AD7928 need a big delay before the next channel to sampling to remove the charge of the capacitor from the earlier conversion??
Please have more details on your set up. What is the sampling rate? What is the value of the capacitor that you mentioned and what is the input voltage amplitude and frequency. There is a article that would explain details on the ADC front end, like how a capacitor affects the ADC and why choosing the right capacitor value is very important. You can refer to the link.
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