In Reply to: Noise problems with ADC (ADS7805) posted by Gregor on 11/07/02 at 8:59 PM:
Here's a few ideas of what might be wrong:
1) Is your input really constant? It might look constant on a digital voltmeter and still have noise which the ADC is seeing. This is because a digital voltmeter integrates the voltage for a relatively long time whereas the ADC samples for brief periods of time.
2) Is there a lot of noise on the reference inputs to the ADC?
3) Is the analog ground free from noise from the digital area of the circuit? There is an application note from TI about designing the circuit layout for low noise grounding of the ADC chip, plus other useful information at:
4) Are the VDD and VSS supplies reasonably free of noise? If the ADC is supplied from a switch-mode supply or regulator, the high-frequency noise could affect the results from the ADC chip.
Hope this helps you to find the problem.