The data sheet mentions that large external capacitors between the input and ground can affect measurement accuracy in unbuffered mode. Can you explain why this is the case? Does it only affect ac input signals, or are dc input signals affected too?
If you use the ADC in unbuffered mode, large RC constants on the input can interact with the internal sampling capacitor of the sigma delta ADC, and effectively starve the sampling capacitor of charging current. This will cause gain errors in the ADC. The solution is either to use buffered mode or ensure that you respect the maximum RC values given in the datasheet.
The reference inputs are also unbuffered so, as with using the analog inputs in unbuffered mode, the RC loading on the reference inputs must be sufficiently low to avoid introducing errors into the conversion process.
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