Texas Instruments | K-type thermocouple measurement circuit with precision ADC | Application notes | Texas Instruments K-type thermocouple measurement circuit with precision ADC Application notes

Texas Instruments K-type thermocouple measurement circuit with precision ADC Application notes
Analog Engineer's Circuit: Data
Converters
SBAA355 – March 2019
K-type thermocouple measurement circuit with
precision ADC
Joseph Wu
Input Measurement
ADC Input
Digital Output ADS1120
T = –270°C, K-type
AINP – AINN = –6.5mV
F300H or –332810
T = 1370°C, K-type
AINP – AINN = 55mV
6E00CH or 2816010
Power Supplies
AVDD, DVDD
AVSS, DGND
3.3V
0V
Design Description
This cookbook design describes a temperature measurement for a single thermocouple using the
ADS1120. The ADS1120 device then takes a separate measurement to measure the temperature of the
thermocouple cold junction for cold-junction compensation (CJC). Calculations for this design are shown
for a single K-type thermocouple with a temperature measurement range from –270°C to 1370°C. This
circuit can be used in applications such as analog input modules for PLCs, lab instrumentation, and
factory automation. For more information about making precision ADC measurements with a variety of
RTD wiring configurations, see A Basic Guide to Thermocouple Measurements.
3.3 V
3.3 V
0.1 PF
0.1 PF
3.3 V
AVDD
1M
RIN
CIN_DIFF
24-Bit
û ADC
AINN
AIN2
Isothermal
Block
PGA
MUX
CIN_CM
Precision
Temperature
Sensor
AIN3
DVDD
Reference
MUX
AINP
AIN1
RIN
1M
REFN0
ADS1120
Internal
Reference
CIN_CM
AIN0
Thermocouple
REFP0
IDAC
Currents
Digital
Filter
and SPI
Interface
CS
SCLK
DIN
DOUT/DRDY
DRDY
Low Drift
Oscillator
PSW
AGND
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CLK
DGND
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Design Notes
1. Use supply decoupling capacitors for both analog and digital supplies. AVDD and DVDD must be
decoupled with at least a 0.1-μF capacitor to GND. See the ADS1120 4-Channel, 2-kSPS, Low-Power,
16-Bit ADC with Integrated PGA and Reference data sheet for details on power-supply
recommendations.
2. When possible, use C0G (NPO) ceramic capacitors for input filtering. The dielectric used in these
capacitors provides the most stable electrical properties over voltage, frequency, and temperature
changes. Because of size, this may not always be practical and X7R capacitors are the next best
alternative.
3. Cold-junction compensation is required for accurate measurement of the thermocouple temperature.
4. The K-type thermocouple is chosen for its large input range and thermocouple voltage. Other
thermocouples have different sensitivities and error tolerances. For measurements with other
thermocouples and a guide to use them, see A Basic Guide to Thermocouple Measurements.
5. Conversion tables and polynomial equations used to determine thermocouple temperature from the
thermoelectric voltage are found at the NIST website at http://srdata.nist.gov/its90/menu/menu.html.
Additionally, some thermocouple conversions may be determined from the Analog engineer's
calculator.
Component Selection
1. Identify the range of operation for the thermocouple.
The K-type thermocouple has a range of approximately −6.5mV to +55mV, if the temperature
measurement range is from −270°C to 1370°C. This range is used to maximize the resolution of the
measurement, considering the full-scale range of the ADC.
2. Determine gain and input range of the ADC.
Calculate the maximum gain allowed without over-ranging the programmable gain amplifier (PGA)
based on the maximum thermocouple output voltage and the reference voltage. Then ensure that the
thermocouple biasing is near mid-supply so that the input signal is in the common-mode input range of
the PGA.
For the ADS1120 device, the PGA can amplify the input signal from gains of 1 to 128 in factors of 2.
With an input range of −6.5mV to +55mV, the maximum gain used is 32 without over-ranging the PGA
using the internal reference voltage of 2.048V.
% of usable ADC range=32 • [55mV–(–6.5mV)] / [2.048V–(–2.048V)] • 100%=48.0%
Number of ADC codes in measurement range = 0.48 • 216 = 31488
With the temperature range of −270°C to 1370°C for the thermocouple, 31488 codes are used to
represent a range of 1640°C. This equates to approximately 0.052°C per ADC code.
3. Set up the resistor biasing to establish the input DC level and burn-out detection.
Equal biasing resistors are tied from either end of the thermocouple to AVSS and AVDD. In normal
operation, the resistors set the DC biasing point of the thermocouple to near mid-supply. If the
thermocouple burns out and becomes an open circuit, the resistors pull apart the thermocouple leads
to either supply. These resistors are set high (often 500kΩ to 10MΩ) to reduce the bias current going
through the thermocouple. Resistor bias current reacting with the thermocouple lead resistance causes
measurement error. However, the resistors must also be low enough to provide sufficient bias current
to overcome any input current from the resistor.
