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What is the input impedance of PSoC® 4 SAR ADC? Is it 2.2K Ohms?
Figure 1. SAR ADC Block Diagram
The value of 2.2 K Ohms mentioned in the PSoC 4 device datasheet is the resistance between the output pin and the sampling capacitor (RSW2+RSW1 as shown in Figure 1). This value does not represent the actual input impedance of the ADC. The actual impedance will be higher than 2.2K Ohms. Regular bursts of charge will go into each ADC’s inputs on each sampling. At low frequencies, the sampling effect will be equivalent to an additional resistance in the input path.
An approximate value of this resistance can be found using 1/(sample rate * 10 pF) .