this pin.
When PDREF = high, and REFIN = low, the on-chip reference buffer and band gap are powered down.
An external reference must be connected to REF and BVDD.
The exposed pad is not connected internally. For increased reliability of the solder joints, it is
recommended that the pad be soldered to the system ground plane.
AI = analog input, AI/O = bidirectional analog; DI = digital input, DO = digital output, and P = power.
Rev. D | Page 8 of 28
Data Sheet
AD7986
TYPICAL PERFORMANCE CHARACTERISTICS
AVDD = DVDD = VIO = 2.5 V, BVDD = 5.0 V, VREF = 5.0 V, external reference (PDREF = high, REFIN = low), unless otherwise noted.
2.5
2.0
POSITIVE INL = +1.57LSB
NEGATIVE INL = –1.25LSB
2.0
POSITIVE DNL = +0.54LSB
NEGATIVE DNL = –0.60LSB
1.5
1.5
1.0
0.5
DNL (LSB)
INL (LSB)
1.0
0
–0.5
0.5
0
–0.5
–1.0
–1.0
–1.5
–2.5
0
196,608
131,072
CODE
65,536
07956-008
–1.5
07956-005
–2.0
–2.0
0
262,144
65,536
Figure 5. Integral Nonlinearity vs. Code
45,000
131,072
CODE
262,144
Figure 8. Differential Nonlinearity vs. Code
45,000
41,811
41,434
38,665
40,000
40,000
35,204
196,608
34,894
35,000
35,000
30,000
30,000
COUNTS
COUNTS
30,897
25,000
20,000
15,000
25,000
20,000
15,000
10,211
10,000
7662
6399
5000
0
0
101
3FF6
1661
3FF8
1418
3FFA
3FFC
CODE IN HEX
68
3FFE
3
0
5000
0
2283
0
4
3FF5
0
Figure 6. Histogram of DC Input at Code Center (External Reference)
1002 34
142
3FF7
3FF9
3FFB
3FFD
CODE IN HEX
0
1
Figure 9. Histogram of DC Input at Code Transition (External Reference)
45,000
40,000
39,395
40,000
1
3FFF
07956-009
8250
07956-006
10,000
37,385
36,210
35,000
35,000
30,000
31,020
29,138
30,000
COUNTS
COUNTS
25,000
25,000
20,000
22,077
18,953
20,000
15,000
12,773
11,107
10,000
10,000
6513
3662
0
0
3FFEC
1
55
2932
547
3FFEE
407 35
3FF0
3FF4
3FF2
CODE IN HEX
3FF6
3FF8
0
0
07956-007
5000
6879
5000
Figure 7. Histogram of DC Input at Code Center (Internal Reference)
0
1
1438
1282
3 150
3FFEB 3FFED 3FFEF
3FF1
3FF3
CODE IN HEX
3FF5
165 16
3FF7
0
07956-010
15,000
3FF9
Figure 10. Histogram of DC Input at Code Transition (Internal Reference)
Rev. D | Page 9 of 28
AD7986
Data Sheet
0
0
–20
fS = 2MSPS
fIN = 20kHz
–40
SNR = 97.0dB
THD = –114.0dB
SINAD = 97.0dB
SNR = 95.5dB
THD = –113.0dB
SINAD = 95.5dB
–40
AMPLITUDE (dB)
–80
–100
–120
–140
–60
–80
–100
–120
–140
07956-111
–160
–180
–200
0
200k
400k
600k
FREQUENCY (Hz)
800k
07956-114
AMPLITUDE (dB)
–60
fS = 2MSPS
fIN = 20kHz
–20
–160
–180
1M
0
Figure 11. FFT Plot (External Reference)
200k
400k
600k
FREQUENCY (Hz)
800k
1M
Figure 14. FFT Plot (Internal Reference)
18
100
–95
125
–100
120
SNR
17
16
THD (dB)
ENOB
90
ENOB (Bits)
–105
115
SFDR (dB)
SINAD
SFDR
–110
110
THD
–115
105
–120
100
2.5
3.0
3.5
4.0
REFERENCE VOLTAGE (V)
4.5
14
5.0
–125
95
2.5
3.0
Figure 12. SNR, SINAD, and ENOB vs. Reference Voltage
3.5
4.0
REFERENCE VOLTAGE (V)
4.5
07956-015
80
07956-212
15
85
5.0
Figure 15. THD and SFDR vs. Reference Voltage
100
–80
–85
–90
95
THD (dB)
–95
90
–100
–105
–110
85
80
1k
10k
100k
FREQUENCY (Hz)
07956-216
–115
07956-013
SINAD (dB)
SNR, SINAD (dB)
95
–120
–125
1M
1k
10k
100k
FREQUENCY (Hz)
Figure 16. THD vs. Frequency
Figure 13. SINAD vs. Frequency
Rev. D | Page 10 of 28
1M
Data Sheet
AD7986
3.0
99
IREF
2.5
97
96
95
94
93
92
2.0
IAVDD
1.5
1.0
0.5
IBVDD
91
–9
–8
–7
–6
–5
–4
INPUT LEVEL (dB)
–3
–2
–1
0
0
–55
07956-032
90
–10
–35
–15
5
25
45
65
85
105
125
TEMPERATURE (C)
Figure 17. SNR vs. Input Level
07956-034
98
SUPPLY CURRENT (mA)
SNR (dB REFERRED TO FULL SCALE)
100
Figure 19. Operating Currents vs. Temperature
3.0
14
IREF
12
SUPPLY CURRENT (µA)
2.0
IAVDD
IDVDD
1.0
8
6
4
IAVDD + IDVDD + IVIO
IBVDD
0.5
2
IVIO
0
2.375
10
2.425
2.475
2.525
2.575
AVDD AND DVDD VOLTAGE (V)
2.625
Figure 18. Operating Currents vs. Supply Voltage
0
–55
–35
–15
5
25
45
65
85
105
TEMPERATURE (°C)
Figure 20. Power-Down Currents vs. Temperature
Rev. D | Page 11 of 28
125
07956-035
1.5
07956-033
SUPPLY CURRENT (mA)
2.5
AD7986
Data Sheet
TERMINOLOGY
Integral Nonlinearity Error (INL)
INL refers to the deviation of each individual code from a line
drawn from negative full scale through positive full scale. The
point used as negative full scale occurs ½ LSB before the first
code transition. Positive full scale is defined as a level 1½ LSB
beyond the last code transition. The deviation is measured from
the middle of each code to the true straight line (see Figure 22).
