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INA126: INA126

Murugavel S
Prodigy 80 points
Part Number: INA126

Tool/software:

Hello TI team,

We have developed two different circuits using INA126 to amplify the thermocouple input signals. One circuit has a cold junction(cCrcuit-B) and the other one has no cold junction(Circuit-A). I have the below queries on the circuits.

CIRCUIT-A

CIRCUIT-B

1. In the circuit-A the VREF is connected to the ground through a 10K resistor. can you please let me know, what is the functionality of the resistor?

2. The circuit-B has cold compensation. Is it mandatory to add the cold junction compensation? Because the Circuit-A board has already reached the production stage. Also, I need to understand that cold compensation is required to mount.

3. The functionality of the circuit is to measure the heater feedback temperature. Regarding this circuit, the microcontroller will make a decision, whether the heater should be turned ON or Not.

  • 0
    TI__Mastermind 29936 points

    Hello Murugavel,

    The 10kOhm resistor on your reference pin is adding significant gain error to your signal. Perhaps it was added to reduce the possibility of current flowing into the device during ground bounce, however, this input pin already has a 40k series resistor. This node should be driven by a low impedance (like GND).

    Just to make sure that I understand your system, you are using a thermistor to generate your cold junction reference point?

    Are you able to include more of your schematic for what is connected to the inputs of the INA126?

    The thermocouple will give you a relative voltage measurement, but does not provide a good absolute voltage/temperature relationship. The cold temp compensation is present to provide a reference point to your ADC for where "25C" or whatever reference temperature you are targeting is. The difference between the reference voltage and the voltage differential across your thermocouple will help determine the absolute temperature you are measuring for your thermocouple.

    Best,
    Gerasimos

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 80 points

    Hello Gerasimos,

    Yes. We are using the cold junction as a reference point. The input to the Instrumentation amplifier is a thermocouple.

    Can you please explain,  can't we meet accuracy without a cold junction?. since we don't have a place-holder to connect the temperature sensor 

  • 0 in reply to Murugavel S
    Prodigy 80 points

    since we don't have a place-holder to connect the temperature sensor(cold junction reference) in circuit-A schematic.

  • 0 in reply to Murugavel S
    TI__Mastermind 29936 points

    Murugavel,

    From my understanding, the determinate portion of the thermocouple behavior is the relationship between temperature and change of voltage, however the starting voltage is undetermined. Therefore, you will be able to know your change in degrees, but not your exact temperature. Exact temperature can be found by setting the starting temperature to be the "reference" point, but will only be accurate if the facility where you are calibrating your voltage has a tightly controlled, known temperature.Otherwise, you must measure a device with a known absolute temperature measurement, such as a thermistor, RTD, IC temperature sensor or other method. The below screenshot is from a thermistor datasheet, and a specification is provided that shows that the thermistor will have an absolute resistance value at an absolute temperature. In this case, if the resistor is measured to be 10kOhms, you know (within one percent or so) that the temperature is 25C. This sets your reference point

    No such specification exists for thermocouples, in their datasheet, they do not specify what the DC value is of the thermocouple when the sensor is at 25C, so you must subtract out the DC portion and only measure the delta.

    There are some reference circuits available on ti.com that can help with the generation of your circuit.

    Example reference design using an RTD: https://www.tij.co.jp/jp/lit/ug/tiduba5/tiduba5.pdf

    An overview of thermocouple measurement: https://www.ti.com/lit/an/sbaa274a/sbaa274a.pdf

    An example design with cold junction compensation: https://www.ti.com/lit/an/sloa204/sloa204.pdf

    Best,
    Gerasimos