CN-0384 - Analog Devices

CN-0384 - Analog Devices
Circuit Note
CN-0384
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/CN0384.
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 Thermocouple 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
The AD7124-4/AD7124-8 establishes the highest degree of
signal chain integration, which includes programmable low
drift excitation current sources, bias voltage generator, and
internal reference. Therefore, the design of a thermocouple
system is simplified when the AD7124-4/AD7124-8 is used
because most of the required system building blocks are
included on-chip.
The circuit shown in Figure 1 is an integrated thermocouple
measurement 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.
Thermocouple measurements using this system show an overall
system accuracy of ±1°C over a measurement temperature range
of −50°C to +200°C . Typical noise free code resolution of the
system is approximately 15 bits.
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 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
low noise programmable gain array (PGA) ensures that signals
of small amplitude can be interfaced directly to the ADC.
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.
CIRCUIT FUNCTION AND BENEFITS
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
toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. Continuedonlastpage)
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©2015 Analog Devices, Inc. All rights reserved.
CN-0384
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
1.8V OUTPUT
ADJ
GND
57.6kΩ
4.7µF
4.7µF
0.1µF
RTD Pt100
4-WIRE
CONFIGURATION
GND
AIN2
IOVDD
AVDD
REGCAPD
0.1µF
1kΩ
0.1µF
REGCAPA
AIN3
0.01µF
COLD
JUNCTION
GND
EN
0.1µF
1kΩ
0.01µF
–
OUT
0.1µF
AD7124-4/
AD7124-8
USB
IOUT1 (AIN1)
1kΩ
DOUT/RDY
AIN6
0.01µF
DIN
0.1µF
1kΩ
SCLK
AIN7
CS
0.01µF
1kΩ
0.01µF
AD7124
ADSP-BF527
POWER
SYNC
LED
CLK
REFIN1(+)
5.11kΩ
0.1%
±15ppm/°C
LED
+
GND
1µF
THERMOCOUPLE
CONNECTOR
T-TYPE
THERMOCOUPLE
IN
SDP-B
STATUS
0.1µF
REFIN1(–)
1kΩ
0.01µF
REFERENCE
BUFFER 250Ω
HEADROOM
AVSS
13470-001
DGND
Figure 1. AD7124-4/AD7124-8 Thermocouple Measurement Configuration Including RTD Cold Junction Compensation
CIRCUIT DESCRIPTION
Temperature Measurement Introduction
Thermocouples are one of the most frequently used sensors for
temperature measurements in industrial applications because of
their low cost, ruggedness, repeatability, as well as wide operating
temperature range and fast response time. Thermocouples are
especially useful for making measurements at high temperatures
(up to 2300°C for C-type thermocouples).
A thermocouple consists of the junction of two wires of
different metal types, as shown in Figure 2.
METAL A
MEASUREMENT
JUNCTION
REFERENCE
JUNCTION
13470-002
METAL B
TO SIGNAL
CONDITIONING
AND ADC INPUT
Figure 2. Thermocouple Connection Showing Measurement and
Reference Junctions
The junction is placed where the temperature is to be measured,
and is referred to as the measurement junction. The other end
of the thermocouple is connected to a precision voltage
measurement unit, and this connection is referred to as the
reference junction, or alternately the cold junction. The
temperature difference between the measurement junction and
the cold junction generates a voltage that is proportional to the
difference between the temperatures of the two junctions. The
signal generated is typically from several microvolts to tens of
millivolts and is dependent on the temperature difference. In
the circuit shown in Figure 1, a T-type thermocouple is used.
T-type thermocouples are capable of measuring temperatures
of −200°C to +400°C with an output range of approximately
−8.6 mV to +17.2 mV. It is important for the signal chain to
present a high impedance and low leakage to the thermocouple
to achieve the highest accuracy.
T-type thermocouples have a sensitivity of approximately
40 μV/°C. Therefore, by using the integrated PGA of the
AD7124-4/AD7124-8, the small thermocouple voltage levels
Rev. 0 | Page 2 of 12
Circuit Note
CN-0384
can be easily sensed and accurately converted to a digital
