CN-0381 - Analog Devices

CN-0381 - Analog Devices
Circuit Note
CN-0381
Devices Connected/Referenced
Circuits from the Lab® reference designs are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0381.
AD7124-4/
AD7124-8
4-Channel/8-Channel, Low Noise, Low
Power, 24-Bit, Sigma-Delta ADCs with PGA
and Reference
ADP1720
50 mA, High Voltage, Micropower Linear
Regulator
Completely Integrated 4-Wire RTD Measurement System Using a Low Power,
Precision, 24-Bit, Sigma-Delta ADC
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
AD7124-4 Evaluation Board (EVAL-AD7124-4SDZ) or
AD7124-8 Evaluation Board (EVAL-AD7124-8SDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is an integrated 4-wire, resistance
temperature detector (RTD) system based on the AD7124-4/
AD7124-8 low power, low noise, 24-bit Σ-Δ ADC optimized for
high precision measurement applications. With a two-point
calibration and linearization, the overall 4-wire system accuracy
is better than ±1°C over a temperature range of −50°C to +200°C.
Typical noise free code resolution of the system is 17.9 bits for
full power mode, sinc4 filter selected, at an output data rate of
50 SPS, and 17.3 bits for low power mode, post filter selected,
and at an output data rate of 25 SPS.
The AD7124-4 can be configured for 4 differential or 7 pseudo
differential input channels, while the AD7124-8 can be configured
for 8 differential or 15 pseudo differential input channels. The
on-chip programmable gain array (PGA) ensures that signals of
small amplitude can be interfaced directly to the ADC.
The AD7124-4/AD7124-8 establishes the highest degree of
signal chain integration, which include programmable low
drift excitation current sources. Therefore, the design of an
RTD system is greatly simplified because most of the required
RTD measurement system building blocks are included on-chip.
The AD7124-4/AD7124-8 gives the user the flexibility to
employ one of three integrated power modes, where the current
consumption, range of output data rates, and rms noise are
tailored with the power mode selected. The current consumed
by the AD7124-4/AD7124-8 is only 255 μA in low power mode
and 930 μA in full power mode. The power options make the
device suitable for non-power critical applications, such as
input/output modules, and also for low power applications such
as loop powered smart transmitters where the complete
transmitter must consume less than 4 mA.
The device also has a power-down option. In power-down mode,
the complete ADC along with its auxiliary functions are powered
down so that the device consumes 1 μA typical. The AD7124-4/
AD7124-8 also has extensive diagnostic functionality integrated
as part of its comprehensive feature set.
Rev. 0
Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog
Devices engineers. Standard engineering practices have been employed in the design and construction
of each circuit, and their function and performance have been tested and verified in a lab environment
at room temperature. However, you are solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices
be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause
whatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)
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©2015 Analog Devices, Inc. All rights reserved.
CN-0381
Circuit Note
GND ADP1720 GND
3.3V OUTPUT
7V TO 9V
VIN
10µF
GND
ADP1720
3.3V OUTPUT
GND
IN
GND
IN
OUT
GND
OUT
GND
EN
GND
EN
GND
GND
0.1µF
4.7µF
4.7µF
4.7µF
ADP1720ARMZ-R7
27kΩ
ADJ
1.8V OUTPUT
GND
57.6kΩ
4.7µF
4.7µF
0.1µF
IN
GND
OUT
GND
EN
GND
1µF
0.1µF
AVDD
Pt100
IOVDD
REGCAPD
RL1
AIN0
RL2
1kΩ
RL3
1kΩ
AIN2
0.01µF
0.1µF
DOUT/RDY
AIN3
DIN
RL4
5.11kΩ
0.1%
±15ppm/°C
CS
1kΩ
0.1µF
LED
CLK
SDP-B
REFIN1(–)
STATUS
0.01µF
DGND
AVSS
13441-001
250Ω
POWER
SYNC
REFIN1(+)
0.01µF
AD7124
ADSP-BF527
LED
SCLK
1kΩ
REFERENCE
BUFFER
HEADROOM
USB
0.1µF
AD7124-4/
AD7124-8
0.01µF
REFERENCE
RESISTOR
0.1µF
REGCAPA
Figure 1. 4-Wire RTD Measurement Configuration
RTDs are frequently used sensors for temperature measurements
in industrial applications. An RTD is made from a pure metal
(examples include platinum, nickel, or copper), which has a
predictable change in resistance as the temperature changes.