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. DNL is the
maximum deviation from this ideal value. It is often specified in
terms of resolution for which no missing codes are guaranteed.
Zero Error
Zero error is the difference between the ideal midscale voltage,
that is, 0 V, from the actual voltage producing the midscale
output code, that is, 0 LSB.
Gain Error
The first transition (from 100 ... 00 to 100 ... 01) must occur at a
level ½ LSB above nominal negative full scale (−4.095984 V for
the ±4.096 V range). The last transition (from 011 … 10 to
011 … 11) must occur for an analog voltage 1½ LSB below
the nominal full scale (+4.095953 V for the ±5 V range). The
gain error is the deviation of the difference between the actual
level of the last transition and the actual level of the first transition
from the difference between the ideal levels.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms
amplitude of the input signal and the peak spurious signal.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to SINAD as follows:
ENOB = (SINADdB − 1.76)/6.02
and is expressed in bits.
Noise-Free Code Resolution
Noise-free code resolution is the number of bits beyond which it is
impossible to distinctly resolve individual codes. It is calculated as
Noise-Free Code Resolution = log2(2N/Peak-to-Peak Noise)
and is expressed in bits.
Effective Resolution
Effective resolution is calculated as
Effective Resolution = log2(2N/RMS Input Noise)
and is expressed in bits.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in decibels.
Dynamic Range
Dynamic range is the ratio of the rms value of the full scale to
the total rms noise measured with the inputs shorted together.
The value for dynamic range is expressed in decibels. It is measured
with a signal at −60 dBF so that it includes all noise sources and
DNL artifacts.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels.
Signal-to-(Noise + Distortion) (SINAD)
SINAD is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components that are less than
the Nyquist frequency, including harmonics but excluding dc.
The value of SINAD is expressed in decibels.
Aperture Delay
Aperture delay is the measure of the acquisition performance
and is the time between the rising edge of the CNV input and
when the input signal is held for a conversion.
Transient Response
Transient response is the time required for the ADC to accurately
acquire the input after a full-scale step function is applied.
Rev. D | Page 12 of 28
Data Sheet
AD7986
THEORY OF OPERATION
IN+
SWITCHES CONTROL
MSB
REF
131,072C
65,536C
LSB
2C
4C
C
SW+
C
BUSY
COMP
REFGND
131,072C
65,536C
2C
4C
C
MSB
CONTROL
LOGIC
C
OUTPUT CODE
LSB
SW–
07956-011
CNV
IN–
Figure 21. ADC Simplified Schematic
CIRCUIT INFORMATION
The AD7986 is a fast, low power, single-supply, precise, 18-bit
ADC using a successive approximation architecture. The AD7986
features different modes to optimize performance according to
the application. In turbo mode, the AD7986 is capable of converting 2,000,000 samples per second (2 MSPS).
The AD7986 provides the user with an on-chip track-and-hold
and does not exhibit any pipeline delay or latency, making it ideal
for multiple multiplexed channel applications.
The AD7986 can be interfaced to any 1.8 V to 2.7 V digital logic
family. It is available in a 20-lead LFCSP that allows space savings
and flexible configurations.
CONVERTER OPERATION
The AD7986 is a successive approximation ADC based on a
charge redistribution DAC. Figure 21 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 18 binary-weighted capacitors that are
connected to the two comparator inputs.
When the acquisition phase is complete and the CNV input goes
high, a conversion phase is initiated. When the conversion phase
begins, SW+ and SW− are opened first. The two capacitor arrays
are then disconnected from the analog inputs and connected to
the REFGND input. Therefore, the differential voltage between
Input IN+ and Input IN− captured at the end of the acquisition
phase is applied to the comparator inputs, causing the comparator
to become unbalanced. By switching each element of the capacitor
array between REFGND and REF, the comparator input varies
by binary-weighted voltage steps (VREF/2, VREF/4 … VREF/262,144).
The control logic toggles these switches, starting with the MSB,
to bring the comparator back into a balanced condition. After
the completion of this process, the device returns to the acquisition
phase, and the control logic generates the ADC output code
and a busy signal indicator.
Because the AD7986 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
CONVERSION MODES OF OPERATION
During the acquisition phase, terminals of the array tied to the
input of the comparator are connected to AGND via SW+ and
SW−. All independent switches are connected to the analog inputs.
Therefore, the capacitor arrays are used as sampling capacitors
and acquire the analog signal on the IN+ and IN− inputs.
