Output impedance of voltage divider?

Question

When connecting a 10S LiPo (42V) to my ADC via a voltage divider, I began wondering how to calculate the output impedance of the divider.

^{simulate this circuit – Schematic created using CircuitLab}

For example, what is the output impedance of the above circuit?

(The ADC's specs recommend a signal impedance of 10kOhm or less, which I simply translated to 0.5mA @ Vcc=5V (worst case) and figured I'd be fine as long as the signal can source/sink more than 0.5mA. But that's not really 'calculating' the impedance...)

Answer 1

The output impedance is the equivalent of the two resistors in parallel.

If you are lazy, you can approximate this by taking just the lower-valued one, in this case the 5k.

Answer 2

Wouter has the right answer, here is the background why.

A voltage source in an ideal sense has zero output resistance. In terms of small signal analysis, this can be replaced by a Ground, grounds have zero resistance and in that case your drawing reverts to:

^{simulate this circuit – Schematic created using CircuitLab}