The most widely used RTDs are platinum Pt100 and Pt1000.
RTDs are capable of high accuracy and good stability when
compared with other types of temperature sensors. The error
due to the resistance of long wire lengths can be eliminated
using the 4-wire connection.
To accurately measure the resistance, a voltage is generated
across the RTD by a constant current source. The AD7124-4/
AD7124-8 offers two such excitation current sources that are
register programmable from 50 μA to 1 mA. Errors in the current
source can be easily cancelled by referring the measurement to
the voltage across a precision reference resistor that is driven with
the same current source, resulting in a ratiometric measurement.
350
300
250
200
150
100
50
0
–200
13441-002
RTD Temperature Measurement Introduction
For the circuit in Figure 1, a Pt100 RTD Class B sensor was used.
Pt100 RTDs measure temperature from −200°C to +600°C. The
resistance of a Class B RTD is typically 100 Ω at 0°C and has a
typical temperature coefficient of ~0.385 Ω/°C (see Figure 2).
Using this information, the voltage generated across the Pt100
RTD can easily be calculated based on the current source selected.
RESISTANCE (Ω)
CIRCUIT DESCRIPTION
0
200
400
TEMPERATURE (°C)
Figure 2. Pt100 RTD Resistance vs. Temperature
Rev. 0 | Page 2 of 10
600
Circuit Note
CN-0381
How the Circuit Works
The AD7124-4/AD7124-8 provides an integrated solution for RTD
measurements; it can achieve high resolution, low non-linearity,
and low noise performance as well as very high 50 Hz and 60 Hz
rejection. The AD7124-4/AD7124-8 consists of an on-chip, low
noise PGA that amplifies the small signal from the RTD with a
gain programmable from 1 to 128, thus allowing direct interface
with the sensor. The gain stage has high input impedance and
limits the input leakage current to 3.3 nA typical for full power
mode and 1 nA typical for low power mode. The following
sections explain the different elements that make up the 4-wire
RTD temperature measurement system.
The AD7124-4/AD7124-8 has diagnostic functions on-chip that
can be used to detect faults in the SPI communication. These
diagnostics include checks on the SPI read and write operations,
ensuring that only valid registers are accessed. An SCLK
counter ensures that the correct number of SCLK pulses is used,
while the CRC functionality checks for changes in bit values
during transmission. When any of these SPI communication
diagnostic functions are enabled and an associated error occurs,
the corresponding flag is set in the error register. All enabled
flags are OR’ed together and control the ERR flag in the status
register. This functionality is particularly useful if the status bits
are appended to the ADC conversions.
Power Supplies
Analog Inputs and Reference
The AD7124-4/AD7124-8 has separate analog and digital
power supplies. The digital power supply, IOVDD, is
independent of the analog power supply and can be 1.65 V to
3.6 V referenced to DGND. The analog power supply, AVDD, is
referred to AVSS and has a range of 2.7 V to 3.6 V for low and
mid-power modes, and 2.9 V to 3.6 V for full power mode. The
circuit shown in Figure 1 operates on a single supply; therefore,
AVSS and DGND are connected together, and only one ground
plane is used. The AVDD and IOVDD voltages are generated
separately using ADP1720 low dropout voltage regulators. The
AVDD voltage is set to 3.3 V, and the IOVDD voltage is set to
1.8 V, using the ADP1720 regulators. Using separate regulators
ensures lowest noise.
The AD7124-4 can be configured for 4 differential or 7 pseudo
differential input channels, while the AD7124-8 can be
configured for 8 differential or 15 pseudo differential input
channels.
Serial Peripheral Interface (SPI)
SPI communications to the AD7124-4/AD7124-8 are handled
by the Blackfin® ADSP-BF527 on the EVAL-SDP-CB1Z board,
as shown in Figure 1. To access the registers of the AD7124-4/
AD7124-8, use the AD7124-4/AD7124-8 EVAL+ Software.
Figure 3 shows the main window of this software. Clicking the
4-WIRE RTD button configures the software for the 4-wire
RTD measurement.