The AD7986 features two conversion modes of operation: turbo
and normal. Turbo conversion mode (TURBO = high) allows the
fastest conversion rate of up to 2 MSPS, and does not power down
between conversions. The first conversion in turbo mode must
be ignored because it contains meaningless data. For applications
that require lower power and slightly slower sampling rates, the
normal mode (TURBO = low) allows a maximum conversion rate
of 1.5 MSPS, and powers down between conversion. The first
conversion in normal mode does contain meaningful data.
Rev. D | Page 13 of 28
AD7986
Data Sheet
Transfer Functions
Table 7. Output Codes and Ideal Input Voltages
Description
FSR − 1 LSB
Midscale + 1 LSB
Midscale
Midscale − 1 LSB
−FSR + 1 LSB
−FSR
011 ... 111
011 ... 110
011 ... 101
1
2
Analog Input
VREF = 4.096 V
+4.095969 V
+31.25 μV
0V
−31.25 μV
−4.095969 V
−4.096 V
Digital Output
Code (Hex)
0x1FFFF1
0x00001
0x00000
0x3FFFF
0x20001
0x200002
This is also the code for an overranged analog input (VIN+ − VIN− above
VREF − REFGND).
This is also the code for an underranged analog input (VIN+ − VIN− below REFGND).
100 ... 010
100 ... 001
100 ... 000
–FSR
TYPICAL CONNECTION DIAGRAM
–FSR + 1 LSB
+FSR – 1 LSB
+FSR – 1.5 LSB
–FSR + 0.5 LSB
ANALOG INPUT
Figure 23 shows an example of the recommended connection
diagram for the AD7986 when multiple supplies are available.
07956-012
Figure 22. ADC Ideal Transfer Function
5V
2.5V
1.8V TO 2.7V
V+
10Ω
0V TO VREF
1.5nF
IN+
V–
DD VIO
BVDD AVDD,
DVDD
TURBO
SDI
AD7986
V+
SCLK
SDO
IN–
CNV
VIO
3- OR 4-WIRE
INTERFACE:
SPI, CS
DAISY CHAIN (TURBO = LOW)
10Ω
REF GND
VREF TO 0V
1.5nF
10µF
V–
07956-016
ADC CODE (TWOS COMPLEMENT)
The ideal transfer function for the AD7986 is shown in Figure 22
and Table 7.
NOTES
1. GND REFERS TO REFGND, AGND, AND DGND.
Figure 23. Typical Application Diagram with Multiple Supplies
Rev. D | Page 14 of 28
Data Sheet
AD7986
ANALOG INPUTS
DRIVER AMPLIFIER CHOICE
Figure 24 shows an equivalent circuit of the input structure of
the AD7986.
Although the AD7986 is easy to drive, the driver amplifier must
meet the following requirements:
The two diodes, D1 and D2, provide ESD protection for the analog
inputs, IN+ and IN−. Take care to ensure the analog input signal
does not exceed the reference input voltage (REF) by more than
0.3 V. If the analog input signal exceeds this level, the diodes
become forward-biased and start conducting current. These
diodes can handle a forward-biased current of 130 mA maximum.
However, if the supplies of the input buffer (for example, the V+
and V− supplies of the buffer amplifier in Figure 23) are different
from those of REF, the analog input signal may eventually exceed
the supply rails by more than 0.3 V. In such a case (for example,
an input buffer with a short circuit), the current limitation can
protect the device.
•
SNRLOSS
REF
D1
RIN
CIN
D2
07956-014
CPIN
Figure 24. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. By using these differential
inputs, signals common to both inputs are rejected.
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of Capacitor CPIN and the network formed by the series connection
of Resistor RIN and Capacitor CIN. Capacitor CPIN is primarily the
pin capacitance. Resistor RIN is typically 400 Ω and is a lumped
component composed of serial resistors and the on resistance of
the switches. Capacitor CIN is typically 30 pF and is mainly the
ADC sampling capacitor.
During the sampling phase, where the switches are closed, the
input impedance is limited to Capacitor CPIN. Resistor RIN and
Capacitor CIN make a one-pole, low-pass filter that reduces
undesirable aliasing effects and limits noise.
When the source impedance of the driving circuit is low, the
AD7986 can be driven directly. Large source impedances
significantly affect the ac performance, especially THD. The
dc performances are less sensitive to the input impedance. The
maximum source impedance depends on the amount of THD
that can be tolerated. The THD degrades as a function of the
source impedance and the maximum input frequency.


44
= 20 log 

π
 44 2 + f −3dB (Ne N )2
2







where:
f–3dB is the input bandwidth, in megahertz, of the AD7986
(19 MHz) or the cutoff frequency of the input filter, if
one is used.
N is the noise gain of the amplifier (for example, 1 in buffer
configuration).
eN is the equivalent input noise voltage of the operational
amplifier, in nV/√Hz.
IN+ OR IN–
GND
The noise generated by the driver amplifier must be kept as
low as possible to preserve the SNR and transition noise
performance of the AD7986. The noise from the driver is
filtered by the AD7986 analog input circuit one-pole, lowpass filter, made by RIN and CIN or by the external filter if
one is used. Because the typical noise of the AD7986 is
44 µV rms, the SNR degradation due to the amplifier is
•
•
For ac applications, the driver must have a THD performance commensurate with the AD7986.
For multichannel multiplexed applications, the driver
amplifier and the AD7986 analog input circuit must settle
for a full-scale step onto the capacitor array at an 18-bit level
(0.0004%, 4 ppm). In the data sheet of the driver amplifier,
settling at 0.1% to 0.01% is more commonly specified. This
may differ significantly from the settling time at an 18-bit
level and must be verified prior to driver selection.
