CN-0383 - Analog Devices

CN-0383 - Analog Devices
Circuit Note
CN-0383
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/CN0383.
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 3-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 3-wire resistance
temperature detector (RTD) system based on the AD7124-4/
AD7124-8 low power, low noise, 24-bit Σ-Δ analog-to-digital
converter (ADC) optimized for high precision measurement
applications. With a two-point calibration and linearization,
the overall 3-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 16.8 bits for low
power mode, post filter selected, 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 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 includes 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
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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
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toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)
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CN-0383
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
IN
GND
OUT
GND
EN
GND
4.7µF
4.7µF
0.1µF
1µF
0.1µF
AVDD
IOVDD
REGCAPD
AIN0
Pt100
RL1
1kΩ
RL2
1kΩ
0.1µF
REGCAPA
USB
AIN2
0.01µF
0.1µF
AD7124-4/
AD7124-8
0.1µF
DOUT/RDY
AIN3
0.01µF
DIN
RL3
AIN1
CS
REFIN1(+)
LED
0.01µF
5.11kΩ
0.1%
±15ppm/°C
POWER
SYNC
1kΩ
AD7124
ADSP-BF527
LED
SCLK
CLK
SDP-B
0.1µF
STATUS
REFIN1(–)
0.01µF
DGND
AVSS
13369-001
1kΩ
Figure 1. 3-Wire RTD Measurement Configuration
CIRCUIT DESCRIPTION
350
RTD Temperature Measurement Introduction
300
200
150
100
50
0
–200
13369-002
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, thereby resulting in a ratiometric
measurement result.
250
RESISTANCE (Ω)
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 resistance
of long wire lengths can be compensated for with a 3-wire
connection.
0
200
400
600
TEMPERATURE (°C)
Figure 2. RTD Resistance vs. Temperature
For the circuit in Figure 1, a Class B Pt100 RTD 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.
Rev. 0 | Page 2 of 11
Circuit Note
CN-0383
How the Circuit Works
The AD7124-4/AD7124-8 provides an integrated solution for
RTD measurement. It can achieve high resolution, low nonlinearity 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 section explain the different elements that
make up the 3-wire RTD temperature measurement system.
Power Supplies
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 from 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 from a single supply; therefore, AVSS and
DGND are connected together, and only one ground plane is
used. The AVDD and IOVDD voltage are generated separately
using ADP1720 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 the lowest noise.
Serial Peripheral Interface (SPI)
SPI communications to the AD7124-4/AD7124-8 is handled by
the Blackfin® ADSP-BF527 on the EVAL-SDP-CB1Z, 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 3-WIRE RTD configures
the software for the 3-wire RTD measurement.
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.
Analog Inputs and Reference
The AD7124-4 can be configured for 4 differential or 7 pseudo
differential channels, while the AD7124-8 can be configured for
8 differential or 15 pseudo differential channels.
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 shown in Figure 1, four analog pins are used to
implement the 3-wire measurement: AIN0, AIN1, AIN2, and
AIN3. AIN2 and AIN3 are configured as a fully differential
input channel and are used for sensing the voltage across the
Pt100. The excitation current source used to excite the RTD
is generated from AVDD and is directed to AIN0. An identical
current is directed to AIN1 and flows through the RL2 lead
resistance, thereby generating a voltage that cancels the voltage
dropped across the RL1 lead resistance. The analog pins and
their configuration are shown in greater detail in Figure 4.
AIN0
Pt100
RL1
1kΩ
AIN2
0.01µF
RL2
0.1µF
1kΩ
AIN3
0.01µF
RL3
AIN1
1kΩ
13369-003
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 are used, while
0.1µF
0.01µF
13369-004
REFIN1(–)
1kΩ
Figure 3. AD7124-4/AD7124-8 EVAL+ Software Configuration Screen
REFIN1(+)
0.01µF
5.11kΩ
0.1%
±15ppm/°C
Figure 4. Analog Inputs for 3-Wire RTD Measurement
With the PGA enabled, the analog input buffers are automatically
enabled. The PGA allows voltages on the input pins to be as low
as AVSS; therefore, headroom resistors are not required for the
analog input pins. The reference buffers are also enabled. These
buffers require headroom. Because the reference resistor is on the
high side, the headroom requirements for the reference resistor
are met; therefore, additional headroom resistors are not required.
