When the ADC is operated with the inputs shorted, different output codes are obtained depending on whether the part is operated in buffered or unbuffered mode. Is this normal?
If an internal offset and gain calibration is performed when the buffer is selected or de-selected, the same code should be obtained from the ADC in both buffered and unbuffered mode within the noise limit of the part. If a calibration is not performed, it is quite possible to see a different output code in buffered and unbuffered mode for the same input voltage. The buffer is similar to an operational amplifier, therefore it will have an offset voltage. When the user switches between buffered and unbuffered mode, a calibration is essential to remove this offset.
It is important to remember that a calibration must be performed whenever the gain or the mode of the buffer is changed. The datasheet includes noise tables that list the expected noise and code spread for the various operating conditions.
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