Table 8. Recommended Driver Amplifiers
Amplifier
AD8021
AD8022
ADA4899-1
AD8014
Rev. D | Page 15 of 28
Typical Application
Very low noise and high frequency
Low noise and high frequency
Ultralow noise and high frequency
Low power and high frequency
AD7986
Data Sheet
VOLTAGE REFERENCE INPUT
The advantages of directly using the external voltage reference are:
The AD7986 allows the choice of a very low temperature drift
internal voltage reference, an external reference, or an external
buffered reference.

The internal reference of the AD7986 provides excellent
performance and can be used in almost all applications.
The SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a larger reference voltage (5 V)
instead of a typical 4.096 V reference when the internal
reference is used. This is calculated by
4.096 
SNR  20 log

 5.0 
Internal Reference, REF = 4.096V (PDREF = Low)
To use the internal reference, the PDREF input must be low. This
enables the on-chip band gap reference and buffer, resulting in
a 4.096 V reference on the REF pin (1.2 V on REFIN).
The internal reference is temperature compensated to 4.096 V ±
15 mV. The reference is trimmed to provide a typical drift of
10 ppm/°C.
The output resistance of REFIN is 6 kΩ when the internal
reference is enabled. It is necessary to decouple this pin with a
ceramic capacitor of at least 100 nF. The output resistance of REFIN
and the decoupling capacitor form an RC filter, which helps to
reduce noise.
Because the output impedance of REFIN is typically 6 kΩ, relative
humidity (among other industrial contaminants) can directly affect
the drift characteristics of the reference. A guard ring typically
reduces the effects of drift under such circumstances. However,
the fine pitch of the AD7986 makes this difficult to implement.
One solution, in these industrial and other types of applications,
is to use a conformal coating, such as Dow Corning® 1-2577 or
HumiSeal® 1B73.
External 1.2 V Reference and Internal Buffer (PDREF = High)
To use an external reference along with the internal buffer, PDREF
must be high. This powers down the internal reference and
allows the 1.2 V reference to be applied to REFIN, producing
4.096 V (typically) on the REF pin.
External Reference (PDREF = High, REFIN = Low)
To apply an external reference voltage directly to the REF pin,
PDREF must be tied high, and REFIN must be tied low. BVDD
must also be driven to the same potential as REF. For example,
if REF = 2.5 V, BVDD must be tied to 2.5 V.

The power savings when the internal reference is powered
down (PDREF high).
Reference Decoupling
The AD7986 voltage reference input, REF, has a dynamic input
impedance that requires careful decoupling between the REF
and REFGND pins. The Layout section describes how this can
be done.
When using an external reference, a very low impedance source
(for example, a reference buffer using the AD8031 or the AD8605),
and a 10 μF (X5R, 0805 size) ceramic chip capacitor are appropriate
for optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR434 reference.
If desired, a reference decoupling capacitor with values as small
as 2.2 μF can be used with minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and REFGND pins.
POWER SUPPLY
The AD7986 uses four power supply pins: an analog supply
(AVDD), a buffer supply (BVDD), a digital supply (DVDD),
and a digital input/output interface supply (VIO). VIO allows
direct interface with any logic between 1.8 V and 2.7 V. To reduce
the number of supplies needed, VIO, DVDD, and AVDD can
be tied together. The AD7986 is independent of power supply
sequencing among all of the supplies. Additionally, it is very
insensitive to power supply variations over a wide frequency range.
Rev. D | Page 16 of 28
Data Sheet
AD7986
DIGITAL INTERFACE
Although the AD7986 has a reduced number of pins, it offers
flexibility in the serial interface modes.
When in CS mode, the AD7986 is compatible with SPI,
MICROWIRE™, QSPI™, and digital hosts. In this mode, the
AD7986 can use either a 3-wire or a 4-wire interface. A 3-wire
interface using the CNV, SCK, and SDO signals minimizes wiring
connections, which is useful, for instance, in isolated applications.
A 4-wire interface using the SDI, CNV, SCK, and SDO signals
allows CNV, which initiates conversions, to be independent of
the readback timing (SDI). This is useful in low jitter sampling
or simultaneous sampling applications.
When in chain mode, the AD7986 provides a daisy-chain feature
using the SDI input for cascading multiple ADCs on a single data
line similar to a shift register. Chain mode is only available in
normal mode (TURBO = low).
The mode in which the device operates depends on the SDI
level when the CNV rising edge occurs. The CS mode is
selected if SDI is high, and the chain mode is selected if SDI is
low. The SDI hold time is such that when SDI and CNV are
connected together, the chain mode is always selected.
In normal mode operation, the AD7986 offers the option of
forcing a start bit in front of the data bits. This start bit can be
used as a busy signal indicator to interrupt the digital host and
trigger the data reading. Otherwise, without a busy indicator,
the user must time out the maximum conversion time prior to
readback.
The busy indicator feature is enabled in CS mode if CNV or
SDI is low when the ADC conversion ends (see Figure 28 and
Figure 32), and TURBO must be kept low for both digital
interfaces.
When CNV is low, reading can occur during conversion and
acquisition, and when split across acquisition and conversion,
as detailed in the following sections.
A discontinuous SCK is recommended because the device is
selected with CNV low, and SCK activity begins to clock
out data.