Rev. 0 | Page 3 of 11
CN-0383
Circuit Note
For the circuit shown in Figure 1, the reference input used is
REFIN1(±). The current through the Pt100 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 Pt100; therefore, any errors
because of variations in the excitation current are removed.
Digital and Analog Filtering
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 excellent 50 Hz and 60 Hz rejection.
The post filter is used to 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 calibration were used. Note that these calibrations
remove only the ADC gain and offset errors, not gain and offset
errors created by the external circuitry.
3-Wire RTD Configuration
The circuit shown in Figure 1 is designed for precision 3-wire
RTD measurement using the AD7124-4/AD7124-8. For the
3-wire RTD measurement, two precision excitation current
sources are required that provide an easy way to cancel the lead
resistance errors produced by RL1 and RL2. Note that the RL3
lead resistance does not affect the measurement accuracy. For
the 3-wire RTD configuration shown in Figure 1, the reference
resistor is placed on the high side of the RTD. For this setup, one
excitation current flows through both the reference resistor and
the RTD; the second current flows through lead-resistance RL2
and develops a voltage that cancels the voltage dropped across
RL1. Because only one excitation current generates the reference
voltage to REFIN1±, and also generates the voltage across the
RTD, the current source accuracy, mismatch, and mismatch
drift has a minimal effect on the ADC transfer function.
A low level voltage is generated across the Pt100 RTD by the
excitation current. This low level voltage can then be amplified
by the on-board PGA of the AD7124-4/AD7124-8, and is then
converted to a precision digital representation using the 24-bit
Σ-Δ ADC. For this 3-wire RTD configuration, both excitation
currents are programmed to 500 µA. For a maximum RTD
temperature of 600°C, the voltage generated across the RTD
using the 500 µA excitation current 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 16, which amplifies the
maximum RTD sensor output voltage to 2.5096 V. To ensure a
true ratiometric configuration for this 3-wire circuit, the
reference voltage to the ADC is generated using an external
precision resistor using the same excitation current as that used
for the Pt100. Using this configuration means that any deviation
in the value of the excitation current is seen across the Pt100 as
well as the reference resistor, and therefore does not alter the
accuracy of the system.
Using the excitation current of 500 µA and the amplified voltage
of the ADC, the reference resistor value is
VRTD MAX/IEXC = 2.51 V/500 µA = 5020 Ω
Therefore, a 5.11 kΩ resistor is chosen, which gives a reference
voltage of
VREF = RREF × IEXC = 5.11 kΩ × 500 µA = 2.555 V
The output compliance of the excitation current source must
also be considered when making 3-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,
which means that 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:
VREF + VRTD = 2.555 V + 156.85 mV = 2.71185 V
The AD7124-4/AD7124-8 configuration for the 3-wire RTD
measurement is as follows:
•
•
•
•
•
•
Differential input: AINP = AIN2, AINM = AIN3
Excitation current: IOUT0 = AIN0 = 500 µA
Excitation current: IOUT1 = AIN1 = 500 µA
Gain = 16
5.11 kΩ precision reference resistor
Digital filtering (sinc4 and post filter)
The general expression to calculate the RTD resistance (R),
where the ADC is operating in bipolar mode, is given by
RRTD =
(CODE − 2N − 1 ) × RREF
G × 2N − 1
(1)
where:
CODE is the ADC code.
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 get an approximate temperature of the RTD. This
method has inaccuracies due to the temperature coefficient of
Rev. 0 | Page 4 of 11
Circuit Note
CN-0383
the RTD changing slightly over the temperature range; however,
it can be a useful method to quickly check the temperature.
