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PGA281: gain resistor 0 ohms

[FAE] Andrew Wang
Genius 10685 points
Part Number: PGA281


Hi Team,

Customer met noise issue in their design, and found PGA281 gain changes during system operation, they want to know if it's ok to use 0 ohm for gain resistors?

Our EVM uses 100k pull down and 1k pull up, any special reason we choose these values?

Thanks!

Andrew

  • +1
    TI__Guru 55466 points

    HI Andrew,

    On the PGA281EVM Switch S1 allows the user to configure the PGA281 gain settings, where the user can pull down the PGA281 digital inputs with 100kOhm by setting the switch low, or pull-up with 1kOhm to set the digital pin high.  I suspect that the pull-down and pull-up resistors with values in the 100kOhm and 1kOhm value were selected to allow flexibility.  For example, on the PGA281EVM, if the user required to interactively control the gain with an external microcontroller, they could drive the digital inputs externally overriding the switch settlings without having to modify the PGA281EVM hardware.

    The PGA281 digital input terminals must not exceed the PGA281 digital supply range by more than 0.5V (or the digital input voltage can not be below -0.5V below ground); or similarly, the current through the inputs must not exceed 10mA under any fault condition.  For example, a fault condition that could exceed the PGA281 absolute maximum ratings could be driving the PGA281 digital inputs high while the PGA281 DVDD digital supply is still at 0-V.  If in the application, the gain is fixed, and the pull-up resistors are connected to the same PGA281 DVDD digital supply powering the PGA281, I see no issue reducing the pull-up and pull-down resistor to a smaller value of 1kohm, reducing any sensitivity to external noise while at the same time still limiting the current through the digital input pins during a fault condition.  

    Thank you and Regards,

     Luis