Note that in the following sections, the timing diagrams
indicate digital activity (SCK, CNV, SDI, and SDO) during
the conversion. However, due to the possibility of performance
degradation, digital activity must occur only prior to the safe
data reading time, tDATA, because the AD7986 provides error
correction circuitry that can correct for an incorrect bit decision
during this time. From tDATA to tCONV, there is no error correction,
and conversion results may be corrupted. Similarly, tQUIET, the
time from the last falling edge of SCK to the rising edge of CNV,
must remain free of digital activity. The user must configure the
AD7986 and initiate the busy indicator (if desired in normal
mode) prior to tDATA. It is also possible to corrupt the sample by
having SCK near the sampling instant. Therefore, it is recommended to keep the digital pins quiet for approximately 20 ns
before and 10 ns after the rising edge of CNV, using a
discontinuous SCK whenever possible to avoid any potential
performance degradation.
Rev. D | Page 17 of 28
AD7986
Data Sheet
DATA READING OPTIONS
There are three different data reading options for the AD7986.
There is the option to read during conversion, to split the read
across acquisition and conversion (see Figure 25 and Figure 26),
and in normal mode, to read during acquisition. The desired
SCK frequency largely determines which reading option to
pursue.
Reading During Conversion, Fast Hosts (Turbo or
Normal Mode)
For turbo mode (2 MSPS),
fSCK = 75 MHz; tDATA = 200 ns; tCNVH = 10 ns; tEN = 10 ns
Number_SCK_Edges = 75 MHz × (200 ns − 10 ns − 10 ns) = 13.5
Thirteen bits are read during conversion (n), and five bits are
read during acquisition (n).
For normal mode (1.5 MSPS),
When reading during conversion (n), conversion results are for
the previous (n − 1) conversion. Reading must only occur up to
tDATA and, because this time is limited, the host must use a
fast SCK.
The required SCK frequency is calculated by
f SCK ≥
To determine how to split the read for a particular SCK frequency,
follow these examples to read data from conversion (n − 1).
Number _ SCK _ Edges
t DATA − t CNVH − t EN
To determine the SCK frequency, follow these examples to read
data from conversion (n − 1).
Turbo mode (2 MSPS),
Number_SCK_Edges = 18; tDATA = 200 ns; tCNVH = 10 ns; tEN = 10 ns
fSCK = 18/(200 ns − 10 ns − 10 ns) = 100 MHz
Normal mode (1.5 MSPS),
Number_SCK_Edges = 18; tDATA = 300 ns; tCNVH = 10 ns; tEN = 10 ns
fSCK = 18/(300 ns − 10 ns − 10 ns) = 64.3 MHz
The time between tDATA and tCONV is an input/output quiet time
where digital activity must not occur, or sensitive bit decisions
may be corrupt.
Split-Reading, Any Speed Host (Turbo or Normal Mode)
To allow for slower SCK, there is the option of a split read where
data access starts at the current acquisition (n) and spans into the
conversion (n). Conversion results are for the previous (n − 1)
conversion.
fSCK = 50 MHz; tDATA = 300 ns; tCNVH = 10 ns; tEN = 10 ns
Number_SCK_Edges = 50 MHz × (300 ns − 10 ns − 10 ns) = 14
Fourteen bits are read during conversion (n), and four bits are
read during acquisition (n).
For slow throughputs, the time restriction is dictated by the
required throughput by the user, and the host is free to run at
any speed. Similar to the reading during acquisition, for slow
hosts, the data access must take place during the acquisition
phase with additional time into the conversion.
Note that data access spanning conversion requires the CNV to
be driven high to initiate a new conversion, and data access is
not allowed when CNV is high. Thus, the host must perform
two bursts of data access when using this method.
Reading During Acquisition, Any Speed Hosts (Turbo or
Normal Mode)
When reading during acquisition (n), conversion results are
for the previous (n − 1) conversion. Maximum throughput is
achievable in normal mode (1.5 MSPS); however, in turbo
mode, 2 MSPS throughput is not achievable.
For the maximum throughput, the only time restriction is that the
reading takes place during the tACQ (minimum) time. For slow
throughputs, the time restriction is dictated by throughput required
by the user, and the host is free to run at any speed. Thus for slow
hosts, data access must take place during the acquisition phase.
Similar to reading during conversion, split-reading must only
occur up to tDATA. For the maximum throughput, the only time
restriction is that split-reading take place during the tACQ
(minimum) + tDATA − tQUIET time. The time between the falling
edge of SCK and CNV rising is an acquisition quiet time, tQUIET.
Rev. D | Page 18 of 28
Data Sheet
AD7986
This can be useful, for instance, to bring CNV low to select other
SPI devices, such as analog multiplexers; however, CNV must be
returned high before the minimum conversion time elapses and
then held high for the maximum possible conversion time to avoid
the generation of the busy signal indicator. When the conversion is
complete, the AD7986 enters the acquisition phase and powers
down. When CNV goes low, the MSB is output onto SDO. The
remaining data bits are clocked by subsequent SCK falling edges.
The data is valid on both SCK edges. Although the rising edge
can capture the data, a digital host using the SCK falling edge
allows a faster reading rate, provided that it has an acceptable hold
time. After the 18th SCK falling edge or when CNV goes high
(whichever occurs first), SDO returns to high impedance.
CS MODE, 3-WIRE WITHOUT BUSY INDICATOR
This mode is usually used when a single AD7986 is connected
to an SPI-compatible digital host. The connection diagram is
shown in Figure 25, and the corresponding timing is given in
Figure 26.