representation. The thermocouple response is approximately
linear over a small portion of the range (0°C to 60°C), as shown
in Figure 3. For accurate measurements over a wide temperature
range, a linearization routine must be applied to the measured
value to ensure accurate temperature values.
20
T-TYPE THERMOCOUPLE
THERMOCOUPLE EMF (mV)
15
10
5
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 power mode and mid power
mode, 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 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 the
lowest noise.
–200
–100
0
100
200
300
400
TEMPERATURE (°C)
13470-003
–5
–10
–300
The AD7124-4/AD7124-8 provides an integrated solution for
thermocouple measurements. The AD7124-4/AD7124-8 can
achieve high resolution, low nonlinearity, and low noise
performance as well as high 50 Hz and 60 Hz rejection. The
device consists of an on-chip, low noise PGA that amplifies the
small signal from thermocouple 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 discuss the
different elements used to develop a thermocouple temperature
measurement system based on the AD7124-4/AD7124-8.
Power Supplies
APPROXIMATELY
LINEAR REGION
0
How the Circuit Works
Figure 3. T-Type Thermocouple Output Voltage vs. Temperature
T-type thermocouples are formed by joining copper and
constantan metals. Other combinations of metals form other
types of thermocouples with various ranges and sensitivities.
For instance, J-type thermocouples are made by joining iron
and constantan and have a range of 0°C to 760°C with a
sensitivity of 55 μV/°C. K-type thermocouples are made by
joining chromel and alumel and have a range of −200°C to
+1260°C with a sensitivity of 39 μV/°C.
Serial Peripheral Interface (SPI)
Cold Junction Compensation (CJC)
The voltage generated by a thermocouple must be converted to
temperature. Converting the voltage measured to an accurate
temperature can be difficult because the thermocouple voltage
is small, the temperature-voltage relationship is nonlinear, and
the cold junction temperature must also be accurately measured.
SPI communication to the AD7124-4/AD7124-8 is 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 4 shows the main window of this software. Clicking
THERMOCOUPLE configures the software for a T-type
thermocouple measurement.
13470-004
The output voltage of the thermocouple represents the difference
between the temperature of the thermocouple and the cold
junction temperature. The cold junction temperature must be
known to ensure an accurate absolute temperature reading from
the thermocouple. The cold junction temperature is measured
with another temperature sensitive device, typically a thermistor,
diode, resistance temperature detector (RTD), or semiconductor
temperature sensor. The temperature-sensing device used for this
circuit is a 4-wire RTD. The cold junction measurement error
contributes directly to the absolute temperature error; therefore,
a high accuracy cold junction temperature measurement is
required. The technique of measuring and compensating for
the cold junction temperature is referred to as cold junction
compensation, or CJC.
Figure 4. AD7124-4/AD7124-8 EVAL+ Software Configuration Screen
Rev. 0 | Page 3 of 12
CN-0384
Circuit Note
THERMOCOUPLE
CONNECTOR
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
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.
T-TYPE
THERMOCOUPLE
+
AVDD
1kΩ
AIN2
0.01µF
–
0.1µF
1kΩ
AIN3
0.01µF
COLD
JUNCTION
RTD Pt100
4-WIRE
CONFIGURATION
IOUT1 (AIN1)
1kΩ
AIN6
0.01µF
0.1µF
1kΩ
AIN7
0.01µF
Analog Inputs
1kΩ
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.
For the circuit shown in Figure 1, two analog input pins are
used to connect the thermocouple (AIN2, AIN3), and three
analog pins are needed for the cold junction compensation
(AIN1, AIN6, AIN7). AIN2 and AIN3 are configured as a fully
differential input channel and measure the voltage generated by
the thermocouple. For this circuit, the thermocouple is floating
as shown in Figure 1. To bias the thermocouple to a known level,
the VBIAS voltage generator is enabled on AIN2 and biases the
thermocouple to
 AVDD  AVSS 
VBIAS  AVSS  

