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    Impedance Calculation

    In the attached image I am showing a voltage divider with the values of 50M (fixed) , 4.7k (fixed) and an unknown impedance Z.

    Is it possible to calculate the value Z only using Vin, 50M and the 4.7k without using Vout directly?

    I am looking for a simplified equation for Z that doesn't explicitly need Vout.

    For this same application I saw a formula that didn't include Vout here so I was curious on the details.

    Thank you for your thoughts.

    Click image for larger version. 

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    Re: Impedance Calculation

    Drive by current source of known value.



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    Re: Impedance Calculation

    Vout is a percentage of Vin. The formula is:

    (4700 + Z)
    divided by
    (50M + 4700 + Z)

    To find Z, make the above formula equal to your desired percentage, then solve for Z algebraically.



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    Re: Impedance Calculation

    Hi,

    Using Brad's formula works for pure ohmic values.

    But for true impedances (and this is how I understand your question) you need graphic (vector) solution.
    I assume even with the value of Vout it is not possible to calculate Z as an unknown impedance.
    (Combination of R L C, or R C, or R L, or L C)

    Klaus

    Added:
    Maybe with:
    * Vin, sinewave, known amplitude, known frequency
    * Vout, amplitude, phase shift
    * and the knwledge it is no R LC impedance
    ...you may be able to calculate Z
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    Re: Impedance Calculation

    Thanks for the posts everyone. A follow-up to what BradtheRad posted.

    My follow-up question is that if Vout is a percentage of Vin how do I know what this percentage is? Do you mean the transfer function, i.e. Vout / Vin ?


    For example, if I capture some data for Vin and Vout should the ratio of Vout/Vin always been the same?

    This is a simple case but I was not make sure I am following you.

    Click image for larger version. 

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    Click image for larger version. 

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    In this case the ratio would be Vout/Vin that is ~0.091


    If one of the resistors was changed I should be able to solve for this because I know the ratio and the other resistor value. Right?

    Thanks.



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    Re: Impedance Calculation

    Hi,

    Now it's a completely different situation:
    * only two resistors
    * no complex impedance

    You now have a simple resistive voltage divider.
    The formula can be found million times in the internet.

    Klaus
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