White noise is being passed through a voltage follower, then filtered through a simple first order low pass RC circuit (C=0.5 micro Farad, varying resistance between 2k-10k, ohm depending on RC value tested). This is then recorded using an A2D device. I am then calculating the power spectra using the following python code applied to the acquired raw data:
def FFTF2(raw_data): result = abs(np.fft.fft(raw_data)*np.conj(np.fft.fft(raw_data))) return result[0:(len(result)/2)]
The result is the following:
This is at an RC of 1ms. As you can see the very low frequencies are missing power. Its as if the signal is going through a bandpass filter. The effect is not as noticeable at higher RC values.
This is the unfiltered noise:
If I use a different op amp in the voltage follower circuit I get the following at different RC values:
RC of 5ms:
RC of 2.5ms:
RC of 1ms:
What could be causing this "cutting" at the lowest frequencies? Does the power spectra of this filtered noise look "ok", or should the decay be smoother?