2


The thermocouple measurement is an absolute measurement;
therefore, a voltage reference is needed, and the AD7124-4/
AD7124-8 internal 2.5 V reference is used.
For the cold junction compensation, one excitation current
source is used to excite the RTD. This current is generated from
AVDD and is directed to AIN1. The analog pins and their
configuration are shown in greater detail in Figure 5.
5.11kΩ
0.1%
±15ppm/° C
REFIN1(+)
0.01µF
0.1µF
REFIN1(–)
1kΩ
REFERENCE
BUFFER 250Ω
HEADROOM
0.01µF
AVSS
13470-005
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.
AD7124-4/
AD7124-8
Figure 5. Analog Input Configuration for Thermocouple Measurement Using
4-Wire RTD for Cold Junction Compensation
For this circuit, the cold junction circuit utilizes the reference input,
REFIN1(±). The current through the 4-wire RTD used for the
cold junction measurement 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 reference 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 5.
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. There are several filter options 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.
Rev. 0 | Page 4 of 12
Circuit Note
CN-0384
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 AD7124-4/AD7124-8 full system configuration for the
thermocouple measurement is as follows:

Thermocouple Configuration
The circuit shown in Figure 1 is designed for precision T-type
thermocouple measurement using the AD7124-4/AD7124-8.
Thermocouple measurements require cold junction compensation.
As shown in Figure 1, a 4-wire Pt100 RTD is used for this purpose.
Using the configuration shown in Figure 1, one precision excitation
current source is required to excite the RTD as part of the cold
junction compensation measurement. The RTD is connected
to analog inputs AIN6, AIN7. The bottom side of the RTD is
connected to a precision reference resistor, which applies an
external reference voltage to the device. The precision reference
resistor is connected between reference input pins REFIN1(±).
This configuration represents a ratiometric configuration, where
any deviation in the excitation current is seen by both the RTD
and the reference resistor, and is therefore removed from the
measurement.
The thermocouple itself is connected to the AIN2, AIN3 analog
inputs. One of the inputs is biased using the internal bias voltage
generator of the ADC. The thermocouple voltage is in the range
of −8 mV to +17.2 mV, which represents a temperature range of
−200°C to +400°C. This low level voltage is amplified by the onboard PGA of the AD7124-4/AD7124-8, which is converted to
a precision digital representation using the 24-bit Σ-Δ ADC. To
ensure that the full range of the ADC is utilized, the PGA gain
is set to 128. This thermocouple measurement is made with
respect to the internal low drift 2.5 V reference.
A 4-wire Pt100 Class B RTD is used for the cold junction
measurement. The excitation current for the Pt100 RTD is
programmed to 500 μA.
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 selected gain. The Circuit Note CN-0381
discusses in detail the following required steps:




Selecting a precision reference resistor
Selecting an appropriate PGA gain for RTD measurement
Headroom resistor selection
Excitation current output compliance

Thermocouple measurement (T-type)
o Differential input (AINP = AIN2, AINM = AIN3)
o Gain = 128
o Internal 2.5 V reference
o Digital filtering (sinc4 and post filter)
Cold junction compensation measurement (4-wire RTD)
o Differential input (AINP = AIN6, AINM = AIN7)
o Excitation current: IOUT1 = AIN1= 500 μA
o Gain = 16
o 5.11 kΩ precision reference resistor
o Digital filtering (sinc4 and post filter)
Thermocouple Temperature Calculation
Once the previous procedure is implemented, the next step is to
work through the thermocouple and cold junction calculations.
Different approaches can be used for the linearization/
compensation, which include


