Texas Instruments | Circuit for driving a switched-capacitor SAR ADC w/ a buffered instrument amp (Rev. A) | Application notes | Texas Instruments Circuit for driving a switched-capacitor SAR ADC w/ a buffered instrument amp (Rev. A) Application notes

Texas Instruments Circuit for driving a switched-capacitor SAR ADC w/ a buffered instrument amp (Rev. A) Application notes
Analog Engineer's Circuit: Data
Converters
SBAA277A – February 2018 – Revised March 2019
Circuit for driving a switched-capacitor SAR ADC with a
buffered instrumentation amplifier
Art Kay
Input
ADC Input
Digital Output ADS8860
–10mV
Out = 0.2V
0A3D H or 262110
5mV
Out = 4.8V
F5C3H or 6291510
Power Supplies
AVDD
DVDD
Vref_INA
V ref
V cc
V ee
5.0V
3V
3.277V
5.0V
15V
–15V
Design Description
Instrumentation amplifiers are a common way of translating low-level sensor outputs to high-level signals
to drive an ADC. Typically, instrumentation amplifiers are optimized for low noise, low offset, and low drift.
Unfortunately, the bandwidth of many instrumentation amplifiers may not be sufficient to achieve good
settling to ADC charge kickback at maximum sampling rates. This document shows how a wide-bandwidth
buffer can be used with an instrumentation amplifier to achieve good settling at high sampling rates.
Furthermore, many instrumentation amplifiers are optimized for high voltage supplies and it may be
required to interface the high voltage output (that is, ±15V) to a lower voltage amplifier (for example, 5V).
This design shows how a current-limiting resistor can protect the amplifier from electrical overstress in
cases where the instrumentation amplifier is outside the input range of the op amp. A related cookbook
circuit shows a simplified approach that does not include the wide-bandwidth buffer (Driving a SwitchedCapacitor SAR With an Instrumentation Amplifier). The simplified approach has limited sampling rate as
compared to the buffered design. Note that the following circuit shows a bridge sensor, but this method
could be used for a wide range of different sensors.
This circuit implementation is applicable in applications such as Analog Input Modules, Electrocardiogram
(ECG), Pulse Oximeters, Lab Instrumentation, and Control Units for Rail Transport.
+15V
-15V
5V
Rg
INA826
Rfilt
25.5
-
Rg 162
++
Ref
-15V
5.0V
3.0V
Vref
AVDD
DVDD
INP
R3 10k
Rg +
+
5.0V
OPA320
Cfilt
1.1n
ADS8860
C1 1n
INM
GND
+15V
+5.0V
R1
11.3k
-15V
-
R2
21.5k
++
OPA192
+15V
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
Copyright © 2018–2019, Texas Instruments Incorporated
1
www.ti.com
Specifications
Specification
Calculated
Simulated
Sampling rate
1Msps
1Msps, settling to –44µV
40μV · 306.7 = 12.27mV
16mV
Offset (ADC Input)
Offset Drift
Noise
(0.4μV/ºC) · 306.7 = 123μV/ºC
N/A
978μV
586μVRMS
Design Notes
1. The bandwidth of instrumentation amplifiers is typically too low to drive SAR data converters at high
data rates (the INA826 bandwidth is 10.4kHz for a gain of 305V/V in this example). Wide bandwidth is
needed because the SAR has a switched capacitor input that needs to be charged during each
conversion cycle. The OPA320 buffer was added to allow the ADC to run at full data rate (ADS8860
1Msps).
2. Select the gain to achieve an input swing that matches the input range of the ADC. Use the
instrumentation amplifier reference pin to shift the signal offset to match the input range. This is
covered in the component selection section.
3. The INA826 gain is scaled so that the op amp input voltage levels are inside the normal operating
range of the amplifier. However, during power up or when a sensor is disconnected the output may
drive to either power supply rail (±15V). The resistor R3 is used to limit the current . This is covered in
the Overvoltage Protection Filter Between Instrumentation Amplifier and Op Amp section of this
document.
4. The buffer amplifier following the voltage divider is required for driving the reference input of most
instrumentation amplifiers. Choose precision resistors and a precision low offset amplifier as the buffer.
Refer to Selecting the right op amp for more details on this subject.
5. Check the common mode range of the amplifier using the Common-Mode Input Range Calculator for
Instrumentation Amplifiers software tool.
6. Select COG capacitors for C1, and Cfilt to minimize distortion.
7. Use 0.1% 20ppm/°C film resistors or better for the gain set resistor Rg. The error and drift of this
resistor will directly translate into gain error and gain drift.
