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Thermocouple & Cold Junction Compensation Measurements with AD74412R/AD74413R

Introduction

The AD74413R is a quad channel software configurable I/O IC solution that provides multiple analog and digital functions to a single pair of screw terminals, configured over a single SPI interface. The part offers functionality Voltage/Current outputs, Voltage/Current Inputs, Digital Inputs and RTD/ Thermocouple measurement capability. This note will focus on how a thermocouple can be measured along with a cold junction compensation measurement.
Refer to figure 1 for a high level block diagram of the required connections for thermocouple measurements.
The thermocouple should be connected between the I/OP and I/ON screw terminals. The AD74413R can be configured in Voltage Input mode, with the ADC range set to the +/-104mV range to measure in a small voltage range required for Thermocouple measurements.
The AD74413R can support cold junction compensation measurements by measuring the response from an external thermistor resistor divider using the onboard diagnostics. The LVIN pin can be used to measure the thermistor voltage within an ADC range of 2.5V. The resistor divider can be placed close to the terminal pins where the thermocouple is connected.
The temperature at the Thermocouple is calculated using a combination of the thermocouple and cold junction measurements.


Figure 1: AD7441xR configured for thermocouple measurements

Implementation

Figure 2 shows a snapshot from the AD74413R evaluation board. This circuit shows the implementation required to use the AD74413R to determine TCOLD_JUNC, the cold – junction temperature:

  • The reference to the AD74413R is applied to the top of a resistor divider. The resistor divider is made up of a 33kΩ resistor, R64 and a 33kΩ thermistor, RT1.
  • The resistance of the thermistor is proportional to the ambient temperature. The resultant voltage is measured from the tap-off point in the middle of the divider, which is connected to the LVIN pin. The LVIN pin can be measured by the ADC in a 2.5V range, which is set by the reference provided.
  • The resistance of the thermistor can be calculated from the measured ADC_CODE:

RRT1 = -(33000* ADC_CODE) / (ADC_CODE – 65535)

Configure one of the AD74413R channels in Voltage input mode with the ADC range set to +/-104.16mV. Connect the thermocouple at the I/OP & I/ON screw terminals. Calculate the thermocouple voltage using the following equation:

V_ADC=V_MIN+ ((ADC_CODE)/65535 ×Voltage Range)

Where:
Vmin is the minimum of the ADC range, -104.16mV
ADC_CODE is the code measured by the ADC
Voltage Range is 208.32 mV
The voltage measured by the ADC can then be combined with the cold junction temperature to calculate the hot junction temperature:

T_TC =V_ADC/S- T_(COLD_JUNC)

where:
TTC is the hot junction temperature
VADC is the voltage measured between the thermocouple terminals
TCOLD_JUNC is the cold junction temperature
S is the Seeback Coefficient, specific to the chosen thermocouple

Register Writes

The following is a set of register writes required to take thermocouple & cold junction temperature measurements.

  • Enable Voltage Input mode on Channel A and set ADC range to +/-104.16mV
    • Write 0x0003 to CH_FUNC_SETUP register (address: 0x01)
    • Write 0x0080to ADC_CONFIG register (address: 0x05)
  • Assign the LVIN pin to the Diagnostic 0 channel
    • Write 0x000E to the DIAG_ASSIGN register (address: 0x24)
  • Enable ADC for continuous conversions on Channel A and Diagnostic Channel 0.
    • Write 0x0211 to the ADC_CONV_CTRL register (address: 0x23) to enable continuous conversions on the ADC
  • Read ADC result (Thermocouple voltage) from ADC_RESULT register (address: 0x26)
    • Read the Diagnostic 0 result from the DIAG_RESULT register (address: 0x2A)



Open Wire Detect

To detect an open-wire on a connected thermocouple, the I/OP terminal can be connected to ground via an on-chip 200kΩ resistor. If the thermocouple lead is open, the terminal will instantly be pulled to 0V (or code midscale of the ADC) in the 104.16mV range. This ground connection can be used periodically to check that the screw is not floating. To enable the 200kΩ to ground, set bit 2 in the ADC_CONFIG register (address 0x05 for channel A).

resources/eval/user-guides/eval-ad7441x/tools/thermocouple.1624629051.txt.gz · Last modified: 25 Jun 2021 15:50 by Bríde Ní Riagáin

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