Look-up table: requires memory for storage, but also
provides a quick, accurate conversion.
Software linear approximation: does not require storage
except for the conversion polynomial coefficients. Requires
processing time to solve the multiple order polynomial.
However, it also yields a very accurate result. This is the
method used for this circuit.
The software linear approximation requires two inputs: the
voltage measured across the thermocouple, and the cold
junction temperature.
The analog input channel (AIN2, AIN3) is used to measure the
voltage across the thermocouple. The formula used to convert
the code representation to a voltage is Equation 1, which assumes
a bipolar configuration of the ADC. The AD7124-4/AD7124-8
software automatically converts the codes to a voltage based on
the configuration implemented.
VTC 
(CODETC  2 N 1 )  VREF
2 N 1  Gain
(1)
where:
VTC is the thermocouple (TC) voltage.
CODETC is the thermocouple (TC) code.
N is the resolution of ADC, 24.
VREF is the reference used for measurement. For this circuit, the
internal reference is used for the thermocouple measurement.
Gain is the chosen gain for TC mode, 128.
Rev. 0 | Page 5 of 12
CN-0384
Circuit Note
The thermoelectric voltage can then be used to calculate the
overall thermocouple temperature. This step involves a power
series polynomial given by Equation 4. For this circuit, a sixthorder polynomial was used, where the T-type thermocouple
polynomial coefficients were taken from the NIST website.
The 4-wire RTD used for the cold junction requires its own
linearization. The general expression to calculate the RTD
resistance (R) where the ADC is operating in bipolar mode is
given by
(2)
Thermocouple Measurements and Results
The following steps are required to calculate the thermocouple
temperature:
Convert the cold junction temperature to a voltage
Calculate the thermoelectric voltage
Convert the thermoelectric voltage to a temperature
representation
The cold junction temperature must be converted to a voltage.
The cold junction temperature is converted using a polynomial
generated by National Institute of Standards and Technology
(NIST) and is outlined in Equation 3.
(3)
For the circuit shown in Figure 1, data was gathered for
different digital filter and power mode configurations of the
AD7124-4/AD7124-8.
The first configuration was with the sinc4 filter, full power mode,
with an output data rate of 50 SPS. These conditions optimize
the AD7124-4/AD7124-8 for best performance in relation to
speed and noise. Figure 6 shows the noise distribution when
a thermocouple is connected between the AIN2, AIN3 input
channel as shown in Figure 1 at ambient temperature. The
corresponding rms noise is typically 70 nV rms or approximately
16.4 noise free bits. The noise performance of the AD7124-4/
AD7124-8 for inputs shorted under the same conditions is
typically 48 nV rms or 17 noise free bits. The increase in the
noise comes directly from the thermocouple that is connected
across the input channel (AIN2, AIN3).
where:
VCJ is the thermoelectric voltage.
ax is the thermocouple type dependent polynomial coefficient.
T is the cold junction temperature (°C).
n is the order of the polynomial.
180
OCCURRENCES
140
The cold junction temperature-to-voltage conversion accuracy can
be increased by increasing the order of the polynomial. However,
the higher the order, the more processing is required. Therefore,
a trade-off is required when carrying out this conversion. For the
calculations implemented for this circuit, an eighth-order
polynomial was used.
120
100
80
60
40
20
8383994
8383975
8383956
8383936
8383917
8383898
8383879
8383859
8383840
8383821
8383801
8383782
0
8383705
The cold junction temperature voltage must be added to the
differential voltage measured across the thermocouple. The
resulting voltage is an approximation of the thermoelectric
voltage generated by the temperature sensing junction of the
thermocouple.
1000 SAMPLES
160
13470-006
The steps involved in converting the RTD voltage to a temperature
and the linearization are outlined in the Circuit Note CN-0381.
VCJ = a0 + a1T + a2T2 + … + anTn
(4)
where:
V is the thermoelectric voltage (microvolts).
ax is the type dependent polynomial coefficient.
T is the temperature (°C).
n is the order of polynomial.
where:
RRTD is the resistance of the RTD.
CODE is the ADC code.
N is the resolution of ADC, 24.
RREF is the reference resistor.
G is the selected gain, 16.



