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Electronics Explorer, Impedance, Nyquist Plot


Rick314

Question

I am trying to understand all the plots shown in the Waveforms Impedance measurement below. It was made on an Electronics Explorer board, AWG1 to a 1 kohm resistor to a 245 nF capacitor to GND.  Scope1 is on the AWG1 output and Scope2 is on the R-C connection. I am measuring a 245 nF capacitor by sweeping from 20 Hz to 20 kHz.  (The capacitor is actually four 0.1 uF ceramic caps in parallel that total a measured 245 nF.)  Notice on the Impedance plot at 20 Hz that Rs = 2.5 kohm and Xs = |Z| = 30 kohm (the two plots are on top of each other).  Notice on the Nyquist plot at 20 Hz that (Rs, Xs) is indicated as (-2.5k, -30k).

I think that since Xs is capacitive reactance it makes sense it has a negative sign.  But then shouldn't the Impedance plot say "|Xs|" (magnitude of Xs) as the plot label instead of "Xs" to indicate that the plot is always positive?

Shouldn't the Nyquist plot indicate Rs as +2.5 kohm (in quadrant 4) instead of -2.5 kohm (in quadrant 3)?

Why does the Inductance plot indicate negative hundreds of Henrys?

190324_Impedance2.png

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Hi @Rick314

I would recommend using the first circuit option, W1-C1-DUT-C2-R-GND
This model provides better results probably because the second scope probe impedance is in parallel with the reference resistor. In the second model the probe is parallel with DUT.

With logarithmic scale the absolute values can be represented. Use linear scale to see the sign.

Perform (at least open) compensation to have better results.

Here you can see results with 200nF from:
- blue: W1-C1-R-C2-DUT-GND  without compensation, similar to your results
- red: W1-C1-R-C2-DUT-GND compensated
- green: W1-C1-DUT-C2-R-GND without compensation
 

image.thumb.png.64657241f807796e90305d49abbdc668.png

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Thank you @attila but I am not trying to get better results or workarounds to problems.  I am trying to submit what appear to be WaveForms software defects. I have tried to re-word them more directly. Feel free to correct me if something I am saying here seems wrong.

  1. A user interface defect: When the negative sign is removed from a measured value for any reason, the plot label of the value should be "|Parameter|" (magnitude of Parameter), not just "Parameter". So in my example, the Impedance window button label should be "|Xs|", not "Xs". This same principle applies to all situations where negative values are plotted as positive.
  2. A plot error: At 20 Hz, the value of Rs is +2.5 kohm, but the Nyquist plot reports it as -2.5 kohm. The Nyquist plot is incorrect.
  3. A plot error: The Inductance plot shows large negative values such as "-200 Henries". Resistance, Inductance and Capacitance values should always be positive. If this isn't the case then it should be explained in the Impedance tool documentation what negative values for these model components means.
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Hi @Rick314

1. For logarithmic scale the labels will have absolute indicator.

2. In this analysis both Rs and Rs are negative. The Nyquist plot shows this accordingly.
This might be due to measurement inaccuracy, compensation or higher/lower reference resistor is needed.

image.png.7848dce3d61c150610acf4e8939230a9.png

image.png.8bd065c40dc588fdb6aa6f11867d544a.png

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