39.6.4 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 ($R_{\text{SAMPLE}}$) and a capacitor ($C_{\text{SAMPLE}}$). In addition, the source resistance ($R_{\text{SOURCE}}$) must be taken into account when calculating the required sample and hold time. The next figure shows the ADC input channel equivalent circuit.

To achieve n bits of accuracy, the $C_{\text{SAMPLE}}$ capacitor must be charged at least to a voltage of

The minimum sampling time $t_{\text{SAMPLEHOLD}}$ for a given $R_{\text{SOURCE}}$can be found using this formula:

for a 12 bits accuracy: $t_{\text{SAMPLEHOLD}}\ge \left({R_{\text{SAMPLE}}+R}_{\text{SOURCE}}\right)\times \left(C_{\text{SAMPLE}}\right)\times 9.02$