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9th October 2019, 01:09 #1
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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.

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9th October 2019, 01:55 #2
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Re: Impedance Calculation
Drive by current source of known value.

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9th October 2019, 06:44 #3
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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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9th October 2019, 07:29 #4
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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 ZPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

9th October 2019, 17:34 #5
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Re: Impedance Calculation
Thanks for the posts everyone. A followup to what BradtheRad posted.
My followup 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.

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9th October 2019, 22:48 #6
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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.
KlausPlease donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.
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