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INA134: input differential impedance

justifice zhou
Intellectual 400 points
Part Number: INA134
Other Parts Discussed in Thread: OPA2192

Hi,

INA134 differential input impedance is 50kOhm stated in datasheet.

And in the blog below:

Overlooking the obvious: the input impedance of a difference amplifier - Precision Hub - Archives - TI E2E support forums  

John Caldwell's reply said: The differential input impedance can be done "by inspection" by remembering that there is a "vitual short" between the two op amp inputs, and therefore R1 and R3 are in series for differential input voltages, 25k + 25k = 50k.

If that's the case, then for a general OP amp, the differential input impedance will be 0 since it's virtual shorted between IN+ and IN-.

But in OPA2192's datasheet, the differential input impedance is 100Mohm.

It makes me confused about the input impedance of an amp.

  • +1
    TI__Guru 63580 points

    There is a world of difference between operation of difference amplifier and operational amplifier and for this reason they may NOT be measured the same way.

    In the case of difference amplifier, the differential voltage between the input stays close to zero (at virtual short, see VM1) regardless of input voltage, VG1, and thus the circuit operates linearly - see top schematic.

    For this reason the input resistance could be measured by determining slope of the line (Rin = deltaVin/delta_AM1).

    However, in case of an op amp, applying the voltage directly between the inputs will result in the output slamming against its rail and thus a non-linear operation - see the lower  schematic. For this reason the only way to measure the input resistance would be to determine the slope of the line in red circle where VF2 is away from its rails - doing so you would find out that it is a strong function of the open-loop gain, AOL.

    Diif Amp vs Op Amp Input resistance.TSC

  • 0 in reply to Marek Lis
    Intellectual 400 points

    Marek,

    thank you very much for the clarification.

    Then can I say that for a general Op Amp, the differential input impedance is not a useful parameter since it seems that it will not influence anything in a schematic?

  • +1 in reply to justifice zhou
    TI__Guru 63580 points

    Yes, that is correct - in the case of op amp its differential input resistance is inconsequential because of the virtual short between the input terminals. However, when it comes to the op amp common-mode input resistance, it may play a role and can be calculated from the change of IB with change of the input common-mode voltage, Vcm - see below.