8. The TI Precision Labs – ADCs training video series methods for selecting the charge bucket circuit Rfilt
and Cfilt. Refer to Introduction to SAR ADC Front-End Component Selection for details on this subject.
2
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Copyright © 2018–2019, Texas Instruments Incorporated
www.ti.com
Component Selection
1. Find the gain set resistor for the instrumentation amplifier to set the output swing to 0.2V to 4.8V.
.
Vout _ max Vout _ min
Gain
Vin _ max Vin _ min
Gain 1
Rg
4.9V 0.2V
5mV ( 10mV )
306.7
49.4k :
Rg
49.4k :
Gain 1.0
49.4k :
(306.7) 1.0
151.6: or 162: for standard 0.1% resistor
2. Find the INA826 reference voltage (Vref) to shift the output swing to the proper voltage level
.
Vout
Gain ˜Vin Vref _ INA
Vref _ INA Vout
Gain ˜ Vin
4.8V
§ 49.4k : ·
¨1
¸· 5mV
162: ¹
©
3.27V
3. Select standard value resistors to set the INA826 reference voltage (Vref_INA = 3.27V). Use the Analog
Engineer's Calculator ("Passive\Find Voltage Divider" section) to find standard values for the voltage
divider.
Vref _ INA
R2
R1
R2
21.5k :
· 5V
11.3k : 21.5k :
·Vin _ div
3.277V
The resistors given are
standard value
resistors and can be
scaled by factors of 10.
Enter the input and
output for the
voltage divider.
4. Use the Common-Mode Input Range Calculator for Instrumentation Amplifiers to determine if the
INA826 is violating the common mode range.
Select INA826.
Enter supply voltage, gain,
and INA reference input.
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Enter the common
mode input to the INA
(Vcm = 0V here)
The linear output range
is given here.
-14.9V < Vout < 14.85V
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
Copyright © 2018–2019, Texas Instruments Incorporated
3
www.ti.com
DC Transfer Characteristics
The following graph shows a linear output response for inputs from –5mV to +15mV. Refer to Determining
a SAR ADC’s Linear Range when using Instrumentation Amplifiers for detailed theory on this subject. In
cases where the INA826 output exceeds the op amp input range, the ESD diodes turn on and limit the
input. The resistor R3 protects the amplifier from damage by limiting the input current (see the
Overvoltage Protection Filter Between Instrumentation Amplifier and Op Amp section). The op amp output
is inside the absolute maximum rating of the ADS8860 (–0.3V < VIN < REF +0.3V).
5.19
Output
(5mV, 4.8227V)
2.50
(-10mV, 0.235V)
Vadc
-189m
-15m
-10m
-5m
0
5m
10m
AC Transfer Characteristics
The bandwidth is simulated to be 11.45 kHz in this configuration. In this bandwidth it is not possible to
drive the SAR converter at full speed. See the TI Precision Labs video series Op Amps: Bandwidth 1 for
more details on this subject.
60
Gain (dB)
40
Gain(dB) = 49.7dB
Gain(V/V) = 305.8 V/V
20
fc = 11.45kHz
0
-20
10
4
100
1k
10k
Frequency (Hz)
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
100k
1MEG
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Copyright © 2018–2019, Texas Instruments Incorporated
www.ti.com
Transient ADC Input Settling Simulation
The OPA320 buffer (20MHz) is used because it is capable of responding to the rapid transients from the
ADC8860 charge kickback. This type of simulation shows that the sample and hold kickback circuit is
properly selected. Refer to Introduction to SAR ADC Front-End Component Selection for detailed theory
on this subject.
Output
1.00m
0.00
Error = -44µV
-1.00m
1.25u
Time (s)
1.00u
1.50u
Noise Simulation
Use a simplified noise calculation for a rough estimate. We neglect the noise from the OPA192 as the
instrumentation amplifier is in high gain so its noise is dominant.
.
2
En
Gain· eNI 2
§ eNO ·
¨
¸ · Kn· f c
© Gain ¹
2
En
305.8 · 18nV / Hz
2
§ 110nV / Hz ·
¨
¸ · 1.57· 11.45kHz
305.8
©
¹
738PV / Hz
Note that the calculated and simulated match well. Refer to TI Precision Labs - Op Amps: Noise 4 for
detailed theory on amplifier noise calculations, and Calculating the Total Noise for ADC Systems for data
converter noise.