The AD7124-4/AD7124-8 has on-chip diagnostics that can be
used to check that the voltage level on the analog pins are
within the specified operating range. The positive (AINP) and
negative (AINM) analog inputs can be separately checked for
overvoltages and undervoltages, as well as ADC saturation. An
overvoltage is flagged when the voltage on the analog input
exceeds AVDD, while an undervoltage is flagged when the
voltage on the analog input goes below AVSS.
For the circuit in Figure 1, three analog pins are used to
implement the 4-wire measurement: AIN0, AIN2, and AIN3.
AIN2 and AIN3 are configured as a fully differential input
channel and are used for sensing the voltage across the RTD.
The excitation current source used to excite the RTD is
generated from AVDD and is directed to AIN0. The analog pins
and their configuration are shown in greater detail in Figure 4.
AD7124-4/
AD7124-8
Pt100
RL1
RL2
IOUT0 (AIN0)
1kΩ
AIN2
0.01µF
RL3
0.1µF
1kΩ
AIN3
0.01µF
RL4
1kΩ
5.11kΩ
0.1%
±15ppm/°C
13441-003
1kΩ
Figure 3. AD7124-4/AD7124-8 EVAL+ Software Configuration Window
Rev. 0 | Page 3 of 10
REFERENCE
BUFFER
HEADROOM
250Ω
REFIN1(+)
0.01µF
0.1µF
REFIN1(–)
0.01µF
13441-004
REFERENCE
RESISTOR
Figure 4. Analog Input Configuration for 4-Wire RTD Measurement
CN-0381
Circuit Note
Digital and Analog Filtering
For the circuit in Figure 1, the reference input used is REFIN1(±).
The current through the RTD also flows through the precision
reference resistor that generates the reference voltage. The voltage
generated across this precision reference resistor is ratiometric
to the voltage across the RTD; therefore, any variations seen in
the excitation current are removed. Because the buffers are
enabled, it is necessary to ensure that the headroom required
for correct operation is met (AVDD − 0.1 V and AVSS + 0.1 V).
The headroom of 0.125 V (500 μA × 250 Ω) is provided by the
250 Ω resistor to ground, as shown in Figure 4.
Differential (~800 Hz cutoff) and common-mode (~16 kHz
cutoff) filters are implemented at the analog inputs, as well as at
the reference inputs. This filtering is required to reject any
interference at the modulator frequency and also any multiples
of this frequency.
The AD7124-4/AD7124-8 offers a great deal of on-chip digital
filtering flexibility. Several filter options are available; the option
selected has an effect on the output data rate, settling time, as
well as 50 Hz and 60 Hz rejection. For this circuit note, the sinc4
filter and the post filter are implemented. The sinc4 filter is used
because it has excellent noise performance across the range of
output data rates, as well as having excellent 50 Hz and 60 Hz
rejection. The post filter provide simultaneous 50 Hz and 60 Hz
rejection, with a 40 ms settling time.
Calibration
The AD7124-4/AD7124-8 provides different calibration modes
that can be used to eliminate offset and gain errors. For this
circuit note, internal zero-scale calibration as well as internal
full-scale calibrations were used.
The value of the external precision resistor is chosen so that
the maximum voltage generated across the RTD equals the
reference voltage divided by the PGA gain.
RREF = VRTD MAX/IEXC = 2.5096 V/500 μA = 5020 Ω
Therefore, a 5.11 kΩ resistor is used, which gives a reference
voltage of
5.11 kΩ × Excitation Current = 5.11 kΩ × 500 μA = 2.555 V
The output compliance of the excitation current source must
also be considered when making 4-wire RTD measurements
using the AD7124-4/AD7124-8. The output compliance is
dependent on the excitation current selected. For this circuit,
500 μA is selected, which has an output compliance voltage of
AVDD − 0.37 V. The AVDD supply voltage for this circuit is 3.3 V;
therefore, the output compliance level for the excitation current
source must be less than 2.93 V. From the previous calculations,
this specification is met, because the maximum voltage on the
AIN0 pin is the voltage across the precision reference resistor
plus the voltage across the RTD plus the voltage across the
headroom resistor.
