I have an RC circuit to which I give sinusoids to recover the impedance. When I give the sinusoids alone, the impedance is recovered by dividing the FFT of voltage by FFT of current. But, when I superimpose the sinusoids over a DC value (large compared to the sine), the results are completely different. I have tried detrending the data and considering the FFT from the second point onwards separately, but none of them help. Why does this happen and how might I remove this effect?
Consult chapter 4 of 'Electrical Engineering-Principles and Applications' by Allan R. Hambley, an excellent book.
One possible reason why this happens is because the actual capacitance of a capacitor is very dependent on the DC bias, even if this DC bias is lower than the rated voltage. In other words: a capacitor of 1µF (nominal) rated for 16V may only be 0.5µF when a DC bias of 10V is applied.
This may all sound surprising, but it is for real and commercial hardware designs take this into account. The effect is more important in some capacitor technologies than others and capacitor manufacturers usually include a capacitance/DCbias curve in their data sheet.
An interesting read about htis topic:
http://www.maximintegrated.com/app-notes/index.mvp/id/5527
Data sheet of a random capacitor from MuRata includes this DC bias dependency:
http://psearch.murata.com/capacitor/product/GRM188R60J475KE19%23.html
Dear Laya Das, It is now low pass filter. Take care of the capacitor value and it will work.
yes, take care about the capacitor value or the sinusoid frequency.
the cutoff frequency of the filter fc=1/2.pi.R.C
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