Total noise (V)
T 586.63u
293.31u
0.00
10
1
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
100
1k
10k
Frequency (Hz)
100k
1MEG
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
Copyright © 2018–2019, Texas Instruments Incorporated
5
www.ti.com
Overvoltage Protection Filter Between Instrumentation Amplifier and Op Amp
The filter between the INA826 and OPA320 serves two purposes. It protects the OPA320 from
overvoltage, and acts as a noise or anti-aliasing filter. The INA826 gain should be scaled so that under
normal circumstances, the output is inside the range of the OPA320 (that is, 0V to 5V). Thus, normally the
overvoltage signals applied to the input of the OPA320 is not seen. However, during power up or in cases
where the sensor is disconnected, the INA826 output may be at either power supply rail (that is, ±15V). In
overvoltage cases, the resistor (R3) will limit current into the OPA320 for protection. The internal ESD
diodes on the OPA320 will turn on during overvoltage events and direct the overvoltage signal to the
positive or negative supply. In the following example, the overvoltage signal is directed to the positive
supply and the transient voltage suppressor (D1, SMAJ5.0A) turns on to sink the current. Note that the
resistor is scaled to limit the current to the OPA320 absolute maximum input current (10mA). See TI
Precision Labs - Op Amps: Electrical Overstress (EOS) for detailed theory on this subject.
.
R3 !
C1
VINA VOpaSupply
0.7V
15V
I ABS _ MAX _ OPA
1
2 ˜ S ˜ R3 ˜ f c
5.0V 0.7V
10mA
1
2 ˜ S ˜ (10k :) ˜ (15kHz )
9.3k : choose 10k: for margin.
1.06nF or 1nF standard value
-15V
5V
Rg
INA826
+15V
0.93mA
+9.3V-
D1
R3 10k
Ref
C1
1n
Rg +
+
5.7V
OPA320
Ref
+15V
6
Copyright © 2018, Texas Instruments Incorporated
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Copyright © 2018–2019, Texas Instruments Incorporated
www.ti.com
Optional Input Filter
The following figure shows a commonly used instrumentation amplifier input filter. The differential noise is
filtered with Cdif, and the common mode noise is filtered with Ccm1 and Ccm2. Note that it is recommended
that Cdif ≥ 10Ccm. This prevents conversion of common mode noise to differential noise due to component
tolerances. The following filter was designed for a differential cutoff frequency of 15kHz.
.
Let Cdif
Rin
Ccm
f cm
f dif
1nF and f dif
1
4 ˜ S ˜ f dif ˜ Cdif
15kHz
1
4 ˜ S ˜ (15kHz ) ˜ (1nF )
5.305k : or 5.23k: for 1% standard value
1
˜ Cdif 100 pF
10
1
1
2 ˜ S ˜ Rin ˜ Ccm 2 ˜ S ˜ ( 5.23k:) ˜ (100 pF )
1
§
4 ˜ S ˜ Rin ˜ ¨ Cdif
©
1
·
Ccm ¸
2
¹
304kHz
1
§
4 ˜ S ˜ ( 5.23k:) ˜ ¨1nF
©
1
·
˜ 100 pF ¸
2
¹
14.5kHz
+15V
CCM1 1n
-15V
Rin1 5.23k
Cdif 10n
Rin2 5.23k
-15V
R1 162
Rg
INA826
Ref
Rg +
+
Vref_INA
CCM2 1n
+15V
Copyright © 2018, Texas Instruments Incorporated
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
Copyright © 2018–2019, Texas Instruments Incorporated
7
www.ti.com
Design Featured Devices
Device
Key Features
Link
Similar Devices
ADS8860
16-bit resolution, SPI, 1-Msps sample rate, single-ended input,
Vref input range 2.5V to 5.0V.
www.ti.com/product/ADS8860
www.ti.com/adcs
OPA192
8-kHz bandwidth, Rail-to-Rail output, 450-nA supply current, unity
gain stable
www.ti.com/product/OPA192
www.ti.com/opamp
INA826
Bandwidth 1MHz (G=1), low noise 18nV/rtHz, low offset ±40µV,
low offset drift ±0.4µV/ºC, low gain drift 0.1ppm/ºC. (typical
values)
www.ti.com/product/INA826
www.ti.com/inas
Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
Link to Key Files
Source Files for this circuit – http://www.ti.com/lit/zip/SBAC184.
Revision History
8
Revision
Date
A
March 2019
Change
Downstyle the title and changed title role to 'Data Converters'.
Added link to circuit cookbook landing page.
Circuit for driving a switched-capacitor SAR ADC with a buffered
instrumentation amplifier
SBAA277A – February 2018 – Revised March 2019
Submit Documentation Feedback
Copyright © 2018–2019, Texas Instruments Incorporated
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2019, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising