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Original question:

ADS122U04: ADS122U04 measurement Question

ADS122U04: Question regarding gain in ADC conversion equation

Jonathan Craig
Prodigy 110 points
Part Number: ADS122U04

Hello,

I have a question regarding ADC gain in the ADS122U04. In our system we are taking a single ended measurement using a gain of 2 and a vref of 2.048V. I am using the equation LSB = (2 x vref / gain) / 2^24,  and then multiplying this by the ADC code to get my result in volts. However, my result in volts appears to be the pre-gain voltage. For example, when i have 0.35V on the input to the ADC, my software reads the voltage as 0.35V not 0.7V. Should it be 0.7V or have i misunderstood the data sheet?

Furthermore, to achieve higher resolution, would using a 2.5V reference and a gain of 4 improve performance if my maximum voltage on the input is 0.6V?

Any help is appreciated.

Kind regards

  • +1
    TI__Guru** 113765 points

    Hi Jonathan,

    As the equation for the LSB takes into account the gain applied (thus reducing the full-scale range) the output code reflects the input voltage value.  So if you apply 0.35V, you will get the result code with respect to the input voltage which is 0.35V.

    As to changing the reference and using more gain you will increase the overall dynamic range but not necessarily improve performance.  Increasing gain does help improve input referred noise (see the noise tables 3 and 4 on page 18 of the datasheet ) but increasing gain also gains any external noise.  As the reference itself directly relates to the noise and stability of the measurement, the reference has to be equally as good or better than the internal reference.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 110 points

    Ok got it. Thanks for your help Bob.