With SDI tied to VIO, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. Once
a conversion is initiated, it continues until completion irrespective
of the state of CNV.
CONVERT
DIGITAL HOST
CNV
VIO
SDI
AD7986
DATA IN
SDO
07956-018
SCK
CLK
Figure 25. CS Mode, 3-Wire Without Busy Indicator Connection Diagram (SDI High)
tCYC
> tCONV
tCONV
SDI = 1
tDATA
tCNVH
tCONV
tDATA
CNV
tACQ
ACQUISITION
(n - 1)
CONVERSION (n – 1)
(I/O QUIET
TIME)
ACQUISITION (n)
(QUIET
TIME)
(I/O QUIET
TIME)
CONVERSION (n)
ACQUISITION
(n + 1)
tQUIET
16
17
1
18
2
16
17
tHSDO
tEN
SDO
tDSDO
tEN
2
1
tDIS END DATA (n – 2)
0
17
tDIS
18
tSCK
16
15
BEGIN DATA (n – 1)
2
Figure 26. CS Mode, 3-Wire Without Busy Indicator Serial Interface Timing (SDI High)
Rev. D | Page 19 of 28
1
tDIS END DATA (n – 1)
0
tDIS
07956-116
SCK
AD7986
Data Sheet
When the conversion is complete, SDO goes from high impedance to low impedance. With a pull-up on the SDO line, this
transition can be used as an interrupt signal to initiate the data
reading controlled by the digital host. The AD7986 then enters
the acquisition phase and powers down. The data bits are then
clocked out, MSB first, by subsequent SCK falling edges. The
data is valid on both SCK edges. Although the rising edge can
capture the data, a digital host using the SCK falling edge allows
a faster reading rate, provided that it has an acceptable hold time.
After the optional 19th SCK falling edge, SDO returns to high
impedance.
CS MODE, 3-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7986 is connected
to an SPI-compatible digital host having an interrupt input.
It is only available in normal conversion mode (TURBO = low).
The connection diagram is shown in Figure 27, and the
corresponding timing is given in Figure 28.
With SDI tied to VIO, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. SDO
is maintained in high impedance until the completion of the
conversion irrespective of the state of CNV. Prior to the minimum
conversion time, CNV can select other SPI devices, such as analog
multiplexers, but CNV must be returned low before the minimum
conversion time elapses and then held low for the maximum
possible conversion time to guarantee the generation of the
busy signal indicator.
If multiple AD7986 devices are selected at the same time, the
SDO output pin handles this contention without damage or
induced latch-up. Meanwhile, it is recommended to keep this
contention as short as possible to limit extra power dissipation.
CONVERT
VIO
DIGITAL HOST
CNV
VIO
47kΩ
SDI
AD7986
DATA IN
SDO
IRQ
TURBO
07956-020
SCK
CLK
Figure 27. CS Mode, 3-Wire with Busy Indicator Connection Diagram (SDI High)
TURBO = 0
SDI = 1
tCYC
tCNVH
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
(QUIET
TIME)
tSCK
tQUIET
tSCKL
1
2
3
17
tHSDO
18
19
tSCKH
tDSDO
SDO
D17
D16
tDIS
D1
D0
Figure 28. CS Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High)
Rev. D | Page 20 of 28
07956-021
SCK
Data Sheet
AD7986
Prior to the minimum conversion time, SDI can select other SPI
devices, such as analog multiplexers, but SDI must be returned
high before the minimum conversion time elapses and then held
high for the maximum possible conversion time to avoid the
generation of the busy signal indicator. When the conversion is
complete, the AD7986 enters the acquisition phase and powers
down. Each ADC result can be read by bringing the SDI input
low, which consequently outputs the MSB onto SDO. The
remaining data bits are then clocked by subsequent SCK falling
edges. The data is valid on both SCK edges. Although the rising
edge can capture the data, a digital host using the SCK falling
edge allows a faster reading rate, provided that it has an acceptable
hold time. After the 18th SCK falling edge, SDO returns to high
impedance and another AD7986 can be read.
CS MODE, 4-WIRE WITHOUT BUSY INDICATOR
This mode is usually used when multiple AD7986 devices are
connected to an SPI-compatible digital host.
A connection diagram example using two AD7986 devices is
shown in Figure 29 and the corresponding timing is given in
Figure 30.
With SDI high, a rising edge on CNV initiates a conversion, selects
the CS mode, and forces SDO to high impedance. In this mode,
CNV must be held high during the conversion phase and the
subsequent data readback. (If SDI and CNV are low, SDO is
driven low.)
CS2
CS1
CONVERT
CNV
SDO
SDI
SCK
AD7986
DIGITAL HOST
SDO
SCK
07956-022
AD7986
SDI
CNV
DATA IN
CLK
Figure 29. CS Mode, 4-Wire Without Busy Indicator Connection Diagram
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSSDICNV
SDI (CS1)
tHSDICNV
tQUIET
SDI (CS2)
tSCK
tSCKL
1
2
3
16
tHSDO
18
19
20
34
35
36
tDSDO
tEN
SDO
17
tSCKH
D17
D16
D15
tDIS
D1
D0
D17
D16
Figure 30. CS Mode, 4-Wire Without Busy Indicator Serial Interface Timing
Rev. D | Page 21 of 28
D1
D0
07956-023
SCK
AD7986
Data Sheet
Prior to the minimum conversion time, SDI can select other SPI
devices, such as analog multiplexers, but SDI must be returned
low before the minimum conversion time elapses and then held
low for the maximum possible conversion time to guarantee the
generation of the busy signal indicator. When the conversion is
complete, SDO goes from high impedance to low impedance.
