As I am trying to learn basic circuit design I would like to build and simulate simple models to test my conceptual understanding. One of the first designs I am trying to tackle is to use an analog input pin on a microcontroller, but it is not clear to me how the pin itself should be modeled (and indeed thus how it behaves in a real circuit).

Can I think of an analog input pin as behaving like a multimeter set to read DC voltage, with the negative lead attached to ground? And does it, like a multimeter, have some large internal resistance?

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As a basic approximation, yes, the input is similar to a multimeter. If you want to take this further for high speed or high accuracy signals, you will find that most microcontroller ADCs use a Successive Approximation Register Architecture to read the voltage. These take some time to read the analog signal, so there needs to be a method to hold the signal on the pin.

A sample and hold circuit is normally used to take a sample of the signal. This involves connecting the input to a small capacitor (pF range I believe) via an op-amp buffer. The measurement is then taken against the voltage held on this capacitor. For a higher accuracy approximation, we can look at the input as the input to a op-amp, configured as a voltage follower. This will be a very high input resistance as it will be a gate of a mosfet, with some capacitance becoming more significant at higher frequencies

So as a simple approximation, a large resistance will be reasonably accurate. For a more complex model, a small gate capacitance with some series resistance from the switch, and a very small inductance from the package and wires connecting to it,

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Thanks for these links! I don't know if or when I would have independently figured out the need for a sample and hold circuit otherwise… –  Kaelin Colclasure Dec 15 '11 at 11:27
Just to clarify, you don't have to build one, there will be one inside the microcontroller. It's only if you want to model the pin that one needs to worry about it. Glad it was useful! –  D_Weight Dec 15 '11 at 15:51