In this design, biasing resistors are chosen to be 1MΩ. This value sets the resistor biasing current to a
low 1.65μA (assuming the resistors bias the thermocouple at center of the 3.3-V supply). However, in
the case of a burned out thermocouple, 1MΩ provides more than enough current to separate the ADC
inputs against any input current of the ADC.
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AVDD
AVDD
AVDD
1M
AVDD
1M
Good
thermocouple
Burned out
thermocouple
+
-6.5mV to
55mV
-
+
Input
Current
Input
Current
ADS1120
ADS1120
AVSS
1M
AVSS
1M
AVSS
AVSS
In the case of the good thermocouple, the thermocouple voltage has a range of –6.5mV to 55mV. The
remaining 3.3V of the VDD supply is dropped equally between the biasing resistors. With 1-MΩ bias
resistors, the thermocouple voltage has a DC operating point near mid-supply at 1.65V.
In the case of a burned out thermocouple, the 1-MΩ bias resistors overcome the input current to pull
apart the input voltage so that the ADC reads a full-scale input reading. Based on the typical
characteristics curves, the absolute input current is 250nA depending upon temperature, input voltage,
and input channel. Assuming this input current at each input, the input current drops the same voltage
across the two bias resistors:
ADC input voltage = 3.3V – (1MΩ • 250nA) – (1MΩ • 250nA) = 2.8V
If the thermocouple burns out, the ADC input voltage is 2.8V, which is much larger that the positive fullscale reading of the ADC. The ADC reports a reading of 7FFFh to indicate a burn-out condition. Note
that this assumes the largest input current. With lower operating temperature or different inputs, the
ADC input may be larger.
4. Select values for the differential and common-mode input filtering for the ADC inputs and reference
inputs.
If there is input filtering, the input current reacts with any series filter resistance to create an error. For
this reason, the input series resistance is kept low to reduce any possible error.
This design includes differential and common-mode input RC filtering. The bandwidth of the differential
input filtering is set to be at least 10 × higher than the data rate of the ADC. The common-mode
capacitors are selected to be 1/10 of the value the differential capacitor. Because of capacitor
selection, the bandwidth of common-mode input filtering is approximately 20 × higher than the
differential input filtering.
With input filtering, differential signals are attenuated at a lower frequency than the common-mode
signals, which are significantly rejected by the PGA of the device. Mismatches in common-mode
capacitors cause an asymmetric noise attenuation, appearing as a differential input noise. With a lower
bandwidth for differential signals, the effects from the mismatch of input common-mode capacitors are
reduced. Input filtering for the ADC inputs and reference inputs are designed for the same bandwidth.
In this design, the data rate is chosen to be 20SPS. For the ADC input filtering, the bandwidth
frequency for the differential and common-mode filtering is approximated in the following equations:
fIN_DIFF = 1 / (2 • π • CIN_DIFF • 2 • RIN)
fIN_CM = 1 / (2 • π • CIN_CM • RIN)
For the ADC input filtering, RIN = 800Ω, CIN_DIFF = 0.47µF, and CIN_CM = 0.047µF. This sets the
differential filter bandwidth to 330Hz and the common-mode filter bandwidth to 4.2kHz.
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5. Use cold-junction compensation to calculate the actual thermocouple voltage based on the coldjunction temperature.
To get a precise measurement from a thermocouple, cold-junction compensation must be performed to
get an accurate temperature measurement. An accurate measurement of the cold-junction where the
thermocouple leads are tied is required. You cannot simply add the temperature of the cold-junction to
the temperature computed from the thermocouple voltage. To accurately determine the thermocouple
temperature, the proper method is to:
1. Convert the cold-junction temperature (TCJ) to a voltage (VCJ)
2. Add the cold-junction voltage to the measured thermocouple voltage (VCJ + VTC)
3. Convert the summed cold-junction voltage and thermocouple voltage to the thermocouple
temperature (TTC)
The following flow diagram shows the conversion method to determine the actual temperature of the
thermocouple based on the ADC measurements.
MCU
ADC
Thermocouple
Voltage
VTC
Cold-Junction
Temperature
TCJ
Convert Temperature
to Voltage
VCJ
6
V
Convert Voltage
to Temperature
T
Conversion tables and polynomial equations used to determine thermocouple temperature from the
thermoelectric voltage are found at the NIST website: http://srdata.nist.gov/its90/menu/menu.html.
Because the ADS1120 device has an accurate internal temperature sensor, a measurement can be
used. The internal temperature sensor has a typical accuracy of 0.25°C for a range of 0°C to 75°C.
