The PGA is connected between the analog input pin and the ADC, with an input impedance depending on the selected gain setting. Refer to the Electrical Characteristics section for details on the input characteristics of the PGA. The equations above are the same for calculating the appropriate sample duration when using the PGA impedance values.
When the PGA is used, it is continuously sampling and will only be in the Hold state when the ADC is sampling the PGA. If the time between conversions is longer than the needed sampling time, this can be utilized to get a shorter total conversion time by setting the SAMPDUR to the minimum supported value.