In the absence of Extra Sensory Perception which would allow me to know what your circuit looks like (particularly the digital output stages of the ADC), I can only suggest that the digital outputs of the ADC can generate glitches, due to either combinational logic, or some sort of timing problem? It could be lots of things...

You didn't even give any clues as to what type of ADC it is. If it's a flash converter, it may be caused by some glitches in the conversion of the thermometer code to a binary output? Almost certainly looking at various other nodes in the circuit and understanding the circuit would allow you to figure this out.

Regards,

Andrew.

It's still likely to be some glitches in the sub-range flash converter thermometer decode, any error correction block, or the output latches/logic. I'm referring to blocks in Fig 3 of http://www.commsdesign.com/main/9810/9810feat2.htm, because we still have virtually no information on your design in which to make an informed diagnosis.

I'm sure if you probed around your design, working back from the output, you ought to be able to find where the glitches are coming from. That's far more likely to be successful than expecting people on a forum to diagnose your problem with such a minimal amount of information to go on!

Best Regards (and good luck),

Andrew.