Why is input impedance not specified on the ADC1203x or ADC1213x?
The input is connected through the multiplexer to a switched capacitor ladder. For a steady DC input value, the input impedance is in the megohms. But a varying input must charge [or discharge] the input capacitive ladder and the value of the capacitance varies with the input voltage and the reference voltage. When the resistance of the input multiplexer is combined with the capacitance of the ladder, the resulting input is not a fixed value but an R-C that must be charged [or discharged]. A low source impedance such as an op amp can drive the R-C to its final value very quickly. But a high source impedance, typical of many transducers, will require more time. That is why the acquisition time is programmable. The source impedance issue is discussed in the data sheet Section 6.
