34.10.4.3 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 ( R SAMPLE ) and a capacitor ( C SAMPLE ). In addition, the source resistance ( R SOURCE ) must be considered when calculating the required S/H time. The figure below shows the ADC input channel equivalent circuit.

Figure 34-5. 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 calculated 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