Sampling error in Lpc1769's Adc
Top left graph looks like 10 kHz to me.

I think you may have your oscillator set incorrectly because what you think may be 75 Hz looks more like 25 kHz (top right picture).

I don't see 100 Hz anywhere. Bottom left (according to the time scale in milli seconds) looks like 50 kHz.

Are you sure you know what you are doing?