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ADS7828: Measurement range when using external reference

Julia Kenel
Prodigy 10 points
Part Number: ADS7828

Hello,

We are using an external reference of 3.3V. In the datasheet, it is specified that the measurement range is from 0V-VDD when using an external reference. However, we do not read the maximum value of bits (4095) when connecting the input channel to 3.3V. We also do not read 0 when connecting the input to ground. Are we not understanding the datasheet correctly? What is the transfer function from bits to voltage?

Thanks,

Julia

  • 0
    TI__Guru**** 180981 points

    Hi Julia,

    Welcome to our e2e forum!  The ADS7828 has an input voltage range from 0-Vref.  If your external reference is 3.3V, that set's your upper limit.  The input selection is potentially part of your issue.  How do you have the COM input (pin 11) connected?  For single ended measurements, that pin is normally set to GND.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Hi Tom,

    COM port is connected to GND, and we do use single ended measurements. Could it be related to the Power-Down selection in the command? We tried with Power Down between A/D Converter Conversions (00) and Internal Reference OFF and A/D Converter ON (01).

    Best regards,

    Julia

  • 0 in reply to Julia Kenel
    TI__Guru**** 180981 points

    Hi Julia,

    The conversion result should not be impacted by the power-down selection.  Can you provide a schematic of your setup?  Maybe a list of expected conversion results vs. actual?