With a pull-up on the SDO line, this transition can be used as
an interrupt signal to initiate the data readback controlled by
the digital host. The AD7986 then enters the acquisition phase
and powers down. The data bits are then clocked out, MSB first,
by subsequent SCK falling edges. The data is valid on both SCK
edges. Although the rising edge can capture the data, a digital host
using the SCK falling edge allows a faster reading rate, provided
that it has an acceptable hold time. After the optional 19th SCK
falling edge or SDI going high (whichever occurs first), SDO
returns to high impedance.
CS MODE, 4-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7986 is connected
to an SPI-compatible digital host with an interrupt input and
when it is desired to keep CNV, which samples the analog input,
independent of the signal that selects the data reading. This
independence is particularly important in applications where
low jitter on CNV is desired. This mode is only available in
normal conversion mode (TURBO = low).
The connection diagram is shown in Figure 31, and the
corresponding timing is given in Figure 32.
With SDI high, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback. (If SDI and CNV are low, SDO is
driven low.)
CS1
CONVERT
VIO
DIGITAL HOST
CNV
47kΩ
AD7986
SCK
DATA IN
SDO
IRQ
TURBO
07956-024
SDI
CLK
Figure 31. CS Mode, 4-Wire with Busy Indicator Connection Diagram
TURBO = 0
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
(I/O QUIET
TIME)
tSSDICNV
SDI
tSCK
tHSDICNV
tSCKL
1
2
3
tHSDO
17
18
19
tSCKH
tDSDO
tDIS
tEN
SDO
D17
D16
D1
Figure 32. CS Mode, 4-Wire with Busy Indicator Serial Interface Timing
Rev. D | Page 22 of 28
D0
07956-025
SCK
tQUIET
Data Sheet
AD7986
CHAIN MODE WITHOUT BUSY INDICATOR
In this mode, CNV is held high during the conversion phase and
the subsequent data readback. When the conversion is complete,
the MSB is output onto SDO, and the AD7986 enters the
acquisition phase and powers down. The remaining data bits
stored in the internal shift register are clocked by subsequent
SCK falling edges. For each ADC, SDI feeds the input of the
internal shift register and is clocked by the SCK falling edge.
Each ADC in the chain outputs the data MSB first, and 18 × N
clocks are required to read back the N ADCs. The data is valid
on both SCK edges. Although the rising edge can capture the
data, a digital host using the SCK falling edge allows a faster
reading rate and consequently more AD7986 devices in the chain,
provided that the digital host has an acceptable hold time. The
maximum conversion rate may be reduced due to the total
readback time.
This mode can daisy-chain multiple AD7986 devices on a
3-wire serial interface. It is only available in normal conversion
mode (TURBO = low). This feature is useful for reducing
component count and wiring connections, for example, in
isolated multiconverter applications or for systems with a
limited interfacing capacity. Data readback is analogous to
clocking a shift register.
A connection diagram example using two AD7986 devices is
shown in Figure 33 and the corresponding timing is given in
Figure 34.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion, selects the chain
mode, and disables the busy indicator.
CONVERT
SDI
CNV
AD7986
A
SCK
SDO
SDI
DIGITAL HOST
AD7986
B
SCK
TURBO
DATA IN
SDO
TURBO
07956-026
CNV
CLK
Figure 33. Chain Mode Without Busy Indicator Connection Diagram
SDIA = 0
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
tQUIET
SCK
1
2
3
16
17
tSSDISCK
tHSCKCNV
18
19
20
DA17
DA16
34
35
36
DA1
DA0
tSCKH
tHSDISCK
tEN
SDOA = SDIB
DA17
DA16
DA15
DA1
DA 0
DB17
DB16
DB15
DB1
DB 0
SDOB
Figure 34. Chain Mode Without Busy Indicator Serial Interface Timing
Rev. D | Page 23 of 28
07956-027
tHSDO
tDSDO
AD7986
Data Sheet
CHAIN MODE WITH BUSY INDICATOR
When all ADCs in the chain have completed their conversions,
the SDO pin of the ADC closest to the digital host (see the
AD7986 ADC labeled C in Figure 35) is driven high. This
transition on SDO can be used as a busy indicator to trigger the
data readback controlled by the digital host. The AD7986 then
enters the acquisition phase and powers down. The data bits
stored in the internal shift register are clocked out, MSB first, by
subsequent SCK falling edges. For each ADC, SDI feeds the
input of the internal shift register and is clocked by the SCK
falling edge. Each ADC in the chain outputs the data MSB first,
and 18 × N + 1 clocks are required to read back the N ADCs.
Although the rising edge can capture the data, a digital host using
the SCK falling edge allows a faster reading rate and consequently
more AD7986 devices in the chain, provided that the digital
host has an acceptable hold time.
This mode can also daisy-chain multiple AD7986 devices on a
3-wire serial interface while providing a busy indicator. This
feature is useful for reducing component count and wiring
connections, for example, in isolated multiconverter applications or for systems with a limited interfacing capacity. Data
readback is analogous to clocking a shift register. A connection
diagram example using three AD7986 devices is shown in
Figure 35, and the corresponding timing is given in Figure 36.
When SDI and CNV are low, SDO is driven low. With SCK
high, a rising edge on CNV initiates a conversion, selects the
chain mode, and enables the busy indicator feature. In this
mode, CNV is held high during the conversion phase and the
subsequent data readback.
