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The AD74413R is a quad channel software configurable I/O IC that provides multiple analog and digital functions to a single pair of screw terminals. The AD74413R is configured over a single SPI interface. The part offers Voltage/Current outputs, Voltage/Current Inputs, Digital Inputs and RTD/Thermocouple measurement capability. This Wiki page 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 generates a voltage that is proportional to the temperature difference between the measurement point and where the thermocouple is connected at the screw terminals, the cold junction. To be able to determine the temperature at the measurement point, the cold junction temperature must be measured along with the voltage generated by the thermocouple.
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 ±104.16mV to measure the small voltage generated by the 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.
Two measurement steps are taken to determine the temperature of the measurement point.
First, the cold junction temperature is measured to establish a reference point for the thermocouple. 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:
RRT1 = -(33000* ADC_CODE) / (ADC_CODE – 65535)
Figure 2: LVIN schematic configuration
The voltage generated by the thermocouple must also be measured.
Configure one of the AD74413R channels in Voltage input mode with the ADC range set to ±104.16mV. Connect the thermocouple to the I/OP & I/ON screw terminals. Calculate the thermocouple voltage using the following equation:
VADC=VMIN + ((ADC_CODE)/65535 × VoltageRange)
Vmin is the minimum of the ADC range, -104.16mV
ADC_CODE is the code measured by the ADC
VoltageRange 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:
TTC =(VADC/S)- TCOLD_JUNC
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
The following is a set of register writes required to take thermocouple & cold junction temperature measurements.
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).