T = a0 + a1V + a2V2 + a3V3 + … + anVn
8383763
G  2N  1
8383744
(CODE  2N  1 )  RREF
8383724
RRTD 
CODES
Figure 6. Histogram of Codes for Thermocouple at Ambient, Sinc4 Filter,
Full Power Mode, 50 SPS
Rev. 0 | Page 6 of 12
Circuit Note
CN-0384
COLD JUNCTION
TEMPERATURE
2
T-TYPE + RTD
ERROR
–40ºC
+25ºC
+105ºC
ERROR (°C)
1
0
–1
–3
–50
13470-007
–2
0
50
100
150
200
THERMOCOUPLE TEMPERATURE SET (°C)
Figure 7. Thermocouple Temperature Accuracy Measurement, Sinc4 Filter,
Full Power Mode, 50 SPS
1000 SAMPLES
100
80
60
40
13470-008
20
8383659
8383610
8383561
8383512
8383464
8383415
8383366
8383317
8383269
8383220
8383171
0
CODES
Figure 8. Histogram of Codes for Thermocouple and Cold Junction
Temperature at Ambient, Post Filter, Low Power Mode, 25 SPS
For this AD7124-4/AD7124-8 configuration (with the post filter
and low power mode selected), the temperature of the RTD was
swept from −50°C to +200°C. For each of the set thermocouple
temperatures, the corresponding voltage across the thermocouple
was measured using the AD7124-4/AD7124-8, as outlined
previously. Also recorded was the cold junction temperature
using the 4-wire RTD. The voltage of the thermocouple along
with the voltage representation of the cold junction temperature
were used to calculate the temperature of the thermocouple.
Figure 9 shows the resulting error between the set and measured
temperatures of the thermocouple after linearization for cold
junction temperatures of −40°C, +25°C, and +105°C. As shown
in Figure 9, the error between the calculated and set temperature
of the thermocouple is well within the root sum square combined
error window of the T-type thermocouple and Pt100 RTD, as
shown in the plot. The T-type thermocouple has a maximum
error of 1°C or 0.75%, and the Pt100 error is ±(0.3 + 0.005 × |T|)
from the IEC751 Standard.
3
The second configuration tested was with the post filter, low
power mode, and a 25 SPS output data rate that gives simultaneous
50 Hz and 60 Hz rejection, allowing the user to trade off settling
time with rejection. Figure 8 shows the noise distribution when
a thermocouple is connected between the AIN2, AIN3 input
channel as shown in Figure 1 at ambient temperature. The
corresponding rms noise is typically 220 nV rms equating to
approximately 14.7 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 170 nV rms
or 15.1 noise free bits. The increase in the noise comes directly
from the thermocouple that is connected across the input
channel (AIN2, AIN3).
2
COLD JUNCTION
TEMPERATURE
T-TYPE + RTD
ERROR
–40ºC
+25ºC
+105ºC
ERROR (°C)
1
0
–1
–2
–3
–50
13470-009
3
120
OCCURRENCES
For the thermocouple configuration where the sinc4 filter and full
power mode were selected, the temperature of the thermocouple
was swept from −50°C to +200°C, while the cold junction was held
at −40°C, +25°C, and +105°C. For each of the set thermocouple
temperatures, the corresponding voltage across the thermocouple
was measured using the AD7124-4/AD7124-8 as previously
outlined. Also recorded was the cold junction temperature using
the 4-wire RTD. The voltage of the thermocouple along with the
voltage representation of the cold junction temperature were
used to calculate the temperature of the thermocouple. Figure 7
shows the resulting error measured between the set temperature
value and measured temperatures of the thermocouple after
linearization, for cold junction temperatures of −40°C, +25°C,
and +105°C. Internal zero-scale and full-scale calibrations were
performed at each cold junction temperature. As shown in
Figure 7, the error between the calculated and set temperature
of the thermocouple is well within the root sum square combined
error window of the T-type thermocouple and Pt100 RTD, as
shown in the plot. The T-type thermocouple has a maximum
error of 1°C, or 0.75%, and from the IEC751 Standard, the
Pt100 error is ±(0.3 + 0.005 × |T|).
0
50
100
150
200
THERMOCOUPLE TEMPERATURE SET (°C)
Figure 9. Thermocouple Temperature Accuracy Measurement, Post Filter,
Low Power Mode, 25 SPS
Rev. 0 | Page 7 of 12
CN-0384
Circuit Note
THERMOCOUPLE
CONNECTOR
COMMON VARIATIONS
Cold Junction Measurement Alternative
The EVAL–AD7124-4SDZ/EVAL-AD7124-8SDZ evaluation
boards have a thermistor on board as part of the overall board
design. This thermistor is a KTY81/110 and has a typical resistance
of 1 kΩ at +25°C, 500 Ω at −40°C, and 1.7 kΩ at +105°C. The
thermistor can be used for measuring the cold junction
temperature. Thermistors are cheaper than 4-wire RTDs, but
are not as accurate. When implementing a thermistor for cold
junction measurements, care must be taken to ensure that the
cold junction measurement works as expected. The following
steps outline some decisions that need consideration:

REGCAPA
AIN3
AD7124-4/
AD7124-8
IOUT1 (AIN1)
1kΩ
THERMISTOR
KTY81-110
1kΩ AT 25°C
DOUT/RDY
AIN6
0.01µF
DIN
0.1µF
1kΩ
SCLK
AIN7
CS
0.01µF
SYNC
1kΩ
Choose the precision reference resistor value.
Choose the appropriate gain.
Choose the excitation current.
Check the output voltage compliance range of the
excitation current.
Check the resistance value of the thermistor for the
different cold junction temperatures.
REFIN1(+)
2kΩ
0.1%
±15ppm/° C
0.01µF
CLK
0.1µF
REFIN1(–)
1kΩ
0.01µF
REFERENCE
BUFFER 250Ω
HEADROOM
DGND
13470-010
AVSS
Figure 10. Thermistor Cold Junction Configuration for Thermocouple
Measurements
Taking all of these steps into consideration, the settings for
implementing this thermistor when measuring the cold junction
temperature as part of the overall temperature measurement
system requires the following register configurations:

