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ADS1120 the value of Rf

Kuramochi Tadahiko
Guru 28715 points
Other Parts Discussed in Thread: ADS1120, ADS1147

Hi,

How much is the value of Rf of ADS1120?

My customer needs the value because Rf affects input offset.

Thanks and Regards,

Kuramochi

  • 0
    TI__Guru** 112935 points

    Hi Kuramochi-san,

    The diagram shown in the datasheet is simplified and the actual circuit is a little different.  As the gain resistors are well matched through process and input is chopper you should see very little offset due to the amplifier.  However, the modulator itself has offset.  This device does not have self offset calibration and must be accomplished by applying the internal short through the device mux and compensating the data result with this value in software.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Guru 28715 points

    Bob-san,

    Thank you for your reply.

    Is the calibrated modulator 's  offset larger than the amplifier's offset?

    Thanks and Regards,

    Kuramochi 

  • 0 in reply to Kuramochi Tadahiko
    TI__Guru** 112935 points

    Hi Kuramochi-san,

    This is a very difficult question to answer as we have no way of probing each section individually.  The input referred offset is stated by +/- 4uV (+/- one sigma) in the ADS1120 datasheet.  As I stated in my earlier answer, the ADS1120 has no self offset calibration to correct for device offset which differs from devices such as the ADS1147.  However offset correction can be done by applying an internal short within the mux (and external short could also be applied) then take a reading.  This value would then be used within firmware/software to make a correction for offset by subtracting the offset value from each subsequent measurement.

    Best regards,

    Bob B