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ADS1112 scaling error

Tony Sander
Prodigy 10 points

I am experiencing a severe scaling error with this device. With the PGA set to x8 (256mV FS) I am getting the MSB at 01111111 with ~+182mV input. With the gain reduced to x4 (512mV FS) the MSB hits 01111111 at ~+285mV.

Do I have damaged device or is this due to the input circuit impedance?

  • 0
    TI__Guru* 97695 points

    Tony,

    I think to answer this question, I'd need a little bit more
    information. I'd want to make sure that the inputs are
    directly measureable with some sort of meter to
    confirm the voltage seen by the ADC. I'd also want
    to know more about the circuit that it's attached to.
    Is this circuit in an EVM or in an application already?

    Is this problem seen in G=1 or G=2?

    One way to make sure if the circuit is responding
    correctly, is to see that the input offset is correct.
    Short the inputs and make sure that the reading is
    consistent and less than 8mV at PGA=1. In different
    gains, the offset error gets progressively smaller.

    This doesn't look like a problem in input impedance,
    unless the source has a very high output impedance.

    If the error is in scaling, there might be a problem
    in the internal reference although this error looks
    pretty large for even an untrimmed reference.

    Joseph Wu