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Analog differential input - how to make it measure 0V when floating

Discussion in 'General Electronics Discussion' started by esm., Feb 24, 2012.

  1. esm.

    esm.

    26
    0
    Feb 19, 2012
    Hello.
    I am using an external ADC (MCP3421), 16-bit resolution, with internal PGA (internal gain) of 8x to read signals from a thermocouple type T. Within my current ADC configuration, the differential input impedance is around 280K.
    But when the inputs are floating, the result of ADC conversions behaves incorrectly (negative and positive aleatory values, sometimes with high values are read from ADC).

    I want this ADC to output conversion values near to zero when the inputs are floating, to detect a sensor fault, or sensor disconnected, for example.

    How could I do a "pull down" to it? because the Vin- input (reference) and Vin+ input (input signal) from the ADC and and the digital ground of the IC are not in the same voltage reference.
     
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Since the source resistance of the thermocouple is likely very low (isn't it just two wires bonded together?), couldn't a 10K resistor directly across the PGA inputs with their 280K input impedance ensure a zero differential input when the TC is disconnected? I don't know what the resistance of your TC is but try to make the shunt resistor high enough that it does not affect the calibration curve too much.
     
  3. esm.

    esm.

    26
    0
    Feb 19, 2012
    I have analog switches between a lot of thermocouples and only one 18bit ADC.
    I can read them correctly currently.

    Suposing that one of these thermocouples get disconnected from the PCB or have its wires broken, I would be able to detect that, but the ADC input voltages cannot swing, it should be stable and near to 0V.

    I have 15 thermocouples (using 16 wires). One wire is the reference for all of them. And the other 15 are the sensors (positive wire). I am reading all the 15 thermocouples sequentially using an 16:1 analog switch (74HC4067) and just one ADC
     
  4. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    So you will pick the value of the shunt resistor not on the value of the thermocouple source resistance but rather based on the channel resistance of the analog switch.
     
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