VREF + VRTD + VHEADROOM = 2.555 V + 156.8 5 mV + 125 mV
= 2.83685 V
The AD7124-4/AD7124-8 configuration for 4-wire RTD
measurements is as follows:





4-Wire RTD Configuration
The circuit shown in Figure 1 is designed for precision 4-wire
RTD measurements using the AD7124-4/AD7124-8. For the
4-wire RTD measurement, one excitation current source is
required. The AD7124-4/AD7124-8 provides two matched
current sources; therefore, either one of these current sources
can be used to excite the RTD. The RTD produces a low-level
voltage signal, which can then be amplified by the on-board
PGA of the AD7124-4/AD7124-8. The amplified voltage is then
converted to a precision digital representation using the 24-bit
Σ-Δ ADC.
For this 4-wire RTD circuit, a Class B RTD is used. If the on-chip
excitation current is programmed to 500 μA, at a maximum
temperature of 600°C, the voltage generated across the RTD is
approximately 156.85 mV. To ensure that the maximum range of
the AD7124-4/AD7124-8 is used, the PGA gain is programmed
to a gain of 16. The PGA amplifies the maximum RTD sensor
output voltage to 2.5096 V.
Differential input: AINP = AIN2, AINM = AIN3
Excitation current: IOUT0 = AIN0 = 500 μA
Gain = 16
Precision reference resistor: 5.11 kΩ
Digital filtering:
o Sinc4 filter (full power mode)
o Post filter (low power mode)
The general expression to calculate the RTD resistance, RRTD, when
the ADC is operating in bipolar differential mode is given by
R RTD 
(CODE  2 N  1 )  R REF
G  2N  1
(1)
where:
CODE is the ADC code output.
N is the resolution of the ADC (24, in this case).
RREF is the reference resistor.
G is the selected gain.
From the specification of the Class B RTD, the resistance changes
by approximately 0.385 Ω/°C. This relationship can be used as a
quick method to obtain an approximate temperature of the RTD.
This method has inaccuracies due to the temperature coefficient
of the RTD changing slightly over the temperature range; however,
it can be a useful method to check the temperature quickly.
To calculate the approximate temperature, use Equation 2.
Temperature (C) 
Rev. 0 | Page 4 of 10
RRTD  100
0.385
(2)
Circuit Note
CN-0381
The RTD transfer function known as the Callender-Van Dusen
equation is made up of two distinct polynomial equations to
provide a more accurate result. Equation 3 is used for temperatures
greater than 0°C, and Equation 4 is for temperatures less than 0°C.
The equation for temperature t ≤ 0°C is
(3)
The equation for temperature t ≥ 0°C is
(4)
where:
t is the RTD temperature (°C).
RRTD(t) is the RTD resistance (Ω).
R0 is the RTD resistance at 0°C (in this case, R0 = 100 Ω).
A = 3.9083 × 10−3.
B = −5.775 × 10−7.
C = −4.23225 × 10−12.
Choosing the configuration for sinc4 filter, full power mode,
and 50 SPS allows the user to operate the AD7124-4/AD7124-8
for best performance relative to speed and noise. Figure 5 shows
the noise distribution when a 4-wire RTD is connected as shown
in Figure 1 at ambient temperature. The corresponding rms noise
is typically 199.37 nV, which is approximately 17.9 noise free bits.
The noise performance of the AD7124-4/AD7124-8 for inputs
shorted using the same filter, gain, power mode, and output
data rate was 100 nV rms or 18.7 noise free bits. The increase in
the noise comes directly from the RTD connection across the
input channel (AIN2, AIN3).
90
There are many different ways to determine the temperature as
a function of the RTD resistance given the transfer function in
Equation 3 and Equation 4. The direct mathematical method is
chosen, due to its accuracy. Taking Equation 3, the temperature
can be calculated as
1000 SAMPLES
70
OCCURANCES
2
(5)
(6)
As an example, if the code read back from the AD7124-4/
AD7124-8 with the temperature set to 25°C is 11270065,
converting this code to a resistance using Equation 1 gives
(11270065  2 23 )  R REF
G  2 23
 109 .704 Ω
Linearization using Equation 5 gives a temperature of 24.921°C.
As a second example, if the code read back from the AD7124-4/
AD7124-8 with the temperature set to −25°C is 10757779,
converting this code to a resistance gives
R RTD 
(10757779  2 23 )  R REF
G  2 23
40
30
 90 .200 Ω
Linearization using Equation 6 gives a temperature of −24.982°C.
13441-005
10
11256475
11256467
11256459
11256451
0
11256419
This method works well for temperatures greater than or equal
to 0°C. To calculate the RTD temperature for temperatures
below 0°C, a best fit polynomial expression is required, and the
polynomial used in this circuit note is a fifth-order polynomial
shown in Equation 6.
