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ADS114S08: If Input Series Resistor Will Influence Gain

Yazhu Liu
Intellectual 1410 points
Part Number: ADS114S08

Hi Team,

My customer is testing ADS114S08, they're using 6.2kohm input series resistor and internal reference, schematic of input stage as below, the gain is set to 1.

During the test they observe that the actual gain is 1.02, is this gain offset caused by the input series resistor? If so, why would this happen? In ADS114S08 function block, the input stage is a MUX, and I assume the Rds_on of the MUX FET won't be large enough to cause this gain offset. Or it's caused by PGA input resistor? Would you please give some comments on that? Thanks.

Best Regards,

Livia

  • 0
    TI__Guru 52296 points

    Hi Livia,

    The gain error could be caused by a number of components. The ADC has a gain error as specified by the datasheet, as does the ADC's internal reference.

    The input resistors are a bit large, but the ADS114S08 has very low leakage so this should not cause a very large error. Also, this would show up as an offset error, not a gain error.

    Has the customer performed an offset and gain calibration to see if this improves the results? I also would have expected an RTD measurement to be performed ratiometrically, not using the ADC's internal reference.

    If you like, you can send me the full schematic and register settings so we can review further. Feel free to contact me offline for additional help.

    -Bryan

  • 0 in reply to Bryan Lizon86
    TI__Intellectual 1410 points

    Hi Bryan,

    I've sent the schematic and register settings to you through email.

    Thanks.

    Best Regards,

    Livia

  • 0 in reply to Yazhu Liu
    TI__Guru 52296 points

    Thanks Livia, we will resolve this offline then