Analog Input Circuitry

The analog input circuitry is illustrated in Figure 1. An analog source applied to ADCn is subjected to the pin capacitance and input leakage of that pin (represented by IH and IL), regardless of whether that channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through the series resistance (combined resistance in the input path).

The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less. If such source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long the source needs to charge the S/H capacitor, which can vary substantially.

Figure 1. Analog Input Schematic