R RTD 
50
20
where r is the RTD resistance, and the other variables are as
defined previously.
TRTD (°C) = −242.02 + 2.2228 × r + (2.5859 × 10−3)r2 −
(48260 × 10−6)r3 − (2.8183 × 10−3)r4 +
(1.5243 × 10−10)r5
60
11256443
A
TRTD (C ) 

r 

A  4 B  1 

R
0 

2B
80
11256435
RRTD(t) = R0(1 + At + Bt2)
For the circuit shown in Figure 1, data was gathered for
different digital filter and power mode configurations of the
AD7124-4/AD7124-8, with the sinc4 filter operating in full
power mode, and the post filter operating in low power mode.
11256427
RRTD(t) = R0[1 + At + Bt2 + C(t − 100°C)t3]
4-Wire RTD Measurements and Results
CODES
Figure 5. Histogram of Codes for RTD at Ambient Temperature, Sinc4 Filter,
Full Power Mode, 50 SPS
For the 4-wire RTD configuration where the sinc4 filter and full
power mode were selected, the temperature of the RTD was swept
from −50°C to +200°C. For each temperature, the corresponding
voltage across the RTD was measured using the AD7124-4/
AD7124-8 as described previously. This voltage was then converted
to a resistance, which was then linearized, and converted to a
temperature as described in the 4-Wire RTD Configuration
section. Figure 6 shows the resulting error between the set
temperature and the measured temperatures of the RTD after
linearization. For each RTD temperature setting, the AD7124-4/
AD7124-8 is kept at 25°C. As shown in Figure 6, the error of the
RTD temperature measured is well within the error window of
the Pt100 Class B RTD after linearization. Figure 6 also shows the
deviation of the RTD error across different AD7124-4/AD7124-8
temperature settings. For each AD7124-4/AD7124-8 temperature
setting, an internal zero-scale and full-scale calibration is carried
out. As shown in Figure 6, the error of the RTD is well within
the expected error of the Class B RTD for all temperature settings
of the AD7124-4/AD7124-8.
Rev. 0 | Page 5 of 10
CN-0381
Circuit Note
100
2.5
1000 SAMPLES
80
Pt100
+25ºC
+105ºC
70
0.5
0
–0.5
60
50
40
30
–1.5
20
13441-006
–1.0
175
0
200
Pt100 TEMPERATURE (°C)
Figure 6. Temperature Accuracy Measurement, Sinc4 Filter,
Full Power Mode, 50 SPS
CODES
Figure 7 shows the error in measured RTD temperature for a one
time, internal zero-scale and full-scale calibration carried out at
25°C. From the plot, it can be seen that carrying out a one time
calibration at 25°C or calibrating at each individual temperature
of the AD7124-4/AD7124-8 gives the same performance.
2.5
2.0
1.5
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
ERROR (°C)
Pt100
+25ºC
+105ºC
1.0
0.5
0
–0.5
–1.0
13441-007
–1.5
–2.0
–2.5
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 TEMPERATURE (°C)
Figure 8. Histogram of Codes for RTD at Ambient Temperature, Post Filter,
Low Power Mode, 25 SPS
The temperature of the RTD was swept from −50°C to +200°C.
For each of the set RTD temperatures, the corresponding voltage
across the RTD was measured using the AD7124-4/AD7124-8
as described previously. The voltage was then converted to a
resistance, which was then linearized and converted to a
temperature as described in the 4-Wire RTD Configuration
section. Figure 9 shows the resulting error between the set and
measured temperatures of the RTD after linearization. For each
RTD temperature setting, the AD7124-4/AD7124-8 is kept at
25°C. As shown in Figure 9, the error of the RTD temperature
measured is well within the error window of the Pt100 Class B
RTD. Figure 9 also shows the deviation of the RTD error across
different AD7124-4/AD7124-8 temperature settings. For each
AD7124-4/AD7124-8 temperature setting, an internal zero-scale
and full-scale calibration was carried out. As shown in Figure 9,
the error of the RTD is well within the expected error of the Class B
RTD for all temperature settings of the AD7124-4/AD7124-8.
