I have got a question about the AD7902 ADC.
How big is the input resistance of the analog inputs?
In the datasheet I found only a value of 400 Ohm, however is this right?
It seems to be a bit too little.
That resistor is that is referred to in the Analog Inputs section of the data sheet is a parasitic resistor that is formed by the input structure and switches of the SAR capacitor array. The input impedance of the circuit itself is much higher. When the device is acquiring, you can think of the ADC input as an RC filter in front of an amplifier (the comparator in the SAR system).
Remember that a SAR ADC presents a dynamic load to the signal source because during conversion, the analog input (including the resistor in question) are disconnected from the outside world, so the signal source would no longer see this RC. When the device returns to track mode (acquisition), the RC is again presented to the signal source.
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