REGCAPD
0.1µF
1kΩ
0.01µF
COLD
JUNCTION
IOVDD
AIN2
Thermocouple measurement settings as outlined
previously (T-type)
o Differential input (AINP = AIN2, AINM = AIN3)
o Gain = 128
o Internal 2.5 V reference
o Digital filtering (sinc4 and post filter)
Cold junction compensation measurement (thermistor)
o Differential input (AINP = AIN4, AINM = AIN5)
o Excitation current: IOUT0 = AIN1 = 500 μA
o Gain = 1
o 2 kΩ precision reference resistor (the thermistor
resistance varies from 500 Ω at −40°C to 1.7 kΩ at
+105°C; it is also required to evaluate the headroom
with this resistance)
Using the setup configuration shown in Figure 10, the reference
to the AD7124-4/AD7124-8 is always approximately 1 V based
on the 500 μA current and the 2 kΩ precision reference resistor.
The performance of the system when the thermistor is used for
cold junction compensation was recorded where the cold junction
was held at 25°C, and the temperature of the thermocouple
swept from −50°C to +200°C. The sinc4 filter in full power mode
and the post filter in low power mode were used. Figure 11 shows
the worst-case error recorded between the set temperature of
the thermocouple, and the calculated temperature using the
linearization technique for both filter types and power modes.
The worst case error recorded was ±1°C.
3
FULL POWER, SINC4 FILTER
LOW POWER, POST FILTER
T-TYPE + RTD
ERROR
2
1
0
–1
–2
–3
–50
13470-011
5.
AVDD
1kΩ
0.01µF
–
ERROR (°C)
1.
2.
3.
4.
+
T-TYPE
THERMOCOUPLE
0
50
100
150
200
THERMOCOUPLE TEMPERATURE (°C)
Figure 11. Thermocouple Temperature Accuracy Measurement Using
Thermistor for Cold Junction Compensation at 25°C
Rev. 0 | Page 8 of 12
Circuit Note
CN-0384
THERMOCOUPLE
CONNECTOR
Bias Voltage
In Figure 1, the internal VBIAS voltage is supplied to the
thermocouple via the AINP or AINM pins. This configuration
for the VBIAS voltage works well when the anti-alias filters are
implemented using the component values shown in Figure 1. If
filters with very large R and C values are used (for example, for
EMC filtering), VBIAS must be taken from a separate dedicated
pin and then applied externally to the thermocouple. This
removes any inaccuracies in the measurements caused by
potential common-mode noise that can be converted to
differential-mode noise.
T-TYPE
THERMOCOUPLE
T-TYPE
THERMOCOUPLE
T-TYPE
THERMOCOUPLE
–
1kΩ
T-TYPE
THERMOCOUPLE
T-TYPE
THERMOCOUPLE
–
T-TYPE
THERMOCOUPLE
THERMISTOR
KTY81-110
1kΩ AT 25°C
T-TYPE
THERMOCOUPLE
1kΩ
0.01µF
0.1µF
1kΩ
0.1µF
1kΩ
AIN7
+
1kΩ
AIN8
0.01µF
0.1µF
1kΩ
AIN9
T-TYPE
THERMOCOUPLE
+
1kΩ
AIN10
0.01µF
–
0.1µF
1kΩ
AIN11
0.01µF
THERMISTOR
KTY81-110
1kΩ AT 25°C
DOUT/RDY
IOUT0 (AIN12)
1kΩ
AIN13
0.01µF
0.1µF
1kΩ
AIN14
0.01µF
SCLK
CS
1kΩ
SYNC
2kΩ
0.1%
±15ppm/° C
CLK
REFIN1(+)
0.01µF
0.1µF
REFIN1(–)
1kΩ
REFERENCE
BUFFER 250Ω
HEADROOM
0.01µF
AVSS
DGND
REFIN1(+)
0.01µF
Figure 13. AD7124-8—Six Thermocouple Measurement System
Including Cold Junction Compensation
0.1µF
0.01µF
AVSS
DGND
13470-012
1kΩ
AIN6
THERMOCOUPLE
CONNECTOR
REFIN1(–)
REFERENCE
BUFFER 250Ω
HEADROOM
CLK
AIN6
1kΩ
SYNC
0.01µF
0.01µF
2kΩ
0.1%
±15ppm/° C
CS
AIN5
1kΩ
DIN
AIN5
SCLK
0.1µF
1kΩ
0.01µF
–
COLD
JUNCTION
IOUT0 (AIN4)
DIN
AIN4
THERMOCOUPLE
CONNECTOR
AD7124-4
AIN3
DOUT/RDY
1kΩ
0.01µF
IOVDD
0.01µF
COLD
JUNCTION
+
–
0.1µF
1kΩ
AIN3
0.01µF
AIN2
0.01µF
0.1µF
1kΩ
0.01µF
AIN1 (IOUT0)
1kΩ
AIN2
THERMOCOUPLE
CONNECTOR
THERMOCOUPLE
CONNECTOR
+
+
–
REGCAPA
0.01µF
1kΩ
0.01µF
REGCAPD
0.1µF
AD7124-8
0.01µF
AIN0
0.01µF
REGCAPA
AIN1 (IOUT0)
THERMOCOUPLE
CONNECTOR
AVDD
1kΩ
+
–
Using this information, the AD7124-4 allows two thermocouples
to be connected and measured with respect to the same cold
junction, as shown in Figure 12. The AD7124-8 allows up to six
different thermocouple measurements with respect to the same
cold junction, as shown in Figure 13.
+
1kΩ
0.01µF
Two analog pins configured differentially to measure the
voltage across the thermocouple
Two analog pins configured differentially to measure the
voltage at the cold junction terminal
One single analog pin to steer the excitation current to the
cold junction compensation circuitry
THERMOCOUPLE
CONNECTOR
REGCAPD
0.1µF
Figure 12. AD7124-4—Two Thermocouple Measurement System
Including Cold Junction Compensation
Rev. 0 | Page 9 of 12
13470-013