Figure 7. Temperature Accuracy Measurement, Sinc4 Filter, Full Power Mode,
50 SPS, One Time 25°C Calibration Only
Rev. 0 | Page 6 of 10
2
+105ºC
Pt100
0
50
1
ERROR (°C)
The second AD7124-4/AD7124-8 configuration tested was the
low power mode, where the post filter and 25 SPS output data
rate were selected. The 25 SPS filter provides simultaneous 50 Hz
and 60 Hz rejection and allows the user to trade off settling time
with power supply rejection. Figure 8 shows the resulting noise
distribution when a 4-wire RTD is connected as shown in
Figure 1 at ambient temperature. The corresponding rms noise is
typically 774 nV, equating to approximately 16.8 noise free bits.
The noise performance of the AD7124-4/AD7124-8 for inputs
shorted with the same filter, gain, power mode, and output data
rate is typically 360 nV rms or 17.3 noise free bits. The increase
in the noise between the two measurements comes directly from
the RTD connection across the input channel (AIN2, AIN3).
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
+25ºC
0
–1
–2
–3
–50
13441-009
150
11271719
125
11271704
100
11271689
75
11271674
50
11271659
25
11271645
0
11271630
–25
11271615
–2.5
–50
10
11271570
–2.0
13441-008
OCCURANCES
1.0
ERROR (°C)
90
11271600
1.5
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
11271585
2.0
–25
25
75
100
125
150
175
Pt100 TEMPERATURE (°C)
Figure 9. Temperature Accuracy Measurement, Post Filter,
Low Power Mode, 25 SPS
200
Circuit Note
CN-0381
Figure 10 shows the error in measured RTD temperature for a one
time, internal zero-scale and full-scale calibration carried out at
25°C. From the plot, it can be seen that carrying out a one time
calibration at 25°C or calibrating at each individual temperature
of the AD7124-4/AD7124-8 gives similar performance.
TERMINAL BLOCK
J6
AIN0
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
AIN2
RL2
AIN3
Pt100
RL3
+25ºC
+105ºC
0.75
AIN1
RL1
1.50
1.25
RL4
AIN4
AIN5
0.25
0
REFIN+
–0.25
RREF
5.11kΩ
–0.50
–0.75
–25
0
25
50
75
100
125
150
175
AGND
Pt100
200
Pt100 TEMPERATURE (°C)
PSW
RL1
Figure 10. Temperature Accuracy Measurement, Post Filter,
Low Power Mode, 25 SPS, 25°C One Time Calibration Only
AIN6
RL2
COMMON VARIATIONS
AIN7
RL3
Multiple 4-Wire RTDs
The steps needed to measure an RTD voltage are
13441-011
RL4
The AD7124-4/AD7124-8 can be used as a measurement system
for multiple 4-wire RTDs. The AD7124-4 can be used to connect
two 4-wire RTDs, while the AD7124-8 can be used to connect
up to five 4-wire RTDs. The same reference input can be used
for all the RTDs, and one current source can be used to excite
all the RTDs. The current is directed to the top side of each of
the RTDs in turn when the RTD temperature measurement is
required. The cross multiplexer on the AD7124-4/AD7124-8
allows multiple channels to be configured separately, where
each channel can be configured for different setups.
Figure 11. AD7124-4 4-Wire RTD Configuration Using Two 4-Wire RTDs
The performance of the 4-wire RTD configuration shown in
Figure 11 was evaluated to ensure that the expected performance
was achieved when more than one RTD was connected. For this
measurement, the temperature of both 4-wire RTDs were swept
from −50°C to +200°C. For each temperature setting, the voltage
across each RTD was recorded. Figure 12 shows the error in
each. As shown in the results, switching the same current from
Channel 0 to Channel 1 has no effect on settling; both RTDs are
well within the expected error window for the Class B RTD.
Set the external reference to REFIN1±.
Enable the IOUT0 current to the RTD to be measured.
Enable the analog input channel that has the RTD
connected across its input.
1.50
1.25
AD7124-4/AD7124-8
TEMPERATURE
1.00
25ºC (AIN2, AIN3)
0.75
25ºC (AIN6, AIN7)
Pt100
0.50
ERROR (°C)
As an example, two 4-wire RTDs were connected to the AD7124-4
as shown in Figure 11. One 4-wire RTD is connected across the
AIN2 and AIN3 analog input pins (Channel 0 configuration),
where the excitation current comes from AIN0, and a second
4-wire RTD is also shown connected across the AIN6 and AIN7
analog input pins (Channel 1 configuration), where AIN1 is used
for the excitation current. Temperature measurements were
then carried out on each RTD in turn using the following steps:
2.