To calculate the approximate temperature, use Equation 2,
where the resistance of the RTD is 100 Ω at 0°C.
RRTD − 100
The RTD transfer function known as the Callender-Van Dusen
equation is made up of two distinct polynomial equations.
Equation 3 is used for temperatures greater than 0°C, and
Equation 4 for temperatures less than 0°C.
The equation for temperature t ≤ 0°C is
RRTD(t) = R0[1 + At + Bt2 + C(t − 100°C)t3]
(3)
The equation for temperature t ≥ 0°C is
RRTD(t) = R0(1 + At + Bt2)
(4)
where:
t is the RTD temperature (°C).
RRTD(t) is the RTD resistance at temperature (t).
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 for
an output data rate of 50 SPS allows the user to operate the
AD7124-4/AD7124-8 for best performance in relation to speed
and noise. Figure 5 shows the noise distribution when a 3-wire
RTD is connected as shown in Figure 1 at ambient temperature.
The corresponding rms noise is typically 199.37 nV rms equating
to approximately 17.9 noise free bits. The noise performance of
the AD7124-4/AD7124-8 for inputs shorted when the same filter,
gain, and output data rate are selected is typically 100 nV rms or
18.7 noise free bits. The increase in the noise comes directly from
the RTD connected across the input channel (AIN2, AIN3).
160
1000 SAMPLES
140
OCCURRENCES
120
TRTD (°C) = −242.02 + 2.2228 × r + (2.5859 × 10 )r −
(48260 × 10−6)r3 − (2.8183 × 10−3)r4 +
(1.5243 × 10−10)r5
−3
60
2
(6)
= 109.704 Ω
Linearization using Equation 5 gives a temperature of 24.921°C.
As a second example, the code read back from the AD7124-4/
AD7124-8 for the temperature set to −25°C is 10757779.
13369-005
11270254
11270249
11270243
11270238
11270233
11270217
0
As an example, the code read back from the AD7124-4/
AD7124-8 for the temperature set to 25°C is 11270065.
Converting this code to a resistance using Equation 1 gives
G × 223
80
20
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. The
polynomial used in this note is a fifth-order polynomial,
shown in Equation 6.
(11270065 − 223 ) × RREF
100
40
(5)
where r is the RTD resistance, and the other variables are as
defined previously.
RRTD =
= 90.200 Ω
For the circuit shown in Figure 1, data was gathered for different
digital filter and power mode configurations of the AD7124-4/
AD7124-8, namely the sinc4 filter operating in full power mode
and the post filter operating in low power mode.
11270228
TRTD (°C) =

r 

A2 − 4 B1 −

R
0 

2B
G × 223
3-Wire RTD Measurements and Results
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. For this circuit note, the direct
mathematical method is chosen because of its accuracy. Using
Equation 3, the temperature can be calculated as
−A+
(10757779 − 223 ) × RREF
Linearization using Equation 6 gives a temperature of −24.982°C.
(2)
0.385
RRTD =
11270222
Temperature (°C) =
Converting this code to a resistance gives
CODES
Figure 5. Histogram of Codes for RTD at Ambient, Sinc4 Filter,
Full Power Mode, 50 SPS
For the 3-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 outlined previously. This voltage was
then converted to a resistance, linearized, and converted to a
temperature as outlined in the 3-Wire RTD Configuration
section. Figure 6 shows the resulting error between the set
temperature and the measured system temperature 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. Figure 6 also shows the
deviation of the RTD error across different AD7124-4/AD7124-8
Rev. 0 | Page 5 of 11
CN-0383
Circuit Note
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.
directly from the RTD connection across the input channel
(AIN2, AIN3).
120
1000 SAMPLES
100
1.25
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
OCCURRENCES
1.50
Pt100
+25ºC
0.75
+105ºC
0.25
60
40
0
20
–0.25
13369-008
ERROR (°C)
0.50
80
–0.50
–25
0
25
50
75
100
125
150
175
Pt100 TEMPERATURE (°C)
Figure 7 shows the error in measured RTD temperature for a
one time, internal zero-scale and full-scale calibrations carried
out at 25°C. The plot shows that carrying out a one time
calibration at 25°C or calibrating at each individual temperature
of the AD7124-4/AD7124-8 gives similar performances.
1.50
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
Pt100
+25ºC
0.75
+105ºC
ERROR (°C)
0.50
0.25
0
–0.25
–0.50
–0.75
13369-007
–1.00
–1.25
–1.50
–50
–25
0
25
50
75
100
125
150
175
11270538
11270525
11270511
11270498
11270484
11270471
Figure 8. Histogram of Codes for RTD at Ambient, Post Filter,
Low Power Mode, 25 SPS
Figure 6. Temperature Accuracy Measurement, Sinc4 Filter,
Full Power Mode, 50 SPS
1.25
11270457
CODES
200
For this AD7124-4/AD7124-8 configuration where the post
filter and low power mode were selected, 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 outlined previously.
This voltage was then converted to a resistance, which was then
linearized and converted to a temperature as outlined in the 3-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 is carried out.
Figure 9 shows that 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.
200
1.50
Pt100 TEMPERATURE (°C)
1.25
Figure 7. Temperature Accuracy Measurement, Sinc4 Filter,
Full Power Mode, 50 SPS, One Time 25°C Calibration Only
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
Rev. 0 | Page 6 of 11
+105ºC
0.50
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 gives 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 3-wire RTD is connected as shown in
Figure 1 at ambient temperature. The corresponding rms noise
is typically 774 nV rms 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
Pt100
+25ºC
0.75
0.25
0
–0.25
–0.50
–0.75
–1.00
13369-009
–1.50
–50
11270444
–1.25
11270430
11270403
13369-006
–1.00
11270417
0
–0.75
–1.25
–1.50
–50
–25
0
25
50
75
100
125
150
175
Pt100 TEMPERATURE (°C)
Figure 9. Temperature Accuracy Measurement, Post Filter,
Low Power Mode, 25 SPS
200
Circuit Note
CN-0383
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. The plot shows that carrying out a one time
calibration at 25°C or calibrating at each individual temperature
of the AD7124-4/AD7124-8 gives similar performances.
1.50
1.25
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
IOUT1 is connected to AIN1. The average of these two voltage
measurements is then used in the overall calculation of the RTD
resistance, and subsequently the temperature is calculated using
Equation 1 through Equation 6. The chopping method greatly
reduces any effects of excitation current mismatch and mismatch
drift. However, there is an impact on the throughput rate since
two measurements are required.
Pt100
+25ºC
0.75
+105ºC
250µA
AIN0
Pt100
0.25
1kΩ
RL1
0
–0.25
AIN2
0.01µF
0.1µF
–0.50
1kΩ
RL2
AIN3
–0.75
0.01µF
–1.00
–25
0
25
50
75
100
125
150
175
AIN1
250µA
200
5.11kΩ
0.1%
±15ppm/°C
Figure 10. Temperature Accuracy Measurement, Post Filter,
Low Power Mode, 25 SPS, 25°C One Time Calibration Only
REFIN1(+)
0.01µF
0.1µF
REFIN1(–)
1kΩ
REFERENCE
250Ω
BUFFER
HEADROOM
COMMON VARIATIONS
Current Source Mismatch and Mismatch Drift
In the Figure 1 circuit, the precision reference resistor was
placed on the high side. The high-side configuration works well
for systems using a single RTD. When multiple RTDs are used,
it is better to place the precision resistor on the low side because
only a single reference resistor is required. With the reference
resistor on the low side, better excitation current matching is
required. Two different techniques can be used to minimize the
errors due to mismatches in the currents:
•
•
1kΩ
500µA
Pt100 TEMPERATURE (°C)
Chopping the excitation currents
Calibration by measuring the excitation currents
0.01µF
Figure 11. AD7124-4/AD7124-8 Configuration for 3-Wire RTD Measurement
Using the Current Chopping Measurement Technique
Measurement data using the excitation current chopping method
was gathered, and the corresponding Pt100 temperature error
recorded as shown in Figure 12. For all RTD temperatures
measured, the temperature error is within the error band of the
Pt100 RTD for different ambient temperatures of the AD7124-4/
AD7124-8. These results show that chopping the excitation
current gives results that are comparable to the data gathered
with the high-side precision reference resistor configuration.
1.50
1.25
Chopping the Excitation Currents
1.00
To chop the currents, a measurement of the RTD voltage is taken
when IOUT0 is connected to AIN0, and IOUT1 is connected
to AIN1, as shown in Figure 11. A second measurement of the
voltage across the RTD is then taken when the currents are
swapped, that is, when IOUT1 is connected to AIN0, and
Pt100
+25ºC
0.75
+105ºC
0.50
ERROR (°C)
The crosspoint multiplexer on the AD7124-4/AD7124-8 allows
easy implementation of the chopping configuration. Figure 11
shows the 3-wire RTD configuration with the precision 5.11 kΩ
reference resistor connected to the low side of the Pt100 RTD.
For this configuration, the current source used as well as the
gain must be reconsidered. Both IOUT0 and IOUT1 are set to
250 µA. Selecting this current ensures that the circuit complies
with the output compliance of the current sources as well as the
reference voltage generated across the precision resistor. To
ensure that the full range of the ADC is used, the gain of the
PGA was set to 32. A resistor is required on the low side of the
reference resistor because the reference buffers are enabled and
require headroom (100 mV).
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
0.25
0
–0.25
–0.50
–0.75
–1.00
13369-012
–1.50
–50
RL3
13369-010
–1.25
13369-011
ERROR (°C)
0.50
–1.25
–1.50
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 TEMPERATURE (°C)
Figure 12. Temperature Accuracy Measurement for Chopping Configuration,
Sinc4 Filter, Full Power Mode, Calibration at Each Temperature
Calibration by Measuring the Excitation Currents
The configuration for calibrating the 3-wire system by
measuring the excitation currents is shown in Figure 13. For
this configuration, the precision reference resistor is connected
Rev. 0 | Page 7 of 11
CN-0383
Circuit Note
to the low side of the RTD. This configuration is similar to that
used for chopping the currents, where both the currents are set
to 250 µA, and the PGA gain is set to 32. However, the main
difference is that an additional differential input channel is
required. The additional input channel enables the measurement
of the two excitation currents. The measurement is implemented
by measuring the voltage drop across the precision reference
resistor with respect to the internal reference when each of the
excitation currents is individually enabled. The measured voltage
is then converted to a current based on the value of the precision
reference resistor value, which is subsequently used to calculate
the ratio of the currents, and then used to calibrate the mismatch.
250µA
AIN0
Pt100
1kΩ
RL1
AIN2
0.01µF
0.1µF
1kΩ
RL2
AIN3
0.01µF
RL3
AIN1
250µA
1kΩ
500µA
5.11kΩ
0.1%
±15ppm/°C
REFIN1(+)
0.01µF
0.1µF
REFIN1(–)
1kΩ
REFERENCE
250Ω
BUFFER
HEADROOM
0.01µF
13369-013
AIN4
AIN5
Figure 13. AD7124-4/AD7124-8 Configuration for 3-Wire RTD Measurement
Calibration by Measuring the Excitation Currents
Figure 14 shows the calibrated temperature error in the RTD
measurements. The results show that the RTD error is within the
expected error band of the RTD, where the error in measurement
is close to the error profile of the RTD itself. To ensure accurate
results, calibration of the currents must take place at regular
intervals over time.
Lead Resistance Considerations
For the 3-wire RTD measurement, the accuracy of the lead
resistance compensation depends on the resistance of each of
the leads being equal (specifically, RL1 = RL2). The voltage
dropped across RL3 does not affect the voltage measured across
the RTD element; therefore, RL3 does not introduce error in the
measurement for the circuits described in this circuit note.
The nominal resistance of 24 AWG copper wire is 0.026 Ω/foot.
A 50 foot length has a 1.3 Ω lead resistance. A 10% error in
matching produces a 0.13 Ω error in the RTD measurement,
assuming perfectly matched compensation and excitation
currents. The RTD temperature coefficient is approximately
0.385 Ω/°C; therefore, the 0.13 Ω lead resistance mismatch
measurement error translates into approximately (0.13 Ω) ÷
(0.385 Ω/°C) = 0.337°C error due to lead resistance mismatch.
Therefore, for accurate 3-wire measurements, the matching
characteristics of the connecting cables must be known precisely.
Assuming perfect lead resistance matching, mismatches in the
excitation currents (IOUT0 and IOUT1) produce an error that
is proportional to the total lead resistance. For instance, a 0.5%
mismatch in the excitation currents (typical specification for the
AD7124-4/AD7124-8) produces a corresponding 0.5% error in
the RTD resistance measurement. The nominal Pt100 RTD
resistance temperature coefficient is 0.385 Ω/°C, which is
equivalent to a temperature change of 2.6°C/Ω. A 0.5% error in
the resistance measurement gives an RTD measurement error of
0.005 × 2.6°C/Ω = 0.013°C/Ω. For a lead resistance of 10 Ω
(~400 feet of 24 AWG copper wire), the error due to the
mismatch in currents is only 0.13°C.
The previous discussion illustrates that, in most practical
applications, the mismatch in the lead resistances creates much
more error than the 0.5% mismatch in excitation currents. As
previously mentioned, the mismatch error in the excitation
currents can be minimized by using either the chopping mode
or by measuring the individual excitation currents.
1.50
1.25
1.00
AD7124-4/AD7124-8
TEMPERATURE
–40ºC
Pt100
+25ºC
0.75
+105ºC
ERROR (°C)
0.50
0.25
0
–0.25
–0.50
–0.75
13369-014
–1.00
–1.25
–1.50
–50
–25
0
25
50
75
100
125
150
175
200
Pt100 TEMPERATURE (°C)
Figure 14. Temperature Accuracy Measurement for Excitation Current
Calibration, Sinc4 Filter, Full Power Mode, Calibration at Each Temperature
Rev. 0 | Page 8 of 11
Circuit Note
CN-0383
CIRCUIT EVALUATION AND TEST
The EVAL-AD7124-4SDZ/EVAL-AD7124-8SDZ evaluation
board is required to test the circuit. In addition, the following
sensor and resistors are required for proper operation:
Equipment Needed
The following equipment is required for the 3-wire RTD
measurement system:
•
•
•
•
•
•
•
•
•
The EVAL–AD7124-4SDZ or EVAL-AD7124-8SDZ
evaluation board
The 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 3-wire RTD
A PC running Windows® XP (SP2), Windows Vista, or
Windows 7 (32-bit or 64-bit)
3-wire Pt100 RTD, Class B
5.11 kΩ precision resistor
250 Ω resistor for buffer headroom (not needed for this
configuration, but included for completeness because it
may be required if a Pt1000 RTD is used with a gain of 1)
To configure the hardware, take the following steps:
•
Set all links on the EVAL-AD7124-4SDZ/EVAL-AD71248SDZ to the default board positions as outlined in the
EVAL-AD7124-4SDZ/EVAL-AD7124-8SDZ user guide.
Power the board with a 7 V or 9 V power source connected
to J5.
Connect the RTD, precision reference resistor, and resistor
for headroom as shown in Figure 16.
•
Software Installation
•
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.
TERMINAL BLOCK
J6
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 run automatically, 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.
AIN0
RL1
AIN3
RL3
AIN4
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
EVALUATION BOARD
AIN5
REFIN+
RREF
5.11kΩ
RHEADROOM
250Ω
Figure 15 shows a functional block diagram of the test setup for
the 3-wire RTD configuration.
