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PGA900: P gain pull-down resistors

BO
Genius 12760 points
Part Number: PGA900


Due to a firmware update, to our product that uses the PGA900, we have been doing some tests in the last few days. One test was sensor failure reaction. We have set the AFEDIAG_CFG register to 0x00, so there should be 4MΩ pull-down resistors on the VINPP and VINPN signals. So we would expect those signals to be close to 0V when disconnected. This is not what we measure however. When disconnected we measure ≈0,32V on VINPP. If we set AFEDIAG_CFG to 0x03 so the pull-down resistors are 1MΩ then we measure ≈0,15V on VINPP.

Do you have an idea why we do not measure 0V here?

Regards Bernd

  • 0
    TI__Mastermind 31655 points

    Hello Bernd,


    There is some non-zero bias current that could potentially flow from the PGAIN amplifier and cause a voltage across the pulldown resistor. The specified value is 5nA, however, which at 4Mohm would only be about 0.02V. This is a typical spec, so I don't know the upper limits, but to reach 0.32V at 4Mohm would require 80nA instead, and 0.15V at 1Mohm would be 150nA. I will have to look into what else could potentially cause this, but that's one source.

    Please have the customer set the PGAIN_OPEN bit in the AFE_CFG register to 0. This will open the connection between the PGAIN inputs and the VINPP and VINPN pins. That will remove any bias current, and then we can see if there is still voltage across the pulldown.


    Regards,