TERMINAL BLOCK
J11
13441-010
–1.25
1.
REFIN–
RHEADROOM
250Ω
–1.00
1.
2.
3.
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
EVALUATION BOARD
IOUT0 is directed to AIN0. The voltage is measured on
Channel 0 (AIN2, AIN3); therefore, Channel 0 must be
enabled. All other channels are disabled for this measurement.
Disable Channel 0, enable Channel 1, and direct the IOUT0
current to AIN1. The voltage is then measured on Channel 1
(AIN6, AIN7).
Rev. 0 | Page 7 of 10
0.25
0
–0.25
–0.50
–0.75
–1.00
13441-012
ERROR (°C)
0.50
–1.50
–50
IOUT0
Pt100
–1.25
–1.50
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 TEMPERATURE (°C)
Figure 12. RTD Temperature Error Recorded for Two Different
RTD Measurement Configurations
CN-0381
Circuit Note
CIRCUIT EVALUATION AND TEST
Configuring the Hardware
Equipment Needed
To configure the hardware, take the following steps:
The following equipment is needed for the 4-wire RTD
measurement system:
1.






EVAL–AD7124-4SDZ or EVAL-AD7124-8SDZ evaluation
board
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
AD7124-4/AD7124-8 EVAL+ Software
Power supply: 7 V or 9 V wall wart
Class B Pt100 4-wire RTD
A PC running Windows® XP (SP2), Windows Vista, or
Windows 7 (32-bit or 64-bit)
Set all links on the EVAL-AD7124-4SDZ/EVAL-AD71248SDZ evaluation board to the default positions, as
described in the EVAL-AD7124-4SDZ/EVAL-AD71248SDZ user guide.
Power the board with a 7 V or 9 V power source connected
to J5.
Connect the RTD, the precision reference resistor, and the
resistor for headroom, as shown in Figure 14.
2.
3.
TERMINAL BLOCK
J6
Pt100
RL1
Software Installation
AIN0
AIN1
A complete software user guide for the AD7124-4/AD7124-8
and the SDP board can be found in the EVAL–AD7124-4SDZ/
EVAL-AD7124-8SDZ user guide and the SDP User Guide.
RL2
AIN3
Software is required to interface with the hardware. This software
can be downloaded from ftp://ftp.analog.com/pub/evalcd/AD7124.
If the setup file does not automatically run, double click
setup.exe from the file. Install the evaluation software before
connecting the evaluation board and SDP board to the USB
port of the PC to ensure that the evaluation system is correctly
recognized when connected to the PC.
RL3
RL4
AIN4
AIN5
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
EVALUATION BOARD
REFIN+
RREF
5.11kΩ
RHEADROOM
250Ω
REFIN–
TERMINAL BLOCK
J11
AGND
PSW
13441-014
After the evaluation software installation is complete, connect the
EVAL-SDP-CB1Z (via Connector B) to the EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ and then connect the EVAL-SDP-CB1Z to
the USB port of the PC using the supplied cable. When the
evaluation system is detected, proceed through any dialog boxes
that appear to complete the installation.
AIN2
Setup and Test
Figure 14. EVAL–AD7124-4SDZ/EVAL-AD7124-8SDZ Evaluation Board
Connector for 4-Wire RTD Measurement
Figure 13 shows a functional block diagram of the test setup.
7V TO 9V SUPPLY
260mA LIMIT
Run the AD7124-4/AD7124-8 EVAL+ Software, and the
window shown in Figure 15 appears.
