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Low Frequency Tone in the Spectrum

LUMSC

I am working on the design of 14 bits- 2nd order sigma delta ADC. I'm plotting the PSD using the spectrum assistant.

input signal 100nA ac with 200nA dc @ 1kHz
sampling frequency 1.6MHz
results plotted for 2^17 samples = 131072 samples 
strobe period set to 625ns
simulation time = strobe period * no. of samples = 82ms
window hanning
There is a tone appears at around 12 Hz. It appears at the beginning of the plot. What can be the reason for this tone?

  • The most likely explanation is that there's actually a 12.195Hz (or there abouts) tone in the signal you're looking at the spectrum. It shouldn't be a vestige of the windowing function itself from what I can see. It's very hard to debug without seeing the raw data, so this is probably something to contact customer support about.

    BTW, your strobeperiod doesn't actually line up with the DFT interval. If you have 131072 samples, and have set the start/stop time to 3m/85m that means you have a sample time of 85m/131072 which is 625.61035ns. This means you'll be getting interpolation error. So either set your stop time to 84.92m (probably the simplest) or use 131200 samples (since IC617 there's no longer a requirement to use a power of 2 for the number of samples).

    Andrew

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