I'm characterizing the impedance of a system. To get the impedance I apply a sum of sinusoidal voltage waves that basically looks like noise. enter image description here

I recorded the output current from the system. As you can see there seems to be some time dependence on the output. I was wondering how to get the time and frequency dependence of the impedance. enter image description here

  • \$\begingroup\$ It’s pretty hard to measure impedance changes of a biological cell due to laser emission on brainwaves at such low frequencies and low capacitance, but if you inject a lower power than the absorbed optical power at much higher frequency then you can get a ceramic band pass filter to improve the impedance amplitude and phase measurement with a signal noise ratio much better than 20dB instead of 0 dB As shown. With impedance matching to 1k converted to whatever this might be called the z11 transform. Or the impedance measurement of return loss s11 \$\endgroup\$ Mar 5, 2021 at 9:00

1 Answer 1


Not enough time resolution to tell.

But it looks inductive, with noisy current sense and insufficient samples shown,

Try reducing noise with a filter and try an XY scope with the same bandwidth to get the dominant phase. Try DSO FFT.

Current starts with an envelope and reduces with high-frequency noise.

It's better to characterize with a Bode Analyzer or swept amplitude phase response with an output relative to input.

  • \$\begingroup\$ Hi thanks for your response, will this work if the impedance is changing in time? \$\endgroup\$
    – BPinto
    Mar 5, 2021 at 2:03
  • \$\begingroup\$ It depends on the frequency, impedance and time domain. Can you show your design specs , photo , measurements and the problem. ?. All I can say is it looks like 1k impedance with attenuation as frequency of noise increases. \$\endgroup\$ Mar 5, 2021 at 2:41
  • \$\begingroup\$ Looking at your previous question on laser induce capacitance changes in a membrane I would consider a touch sensor circuit or a high impedance oscillator that changes frequency with external capacitance tuned by laser injection. Photo diodes conduct more and increase capacitance when the bias current increases. cells may have similar behaviors. But probe inductance and cell pF must be given. (0.5 to 1nH/mm) \$\endgroup\$ Mar 5, 2021 at 2:47
  • \$\begingroup\$ In the end I just used different sine waves and applied the Hilbert transform to get the impedance at each frequency \$\endgroup\$
    – BPinto
    Mar 19, 2021 at 1:55
  • \$\begingroup\$ Can you pls. explain in answer how that works? \$\endgroup\$ Mar 19, 2021 at 2:18

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