PC
USB
Pt100
J4/J5
J1
120
CON A
J6
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
REFERENCE
AND
HEADROOM
RESISTORS
EVAL-SDP-CB1Z
13441-013
J11
Figure 13. Test Setup Functional Diagram



4-wire Pt100 RTD, Class B
5.11 kΩ precision resistor
250 Ω resistor needed for buffer headroom
13441-015
The EVAL-AD7124-4SDZ /EVAL-AD7124-8SDZ evaluation
board is required to test the circuit. In addition, the following
sensor and resistors are required to ensure proper operation:
Figure 15. AD7124-4 EVAL+ Software Window
Rev. 0 | Page 8 of 10
Circuit Note
CN-0381
To configure the AD7124-4/AD7124-8 for 4-wire RTD
measurements, click the 4-WIRE RTD demo mode button
(see Figure 15). Clicking the 4-WIRE RTD button configures
the ADC software as follows:



Channel_0
o AINP_0 = AIN2
o AINM_0 = AIN3
o Setup0
o Enabled = TRUE
Setup_0
o PGA_0 = 16
o AIN_BUFP, AIN_BUFM both = ENABLED
o BIPOLAR = ENABLED
o FS_0 = 384
o FILTER_MODE_0 = SINC4
ADC_Control
o MODE = Continuous Conversion
o POWER_MODE = FULL
IO_CONTROL_1
o IOUT0 Channel Enable = AIN1
o IOUT0 Select = 500 μA
13441-016

Figure 16. Register Map Internal Full-Scale and Zero-Scale Calibration
The board and device are now configured for 4-wire RTD
measurements. Click SAMPLE to start gathering samples from
the AD7124-4/AD7124-8. The Waveform tab and the Histogram
tab show the data gathered from the AD7124-4/AD7124-8.
One additional step is required before the AD7124-4/AD7124-8
is configured for 4-wire RTD measurements: an internal fullscale and zero-scale calibration of the AD7124-4/AD7124-8.
This calibration can be performed via the Register Map tab,
as shown in Figure 16.
1.
2.
3.
4.
5.
Click the ADC_Control register.
Select Low Power mode.
Perform an internal full-scale calibration.
a. Click the Mode bitfield of the ADC control register
b. In the Mode bitfield, select the internal full scale
calibration option.
c. Check the calibration performed by clicking the
Gain0 register from the register tree, and check that
the coefficients have changed.
Perform an internal zero-scale calibration.
a. Click the Mode bitfield of the ADC control register.
b. In the Mode bitfield, select the internal zero-scale
calibration option.
c. Check the calibrations performed by clicking the
Offset0 register in the register tree, and check that the
coefficients have changed.
When calibrations are complete, change the power mode to
the required mode of operation, and ensure that the ADC is
set to continuous conversion mode by selecting Continuous
from the drop-down box in the Mode bit field of the
ADC_Control register.
Rev. 0 | Page 9 of 10
CN-0381
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0381 Design Support Package:
www.analog.com/CN0381-DesignSupport
EVAL-AD7124-4SDZ
EVAL-AD7124-4 User Guide (UG-855)
System Demonstration Platform (EVAL-SDP-CB1Z)
EVAL-AD7124-8 User Guide (UG-856)
AD7124-4 Data Sheet
SDP User Guide
AD7124-8 Data Sheet
Kester, Walt. “Temperature Sensors,” Chapter 7 in Sensor Signal
Conditioning. Analog Devices, 1999.
ADP1720 Data Sheet
McCarthy, Mary. AN-615 Application Note. Peak-to-Peak
Resolution Versus Effective Resolution. Analog Devices.
7/15—Revision 0: Initial Version
EVAL-AD7124-8SDZ
REVISION HISTORY
McNamara, Donal. AN-892 Application Note. Temperature
Measurement Theory and Practical Techniques. Analog Devices.
MT-031 Tutorial. Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”. Analog Devices.
MT-101 Tutorial. Decoupling Techniques. Analog Devices.
Circuit Note CN-0376. Channel-to-Channel Isolated
Temperature Input (Thermocouple/RTD) for PLC/DCS
Applications. Analog Devices.
Circuit Note CN-0382. Ultralow Power Industrial Temperature and
Pressure, 4 mA to 20 mA/HART Transmitter. Analog Devices.
Circuit Note CN-0383. Completely Integrated 3-Wire RTD
Measurement System Using a Low Power, Precision, 24-Bit,
Sigma-Delta ADC. Analog Devices.
Circuit Note CN-0384. Completely Integrated Thermocouple
Measurement System Using a Low Power, Precision, 24-Bit,
Sigma-Delta ADC. Analog Devices.
(Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors.
While you may use the Circuits from the Lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual
property by application or use of the Circuits from the Lab reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the
Lab reference designs are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability,
noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties
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©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN13341-0-7/15(0)
Rev. 0 | Page 10 of 10
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