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INA122: Common Mode Input Impedance

Richard Elmquist
Mastermind 45250 points
Part Number: INA122

I have a customer asking if we any information on the common-mode input impedance for the INA122. He asked specifically for the input impedance to GND, but I believe that he is looking for the common-mode impedance for this part. Do we have this information?

Thanks for your help with this!

Richard Elmquist

  • 0
    TI__Intellectual 1095 points

    Hi Richard,

    The information for the input impedance is included in the data sheet as shown below:  

    What this basically means is that if you change the input common voltage, Vcm, by 10V, the input bias current, IB, will change by 1nA; thus resulting in the common-mode input impedance of 10Gohm (10V/1nA).

  • 0 in reply to Miriam Mesek
    TI__Mastermind 45250 points

    Miriam,

    My customer was really asking what was the impedance between the input and GND. To me this seemed like a question of what was the common mode input impedance? Am I there is a difference and whether this is the proper correct that there are really two impedances when describing the input? Is the impedance specified above simply the impedance between the two inputs? I read a little about this and I just wanted to verify if my understanding was correct. Please let me know if my understanding is correct and if the answer above is the proper answer for the customer.

    Thanks for your help with this!

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    TI__Expert 5830 points
    Hello Richard,

    The impedance Miriam posted is the impedance between the input and GND. The input impedance according to the specification in the datasheet is 10Gohm in parallel with a capacitance of 3pF. Hope this helps!

    Best,

    Errol Leon
  • 0 in reply to Errol Leon6
    TI__Mastermind 45250 points

    Errol,

    Thanks for your response!

    I just wanted to verify this as I have read some application notes on this.

    Thanks for your definitive response!

    Have a great day.

    Richard Elmquist