I'm building an 8-bit SAR analog to digital converter. I'm having trouble with the simple op-amp. The ADC has to be able to take up to 100kHz. The input signal is a 620mV P-P sine wave with an offset of 1.2V. This is the voltage swing of a thermistor that I'm going to use for the input when it's finished. At 100kHz the output signal looks clean with no noise. When I decrease the frequency down from 100kHz the noise will start to show up at the bottom of the sine wave at around 20kHz and increases as the frequency goes down. The source voltage is 5 volts. For the N-channel MOSFET's I'm using ALD212900's and for the P-channel MOSFET I'm using ALD1117. I can't figure out how to get rid of the noise shown in the pictures. I've tested all the gates for ringing and didn't find anything.
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