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Very High Input Impedance Buffer Amp

Arvo Palm-Leis
Prodigy 80 points
Other Parts Discussed in Thread: OPA277, INA116

Hi!

I'm trying to build a buffer amplifier with input impedance ~100 MΩ.  Using the BB OPA277 with ~1MΩ the output drifts about 0.5V.  At 100 MΩ it's almost to the V+ rail.  How can I get the input impedance up?

Thanks!

  • 0
    TI__Guru 67601 points

    Arvo,

    I am not sure from your description how exactly your configuration looks like - is it simply a follower with 100M resistor in series with the positive input terminal?  What do you mean by drift - it is not a temperature drift you have in mind, is it?  Do you mean a voltage drop due to the input bias current passing through 100M input resistor?

    The maximum input bias current of OPA277 is 4nA (see below) thus you should only see 4mV with 1M input resistor, or 400mV with 100M, unless the your ambient temperature is above 85C or the circuit configuration is unstable - check the output with a scope for any sign of oscillation.  Do you factor in the OPA277 finite input impedance?  Do you use decoupling capacitors (0.1uF) on the supply pins?

    If you need more help, please send your complete schematic including all the values of external components and supply voltages so we may evaluate it not only from a DC biasing but also from a stability point of view.

     

     

     

  • 0 in reply to Marek Lis
    Prodigy 80 points

    Thanks for the reply!  Now I know what input bias current does.  Looks like I should be using a INA116.

  • 0 in reply to Arvo Palm-Leis
    Prodigy 120 points

    Hi Arvo,

     

    Can you please share your experience with this design. I also need min. 20 Mohm input impedance for AI range 0-10V, -10~+10V DC.

    Thanks,

     

  • 0
    Prodigy 100 points

    Check the OPA129, it is dielectrically isolated, and in buffer mode can actively cancel it's input capacitance within the amplifier's bandwidth. The IB on this only 100fA too.