# Impedance Calculation

1. ## 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.   Reply With Quote

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2. ## Re: Impedance Calculation

Drive by current source of known value.  Reply With Quote

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3. ## 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.  Reply With Quote

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4. ## 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

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  Reply With Quote

5. ## Re: Impedance Calculation

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.  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.  Reply With Quote

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6. ## 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  Reply With Quote

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