33.7.3.1 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 in order to achieve maximum accuracy. Seen externally the ADC input consists of a resistor ( R SAMPLE ) and a capacitor ( C SAMPLE ). In addition, the source resistance ( R SOURCE ) must be taken into account when calculating the required sample and hold time. The figure below shows the ADC input channel equivalent circuit.

Figure 33-3. ADC Input

To achieve n bits of accuracy, the C SAMPLE capacitor must be charged at least to a voltage of

V CSAMPLE V IN × 1 2 n + 1

The minimum sampling time t SAMPLEHOLD for a given R SOURCE can be found using this formula:

t SAMPLEHOLD R SAMPLE + R SOURCE × C SAMPLE × n + 1 × ln 2

for a 12 bits accuracy: t SAMPLEHOLD R SAMPLE + R SOURCE × C SAMPLE × 9.02

where

t SAMPLEHOLD = 1 2 × f ADC