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LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source)
Hans Polak:
Wim,
I don't know how you calculate the capacitance, but for a DUT it is unknown whether a capacitance is in series or in parallel.
But indeed, when using a 10nF+100R in series, FRA Imp Viewer is doing a proper job ?
The algorithm that I use calculates the capacitance of the DUT no matter where this cap is located and gives the right answer for a certain freq band.
See images below for 100R+10nF in parallel and 100R+10nF in series.
Hans
_Wim_:
--- Quote from: Hans Polak on May 13, 2021, 11:20:09 am ---I don't know how you calculate the capacitance, but for a DUT it is unknown whether a capacitance is in series or in parallel.
--- End quote ---
I use what I think is called the I-V method (see here: https://www.eevblog.com/forum/testgear/rlc-impedance-viewer-for-picoscope-bode-plot-data/msg2238570/#msg2238570)
If I compare the 100 ohm || 22 nF measurement with the result I get using the analog discovery in "series" capacitance mode (Cs), the results my app get are very similar. The analog discovery also has parallel capacitance mode, which gives your result. So I do think the way it is currently implemented is also a correct way, although there are probably several ways to do it.
My older HP LCR meter cannot test the 100 ohm + 22nF is series (gives no reading, I get only a result in parallel mode), so I cannot compare with that one.
_Wim_:
Result using my app (only series mode available)
Hans Polak:
Wim,
Thanks for explaining.
My point is that you never know whether a DUT has a serial or a parallel cap.
So making a choice between series or parallel can both be wrong when several caps at different places are part of the DUT.
Nevertheless I will have a look at your math, that seems a bit complicated.
After all, with channel A as reference and Channel B after series resistor R as the point to measure gain and phase, B=A*Z/(Z+R)
Assuming both channels are normalised to A=1, you get B=Z/(Z+R) or Z=R*B/(1-B)
B being a complex signal, whose Re and Im parts can be calculated with the measured phase, you get all the data you want.
So because we used different ways for calculation, I will try to find out at what point they differ, that will make the discussion easier.
Hans
Hans Polak:
I know what's wrong, and this has nothing to do with serial or parallel caps.
Let's assume Z = A+jB (so this already includes the serial resistor R)
Resr = Re(Z) = A
JwL= im(Z) = jB , so L=B/w
Impedance = sqrt(A^2+B^2)
So far that's what you did and that's all O.K
But now for the cap, where you used:
1/jwC = Im(Z) = jB, so wC=1/B and C=1/Bw
And that's wrong, let me explain
if 1/jwC=A+jB, then jwC = Im(1/(A+jB)) = Im (A-jB)/[(A+jB)*(A-jB)] = Im(A-jB)/(A^2+B^2) = jB/(A^2+B^2)
C = [B/(A^2+B^2)]/w
Quite a difference from the 1/B*w that you are using.
This the correct way to calculate C.
Hans
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