Inputs and Sample and Hold Acquisition Times

The analog voltage source must be able to charge the Sample-and-Hold (S/H) capacitor in the ADC to achieve maximum accuracy. Seen externally the ADC input consists of a resistor ( RSAMPLE ) and a capacitor ( CSAMPLE ). In addition, the source resistance ( RSOURCE ) must be considered when calculating the required S/H time. The figure below shows the ADC input channel equivalent circuit.

Figure 1. ADC Input

To achieve ‘n’ bits of accuracy, the CSAMPLE capacitor must be charged at least to a voltage of

VCSAMPLE≥VIN×1−2−n+1

The minimum sampling time tSAMPLEHOLD for a given RSOURCE can be calculated using this formula:

tSAMPLEHOLD≥RSAMPLE+RSOURCE×CSAMPLE×n+1×ln⁡2

For a 12 bits accuracy: tSAMPLEHOLD≥RSAMPLE+RSOURCE×CSAMPLE×9.02

where,

tSAMPLEHOLD=12×fADC