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ADS122C04: ADS122C04 single ended mode

Ken Stutzman
Prodigy 30 points
Part Number: ADS122C04

The ADS122C04 data sheet says to bypass the PGA in single ended mode.  In our single ended photodiode application, enabling the PGA seems to reduce noise (possibly due to the internal chopper amp?).  What are the consequences of using the PGA in single ended mode?  Ken

  • 0
    TI__Guru** 112725 points

    Hi Ken,

    Actually the ADC noise is slightly better using the PGA disabled.  Compare the noise tables in the ADS122C04 datasheet.  I suspect that there may be something more related to the input impedance and charge distribution from the switched cap input.  You may be able to correct some of the effect by altering the filter components and increasing the input cap value. 

    When the PGA is enabled there becomes an issue when AVSS and the input is at AGND.  The op amp cannot drive all the way to the supply rails, so AGND is outside of the input range when using a unipolar supply configuration.  You will see a non-linear effect and the error can change over temperature.

    If you really need to use the PGA, then I would suggest using bipolar supplies for the analog supply.  This mode of operation would also require you to use one of the ADS122C04 inputs with a connection to AGND as you would not be able to use the internal mux connection for the AINN input connected to AVSS.

    Best regards,

    Bob B