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AD7980 noise


I am using the AD7980 with a 2.5V reference to capture data from a sensor. The sensor signal goes through some op amps then into the ADC. If I ground the input to the analog circuit that feeds the ADC, and then acquire 512 samples of data, I get values that vary by 5 ADC counts (so about 200uV of variation). Would this be typical or considered high? The company I am doing the circuit for has specified 2 counts of noise with no sensor input. I have been trying to see how I can reduce the noise, but it seems quite difficult. The Reference and ADC power supplies are low noise (ADP7102 and ADR4525). The PCB is laid out pretty well with ground planes. Does anyone have any thoughts of if its possible to reduce the noise and how I can figure out where its coming from?



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