This accuracy is ideal for the cold-junction measurement. However, the device requires a good thermal
contact to the connection for the thermocouple cold-junction. Any error in the cold-junction
measurement yields an error in the resulting temperature measurement. Note that the ADS1120 device
does not specify a maximum error for the temperature sensor.
For more information about thermocouples and the cold-junction compensation measurement, see
A Basic Guide to Thermocouple Measurements.
6. Cold-junction compensation can be made through alternate methods.
Because the ADS1120 device has an additional analog input channel, internal voltage reference, and
an external reference input, alternate cold-junction measurements may be used.
As an example, while AIN0 and AIN1 are used for the thermocouple, AIN2 and AIN3 may be used to
measure an RTD. This would require a reference resistor and a connection to the external reference
inputs. The ADS1120 device has current IDAC sources that may be used for excitation of the RTD and
reference resistor. Alternately, AIN2 and AIN3 may be used to measure other temperature sensors like
the LMT70. The LMT70 precision analog temperature sensor outputs a voltage based on the ambient
temperature of the device.
For more information about RTDs, see A Basic Guide to RTD Measurements. For more information
about the LMT70, see the LMT70, LMT70A ±0.05°C Precision Analog Temperature Sensor, RTD and
Precision NTC Thermistor IC data sheet.
4
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0
0
±40
±40
Magnitude (dB)
Magnitude (dB)
7. An extra input channel and other ADS1120 features are available for other measurements.
The circuit schematic is drawn with AIN2 and AIN3 unused and may be used for other measurements.
If the device cannot be placed at the cold-junction temperature, an alternate cold-junction
measurement must be made. As mentioned in the previous section, AIN2 and AIN3 can be used with
the external reference to make an RTD measurement if the internal temperature sensor is not used.
IDAC current sources may be enabled in the ADS1120 device for excitation of the RTD and the
reference resistor. However, in this design, the configuration register and pseudocode are set when the
internal temperature sensor is enabled, and the IDACs are disabled.
Additionally, AIN3 is internally connected to a low-side power switch (PSW). This switch may be used
in resistive bridge applications to disconnect a bridge when not in use, reducing the current
consumption. For this circuit, the PSW is unused in this design and disabled.
The ADS1120 device has a set of burn-out current sources (BCS) which can be used enable current
sources at the ADC inputs to pull apart the input voltage when the thermocouple is burned out. In this
design, the bias resistors serve the same purpose and the BCS is disabled. However, if the
thermocouple is biased from a voltage, without biasing resistors, the BCS may be necessary for
burnout detection.
The ADS1120 device uses a finite impulse response digital filter that can be configured for different
data rates and different frequency responses. At data rates of 5SPS and 20SPS, the filter can be
configured to reject 50-Hz or 60-Hz line frequencies or to simultaneously reject 50 Hz and 60 Hz. Two
bits (50/60[1:0]) in the configuration register are used to configure the filter accordingly. At 20SPS, the
digital filter frequency responses are shown. The 50/60[1:0] bits are enabled in this design.
For more information about features in the ADS1120 device, see the ADS1120 4-Channel, 2-kSPS,
Low-Power, 16-Bit ADC with Integrated PGA and Reference data sheet.
±80
±120
±160
±80
±120
±160
±200
±200
0
20
40
60
80
100 120 140 160 180 200
Frequency (Hz)
46
48
50
52
54
56
58
60
62
Frequency (Hz)
C049
64
C050
Simultaneous 50-Hz and 60-Hz rejection,
50/60[1:0] = 01
Simultaneous 50-Hz and 60-Hz rejection,
50/60[1:0] = 01
Filter Response (DR = 20SPS)
Detailed View of Filter Response (DR = 20SPS)
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Measurement Conversion
Conversions for the thermocouple voltage are relatively straightforward based on the full-scale range
setting of the ADC.
Measurement 1 (Thermocouple):
OutputCode1 = [215 • (VAIN0 – VAIN1) / (VREF / Gain)]
Thermocouple Voltage = VAIN0 – VAIN1 = [OutputCode1 • (2.048V / 32) / (215)]
If the internal temperature sensor is used, conversions for the internal temperature sensor require some
data manipulation. Temperature data from the ADC are represented as a 14-bit result that is left-justified
within the 16-bit conversion result. Data are output starting with the most significant byte (MSB). When
reading the two data bytes, the first 14 bits are used to indicate the temperature measurement result. One
14-bit LSB equals 0.03125°C and negative numbers are represented in binary twos complement format.
Measurement 2 (Internal Temperature Sensor):
OutputCode2 = [(Temperature) / (0.03125°C)]
Temperature = [(OutputCode2) • (0.03125°C)]
OutputCode2 is the first 14 bits of the two-byte output data from the ADC.