AIN0
THERMOCOUPLE
CONNECTOR
The AD7124-4 and AD7124-8 can be used for multiple
thermocouple measurements. Thermocouple measurements
require

IOVDD
AVDD
1kΩ
0.01µF
–
Multiple Thermocouple Measurement System

+
CN-0384
Circuit Note
The EVAL–AD7124-4SDZ/ EVAL-AD7124-8SDZ evaluation
board is needed to test the circuit. In addition, the following
sensor and resistors are required for proper operation:
CIRCUIT EVALUATION AND TEST
Equipment Needed
The following equipment is required for the thermocouple
measurement system:





EVAL–AD7124-4SDZ/EVAL-AD7124-8SDZ evaluation
board
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
AD7124-4/AD7124-8 EVAL+ Software
Power supply: 7 V or 9V wall wart
T-type thermocouple
A PC running Windows® XP (SP2), Windows Vista, or
Windows 7 (32-bit or 64-bit)
T-type thermocouple
2 kΩ precision resistor
250 Ω resistor needed for buffer headroom
Configuring the Hardware
To configure the hardware, do the following:

Set all links on the EVAL-AD7124-4SDZ/EVAL-AD71248SDZ to the default board positions as outlined in the
evaluation board user guide.
Power the board with a 7 V or 9 V power source connected
to J5.
Connect the thermocouple, precision reference resistor,
and resistor for headroom as shown in Figure 15.

Software Installation

A complete software user guide for the EVAL–AD7124-4SDZ/
EVAL-AD7124-8SDZ and SDP boards can be found in the
EVAL–AD7124-4SDZ/EVAL-AD7124-8SDZ user guide and
the SDP User Guide, respectively.
TERMINAL BLOCK
J6
500µA
AIN0
Software is required to interface with the hardware, and 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.
IOUT0 (AIN1)
AIN3
AIN4
After the evaluation software installation is complete, connect the
SDP board (via Connector A) to the evaluation board, and then
connect the evaluation 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.
AIN5
REFIN(+)
RREF
5.11kΩ
Setup and Test
REFERENCE
AND
HEADROOM
RESISTORS
AGND
PSW
T-TYPE
THERMOCOUPLE
USB
REFIN1(+)
J4/J5
J6
–
COLD
JUNCTION
R28
THERMISTOR
+
J1
120
THERMOCOUPLE
CONNECTOR
A2
CON A
REFIN1(–)
AGND
TERMINAL BLOCK
J11
PC
500µA
AIN4
AIN5
REFIN(–)
RHEADROOM
250Ω
Figure 14 shows the functional block diagram of the test setup. To
allow quick setup for thermocouple measurements, the on-board
thermistor is used to implement the cold junction measurements.
7V TO 9V SUPPLY
260mA LIMIT
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
EVALUATION BOARD
AIN2
13470-015