CONVERT
SDI
CNV
AD7986
SDO
SDI
CNV
AD7986
SDO
SDI
AD7986
B
A
SCK
SDO
DATA IN
C
SCK
TURBO
DIGITAL HOST
SCK
TURBO
IRQ
TURBO
07956-028
CNV
CLK
Figure 35. Chain Mode with Busy Indicator Connection Diagram
TURBO = 0
tCYC
ACQUISITION
tCONV
tACQ
ACQUISITION
CONVERSION
tQUIET
SCK
tSCKH
1
2
tSSDISCK
tHSCKCNV
tEN
SDOA = SDIB
3
4
tSCK
17
18
tHSDISCK
DA17 DA16 DA15
19
20
21
35
36
37
38
39
tSCKL
D A1
SDOB = SDIC
55
DA0
tDSDOSDI
DB17 DB16 DB15
DB1
DB0 DA17 DA16
DA1
DA0
DC17 DC16 DC15
D C1
DC0 DB17 DB16
DB1
DB0 DA17 DA16
tDSDOSDI
SDOC
54
tDSDOSDI
tHSDO
tDSDO
tDSDOSDI
53
tDSDOSDI
Figure 36. Chain Mode with Busy Indicator Serial Interface Timing
Rev. D | Page 24 of 28
DA 1
DA0
07956-029
CNV = SDIA
Data Sheet
AD7986
APPLICATION HINTS
LAYOUT
Design the printed circuit board (PCB) that houses the AD7986 so
the analog and digital sections are separated and confined to
certain areas of the board. The pinout of the AD7986, with the
analog signals on the left side and the digital signals on the right
side, eases this task.
Avoid running digital lines under the device because these couple
noise onto the die, unless a ground plane under the AD7986 is
used as a shield. Fast switching signals, such as CNV or clocks,
must not run near analog signal paths. Crossover of digital and
analog signals must be avoided.
At least one ground plane must be used. It can be common or
split between the digital and analog sections. In the latter case,
the planes must be joined underneath the AD7986 devices.
The AD7986 voltage reference input (REF) has a dynamic input
impedance and must be decoupled with minimal parasitic inductances. This is done by placing the reference decoupling ceramic
capacitor close to, ideally right against, the REF and REFGND pins
and connecting them with wide, low impedance traces.
Finally, the power supplies, VDD and VIO of the AD7986, must
be decoupled with ceramic capacitors, typically 100 nF, placed
close to the AD7986 and connected using short, wide traces to
provide low impedance paths and to reduce the effect of glitches
on the power supply lines.
EVALUATING THE PERFORMANCE OF THE AD7986
The evaluation board package for the AD7986, EVALAD7986FMCZ, includes a fully assembled and tested evaluation
board and software for controlling the board from a PC, via the
controller board, EVAL-SDP-CH1Z.
Rev. D | Page 25 of 28
AD7986
Data Sheet
AVDD
BVDD
REF
REF
REF
PADDLE
1
2
GND
GND
GND
GND
3
4
DVDD
5
GND
6
GND
07956-030
GND
VIO
Figure 37. Example Layout of the AD7986 (Top Layer)
5V
EXTERNAL
REFERENCE
(ADR435 OR ADR445)
REF
REF
AVDD CAVDD
CBVDD BVDD
REF
GND
CREF
GND
GND
GND
GND
CDVDD
DVDD
GND
GND
VIO
CVIO
VIO
07956-031
GND
Figure 38. Example Layout of the AD7986 (Bottom Layer)
Rev. D | Page 26 of 28
Data Sheet
AD7986
OUTLINE DIMENSIONS
PIN 1
INDICATOR
4.10
4.00 SQ
3.90
0.30
0.25
0.20
0.50
BSC
20
16
15
PIN 1
INDICATOR
1
EXPOSED
PAD
2.65
2.50 SQ
2.35
5
11
0.80
0.75
0.70
SEATING
PLANE
0.50
0.40
0.30
10
6
0.25 MIN
BOTTOM VIEW
0.05 MAX
0.02 NOM
COPLANARITY
0.08
0.20 REF
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
COMPLIANT TO JEDEC STANDARDS MO-220-WGGD.
061609-B
TOP VIEW
Figure 39. 20-Lead Lead Frame Chip Scale Package [LFCSP]
4 mm × 4 mm Body and 0.75 mm Package Height
(CP-20-10)
Dimensions shown in millimeters
ORDERING GUIDE
Model 1, 2, 3
AD7986BCPZ
AD7986BCPZ-RL7
EVAL-AD7986FMCZ
EVAL-SDP-CH1Z
Temperature
Range
−40°C to +85°C
−40°C to +85°C
Package Description
20-Lead Lead Frame Chip Scale Package LFCSP, Tray
20-Lead Lead Frame Chip Scale Package LFCSP, 7” Tape and Reel
Evaluation Board
Controller Board
Package
Option
CP-20-10
CP-20-10
Ordering
Quantity
490
1,500
Z = RoHS Compliant Part.
The EVAL-AD7986FMCZ can be used as a standalone evaluation board or in conjunction with the EVAL-SDP-CH1Z for evaluation and/or demonstration purposes.
3
The EVAL-SDP-CH1Z allows a PC to control and communicate with all Analog Devices, Inc. evaluation boards ending in the FMC designator.
1
2
Rev. D | Page 27 of 28
AD7986
Data Sheet
NOTES
©2009–2016 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D07956-0-3/16(D)
Rev. D | Page 28 of 28
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