Register Settings
Measurement 1 (Thermocouple):
Measurement 1, Configuration Register 0
Bit
Field
Setting
7:4
MUX[3:0]
0000
Description
AINP is AIN0 and AINN is AIN1 for thermocouple measurement
3:1
GAIN[2:0]
101
Gain = 32
0
PGA_BYPASS
0
PGA enabled
Measurement 1, Configuration Register 1
Bit
Field
Setting
Description
7:5
DR[2:0]
000
Data rate = 20SPS
4:3
MODE[1:0]
00
Normal mode
2
CM
0
Single-shot conversion mode
1
TS
0
Temperature sensor disabled
0
BCS
0
Burn-out current sources off
Measurement 1, Configuration Register 2
Bit
Field
Setting
7:6
VREF[2:0]
00
Internal 2.048V reference selected
5:4
50/60[1:0]
01
Simultaneous 50-Hz and 60-Hz rejection enabled
Switch is always open (used in bridge applications)
3
PSW
0
2:0
IDAC[2:0]
000
Description
IDAC current sources off
Measurement 1, Configuration Register 3
6
Bit
Field
Setting
7:5
IMUX1[2:0]
000
Description
IDAC1 disabled
4:2
IMUX2[2:0]
000
IDAC2 disabled
1
DRDYM
0
Only the dedicated DRDY pin is used to indicate when data are ready
0
Reserved
0
Always write 0
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Measurement 2 (Internal Temperature Sensor):
Measurement 2, Configuration Register 1
Bit
Field
Setting
7:5
DR[2:0]
000
Description
Data rate = 20SPS
4:3
MODE[1:0]
00
Normal mode
2
CM
0
Single-shot conversion mode
1
TS
1
Temperature sensor enabled, bypasses PGA and sets the gain to 1
0
BCS
0
Burn-out current sources off
Pseudo Code Example
The following shows a pseudo code sequence with the required steps to set up the device and the
microcontroller that interfaces to the ADC in order to take subsequent readings from the ADS1120 in
single-shot conversion mode:
Configure microcontroller for SPI mode 1 (CPOL = 0, CPHA = 1)
Set CS low;
Send 43h; //WREG starting at register 00h, four bytes
0Ah 00h 10h 00h; // Register 0: AINP=AIN0, AINN=AIN1, PGA=32, PGA enabled
// Register 1: DR=20SPS, Normal mode, Single shot conversion, Temp sensor and BCS disabled
// Register 2: Internal 2.048V ref, 50Hz/60Hz rejection enabled, PSW and IDACs disabled
// Register 3: IDAC disabled, Only /DRDY pin indicates when data are ready
Set CS high;
Loop
{
Set CS low;
Send 44h; //WREG starting at register 01h, one byte
00h; // Register 1: Data Rate=20SPS, Normal mode, Single shot conversion,
// Temp sensor and BCS disabled
Set CS high;
Set CS low;
Send 08h; // Send START/SYNC command to start conversion
Set CS high;
Wait 51 ms for /DRDY to go low; // At 20SPS data rate, nominally 50ms ±2% accuracy
Set CS low;
Send 10h; // RDATA command to read data for thermocouple
00h 00h; // Clock out 16 bits of data
Set CS high;
Set CS low;
Send 44h; //WREG starting at register 01h, one byte
02h; // Register 1: Enable temperature sensor
Set CS high;
Set CS low;
Send 08h; // Send START/SYNC command to start conversion
Set CS high;
Wait 51ms for /DRDY to go low; // At 20SPS data rate, nominally 50ms ±2% accuracy
Set CS low;
Send 10h; // RDATA command to read data for thermocouple
00h 00h; // Clock out 16 bits of data
Set CS high;
Convert thermocouple ADC data to voltage; // Cold junction compensation to determine
// thermocouple temperature
Convert ADC temp sensor data to temperature;
Convert ADC temp sensor data to thermoelectric voltage; // By lookup table or calculation
Add thermocouple voltage to temperature thermoelectric voltage;
Convert resulting voltage for thermocouple to temperature; // By lookup table or calculation
}
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Design Featured Devices
Device
ADS1120
Key Features
ADS1120 4-channel, 2-kSPS, low-power, 16-bit ADC
with integrated PGA and reference
Link
Other Possible Devices
http://www.ti.com/product/ADS1120
Link to similar devices
Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
Additional Resources
• Texas Instruments, ADS1120 Evaluation Module
• Texas Instruments, ADS1x20EVM User's Guide
• Texas Instruments, ADS1x20 Example Code
• Texas Instruments, A Basic Guide to Thermocouple Measurements
• Texas Instruments, A Basic Guide to RTD Measurements
For direct support from TI Engineers use the E2E community:
e2e.ti.com
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