7V TO 9V SUPPLY
260mA LIMIT
AIN2
RL2
After the evaluation software installation is complete, connect
the SDP board (via Connector A) to the EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ and then connect the SDP board 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.
Setup and Test
AIN1
Pt100
REFIN–
TERMINAL BLOCK
J11
AGND
PSW
13369-016
PC
USB
Pt100
Figure 16. EVAL-AD7124-4SDZ/EVAL-AD7124-8SDZ Evaluation Board
Connector for 3-Wire RTD Measurement
J4/J5
J1
120
Run the AD7124-4/AD7124-8 EVAL+ Software. Figure 17
shows a screenshot of the main window of the software.
CON A
J6
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
RREF
J11
13369-015
EVAL-SDP-CB1Z
Figure 15. Test Setup Functional Diagram
Rev. 0 | Page 9 of 11
CN-0383
Circuit Note
One additional step is required before the AD7124-4/AD7124-8
is configured for 3-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 18.
From the register tree, select 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 that the calibration performed by selecting 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 that the calibration performed by selecting the
Offset0 register from 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 bitfield of the
ADC_Control register.
13369-017
1.
2.
3.
4.
Figure 17. AD7124-4/AD7124-8 EVAL+ Software Main Window
To configure the AD7124-4/AD7124-8 for 3-wire RTD
measurements, click the 3-WIRE RTD demo mode button (see
Figure 17). Clicking this button configures the ADC software
for optimized performance. Some of the register settings are as
follows:
2.
3.
4.
Channel_0
a. AINP_0 = AIN2
b. AINM_0 = AIN3
c. Setup0
d. Enabled = TRUE
Setup_0
a. PGA_0 = 16
b. AIN_BUFP, AIN_BUFM both = ENABLED
c. BIPOLAR = ENABLED
d. FS_0 = 384
e. FILTER_MODE_0 = SINC4
ADC_Control
a. MODE = Continuous Conversion
b. POWER_MODE = FULL
IO_CONTROL_1
a. IOUT0 Channel Enable = AIN0
b. IOUT0 Select = 500 μA
c. IOUT1 Channel Enable = AIN1
d. IOUT1 Select = 500 μA
5.
13369-018
1.
Figure 18. Register Map Internal Full-Scale and Zero-Scale Calibration
The board and device are now configured for 3-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.
Rev. 0 | Page 10 of 11
Circuit Note
CN-0383
LEARN MORE
Data Sheets and Evaluation Boards
CN-0383 Design Support Package:
www.analog.com/CN0383-DesignSupport
EVAL-AD7124-4SDZ
SDP User Guide
System Demonstration Platform (EVAL-SDP-CB1Z)
EVAL-AD7124-4 User Guide (UG-855)
AD7124-4 Data Sheet
EVAL-AD7124-8 User Guide (UG-856)
AD7124-8 Data Sheet
AN-892 Application Note. Temperature Measurement Theory
and Practical Techniques. Analog Devices.
ADP1720 Data Sheet
Kester, Walt. “Temperature Sensors,” Chapter 7 in Sensor Signal
Conditioning. Analog Devices, 1999.
7/15—Revision 0: Initial Version
EVAL-AD7124-8SDZ
REVISION HISTORY
McCarthy, Mary. AN-615 Application Note. Peak-to-Peak
Resolution Versus Effective Resolution. 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-0381. Completely Integrated 4-Wire RTD
Measurement System Using a Low Power, Precision, 24-Bit,
Sigma-Delta ADC. Analog Devices.
Circuit Note CN-0382. Ultralow Power Industrial Temperature and
Pressure 4 mA to 20mA/HART Transmitter. 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 that
may result from their use. Analog Devices reserves the right to change any Circuits from the Lab reference designs at any time without notice but is under no obligation to do so.
©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN13369-0-7/15(0)
Rev. 0 | Page 11 of 11
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