Figure 15. Evaluation Board Connector for Thermocouple Measurement
J11
EVAL-SDP-CB1Z
THERMISTOR
EVAL-AD7124-4SDZ/
EVAL-AD7124-8SDZ
13369-014
TC AIN2 +
A2
COMNNECTION ANI3
–
Figure 14. Test Setup Functional Diagram
Rev. 0 | Page 10 of 12
Circuit Note
CN-0384

Configuring the Software
Run the AD7124-4/AD7124-8 EVAL+ Software. Figure 16
shows a screenshot of the main window of the software.
IO_CONTROL_1 (excitation for RTD)
o IOUT1 Channel Enable = AIN1
o IOUT1 Select = 500 μA
IO_CONTROL_2 (biasing the thermocouple)
o VBIAS2 = True

One additional step is required before the AD7124-4/AD7124-8
is configured for thermocouple measurements: an internal fullscale and zero-scale calibration of the AD7124-4/AD7124-8 is
required for the thermocouple channel. This can be performed
via the Register Map tab, as shown in Figure 17.
From the register tree, select the ADC_Control register.
Enable Channel 0 only.
Select low power mode.
Carry out an internal full-scale calibration.
a. Click the Mode bitfield of the ADC control register.
b. Select internal full-scale calibration.
c. Check that the calibration has been performed by
selecting the Gain0 register from the register tree, and
check that the coefficients have changed.
Carry out an internal zero-scale calibration.
a. Click the Mode bitfield of the ADC control register.
b. Select internal zero-scale calibration.
c. Check that the calibration has been performed by
selecting the Offset0 register in the register tree, and
check that the coefficients have changed.
13470-016
1.
2.
3.
4.
Figure 16. AD7124-4/AD7124-8 EVAL+ Software Main Window







Channel_0 (thermocouple)
o AINP_0 = AIN2
o AINM_0 = AIN3
o Setup = Setup0
o Enabled = TRUE
Channel_1 (thermistor cold junction measurement)
o AINP_1 = AIN4
o AINM_0 = AIN5
o Setup = Setup1
o Enabled = TRUE
CONFIG_0 (thermocouple)
o PGA_0 = 128
o AIN_BUFP, AIN_BUFM both = ENABLED
o BIPOLAR = ENABLED
o REF_SEL = Internal Reference
CONFIG_1 (thermistor cold junction measurement)
o PGA_0 = 1
o AIN_BUFP, AIN_BUFM both = ENABLED
o BIPOLAR = ENABLED
o REF_SEL = External Reference
FILTER_0 (thermocouple)
o Filter = Sinc4
o FS_0 = 384
FILTER_1 (thermistor cold junction)
o Filter = Sinc4
o FS_0 = 384
ADC_Control
o MODE = Continuous Conversion
o POWER_MODE = FULL POWER
o REF_EN = Enabled
5.
13470-017
To configure the AD7124-4/AD7124-8 for thermocouple
measurements, click the THERMOCOUPLE demo mode button
in the main window, as shown in Figure 16. Clicking this button
configures the ADC software for optimized performance. Some
of the register settings are as follows:
Figure 17. Register Map Internal Full-Scale and Zero-Scale Calibration
A calibration is not required for the thermistor channel because
the gain error at a gain of 1 is factory calibrated.
The board and device are now configured for thermocouple
measurements, which includes cold junction compensation
measurement using the thermistor positioned on the evaluation
board. 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.
For more accurate cold junction measurements, a 4-wire RTD can
be connected, as outlined in the previous sections. To use a 4-wire
RTD, the current from AIN1 must be disconnected from the
thermistor and connected to the 4-wire RTD, as shown in Figure 1.
Rev. 0 | Page 11 of 12
CN-0384
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0384 Design Support Package:
www.analog.com/CN0384-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
Mary McCarthy. 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.
CN-0376 Circuit Note. Channel-to-Channel Isolated
Temperature Input (Thermocouple/RTD) for PLC/DCS
Applications. Analog Devices.
CN-0381 Circuit Note. Completely Integrated 4-Wire RTD
Measurement System Using a Low Power, Precision, 24-Bit,
Sigma-Delta ADC. Analog Devices.
CN-0382 Circuit Note. Isolated 4 mA to 20 mA/HART Temperature
and Pressure Industrial Transmitter Using a Low Power, Precision,
24-Bit Sigma-Delta ADC. Analog Devices.
CN-0383 Circuit Note. Completely Integrated 3-Wire RTD
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
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©2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN13470-0-7/15(0)
Rev. 0 | Page 12 of 12
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