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

Discussion in 'The Projects Forum' started by esm., Feb 24, 2012.

  1. esm.

    Thread Starter Member

    Jan 20, 2012
    30
    0
    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. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi esm,

    You will have to post a schematic in order to get some good advice.

    Regards,
    Ifixit
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Thermocouples are made by welding together two conductors of different metals. The weld point forms the junction and the measurement spot. As this is metal to metal, the impedance is very very low.

    Your amp has a very very high input impedance and is thus prone to all sorts of pick-up.

    So just drop a resistor between the Vin+ and Vin- inputs. Ideally you can calculate the value you can tolerate by looking at it as a simple voltage divider between the source R of the thermocouple and the resistor value. Practicably just put between 10 and 1,000 ohms there and try it out.

    Lower resistance is less noise but less accurate.
     
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