Switched Capacitor

The analog input voltage VAIN and the source resistance RS are shown on the left side of the diagram. It is recommended that the source resistance be no greater than 2.5K. The analog signal is applied to the pin labeled ANx. There is a small capacitance (5pF) and a leakage current to the ground of approximately 500nA. RIC is the interconnect resistance, which has a value of less than 1K. The sampling process is shown with switch SS having a resistance RSS whose value depends on the voltage as shown in the small graph at the bottom of Figure 2.45. The value of RSS is approximately 7K at 5V supply voltage. Figure 2.45: Analog input model of the A/D converter The A/D converter is based on a switched capacitor principle, and capacitor CHOLD shown in Figure 2.45 must be charged fully before the start of a conversion. This is a 120pF capacitor which is disconnected from the input pin once the conversion is started. The acquisition time can be calculated by using Equation (2.7), provided by Microchip Inc: TACQP= Amplifier settling timeP+ Holding capacitor charging time + temperature coefficientPPP (2.7) The amplifier settling time is specified as a fixed 2ls ...»