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PGA280 Souce Impedance

David Musoke
Intellectual 750 points

Other Parts Discussed in Thread: PGA280

Hi:

I plan to the PGA280 to read multiple(256) wide ranging voltages accross resistances of 1 ohm to 10Kohm (0.1mV to 5V). These would be at the inputs of several multiplexers. Will this create a problem for the PGA280? Does it expect a constant input impedance source? If so, then i'd need a buffer and which low noise/bias current amp would you recommend?

Thanks,

David

  • 0
    TI__Guru 55466 points

    Hello David,

    The PGA280 has high input impedance (>1GOhm typical).  The typical input bias current is+/-0.3nA at gain 1V/V and 0.8nA at gain128V/V.  Maximum input current is +/-2nA. The input offset current is +/-0.5nA.  Figure 22 shows the input bias current vs temperature. 

    Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Intellectual 750 points

    Luis:

    I presume then that no input buffer is needed due to the PGA280's  high input impedance.

    Thanks & Regards,

    David

  • 0 in reply to David Musoke
    TI__Guru 55466 points

    Hi David,

    I am assuming this is the application where you are measuring voltages accross resistors;  using a very accurate current excitation source...  Depending on the magnitude of the current source you are using; the input bias current will create a small error.  If for example you are using a 500uA of current to measure a voltage of 5V accross a 10kOhm resistor; 2nA of input bias current will contribute to a negligible  (0.0004%) error in the measurement; for the 1 Ohm case with 100uA to measure 0.1mV;  the error is 0.002%.  The error caused by the input bias current will depend on the ratio of the excitation current source and the input bias current. 

    Best Regards,

    Luis

     

     

  • 0 in reply to Luis Chioye
    Intellectual 750 points

    Yes Luis ...good memory and thanks for the answer!

    I assumed, with the large numbers of emails you receive, you'd have forgotten about my application by now. I will be using multiple ADCs in parallel (24-bit sigma delta types) that you had recommeded earlier to ease up on the bandwidth requirements so as to allow the PGA280 to have enough time to settle, within its 40us spec.

     I'm still shopping for an ultra -precision current source (0 to 5mA) to handle loads from 1 ohm to 10K. Do you have any suggestions on how to build one? (It has to be load indepedent with very high output impedance obviously like any true current source). We have the classic opamp+FET combination in our current design which doesn't measure up as its load dependent and not accurate enough for our specs.

     

    Regards,

    David