I have a simple signal chain with a differential sinusoid as the input. This passes through the first amp which converts the signal to a single ended signal. The next block is a 4 pole low pass filter with an Fc~100kHz. After this LPF I have my ADC. My ADC is sampling at 500ksps.
(input signal 100kHz differential) -> (instrumentation amp) -> (LPF 100k Fc) -> (500ksps ADC)
I know I don't have a lot of oversampling (5 samples per period in this example), but I'm seeing 5Hz oscillations in the data and I'm trying to understand exactly what is going on since my LPF when measured with an oscilloscope seems to be working great. It seems with 5 samples per period there isn't enough data to re-create the signal very well but it is interesting that the frequency I'm seeing appear as an error is 5Hz which is my sample rate divided by my input signal frequency (500kHz/100kHz).
This is probably something obvious, but in reading about anti-aliasing all I see is problems due to under sampling and not having a LPF. Does anyone know what is going on here and what topic I should research to get a better handle on this?
Below is a picture of the datafile showing the high frequency sine wave riding on the 5Hz sinewave which I don't want. In this screenshot you can see the cursors marking an oscillation period of 200mS. This is shown in the software box labeled "Delta-X".