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ADS122C04: Thermocouple INPUT at Power off

user5163865
Genius 3980 points
Part Number: ADS122C04

Hi all,

Our customers are concerned about:
Q: The thermocouple electromotive force is applied even when the ADC is power off.

Since the electromotive force of Thermocouple is very small, I think that there is no problem if it is within the absolute maximum rating, but is there any countermeasure for TI ADC (eg ADS122C04)?

Even if there is no countermeasure, if it is within +/- 300mV from the ABS MAX specification of the device, I think there is no problem. Is it correct?

Best regards,

Hiroshi

  • 0
    TI__Guru** 113765 points

    Hi User5163865,

    There are two important considerations with respect to an external sensor connected to the inputs of the ADS122C04.  The first one is what you have already mentioned with respect to the input voltage staying within +/- 300mV of the analog supply rails.  The second one is also an ABS MAX rating where the input current must be no more than 10mA.

    The most common TC types cannot generate voltages anywhere close to 300mV, so the TC itself should not be an issue.  However, the TC is made up of wires that can act as an antenna and can pick up voltage noise from EMI/RFI that can be much greater than 300mV.  These transients must be limited so that the 10mA input current is not exceeded.  Most commonly this requires a series resistance to act as a current limit.  The input resistance can act as both a current limit and when added with a capacitor can also act as an anti-aliasing filter.

    You may also require additional components to limit the voltage at the analog inputs by using a diode array or TVS diodes.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 3980 points

    Hi Bob B-san,

    Thank you for your support.

    Our customer also understood your explanation.
    Best regards,
    Hiroshi