Highest recommended analog input impedance

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Xav

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Hello,

I am wondering what the highest recommended input impedance is when reading with the analog pins. I am planning to measure the voltage from 4xAA batteries with two 10K and one 2.2K to divide the voltage from 6V down to 3.3V. I'd like to scale up the resistors to say two 100K and one 22K (to reduce power loss), but I'm not sure the additional impedance wouldn't mess with the analog read.

And then if the impedance is too high- could I just cheat by adding a delay for the analog caps to charge before reading?

Thanks,

Xav
 
Using a storage cap is the normal trick here for steady state levels like batteries. Don't know of any math that would allow prediction of the needed cap size, beyond some experimentation with a bench supply and some test resistors.
 
I think I'm missing something- could you please explain how the cap would help me out here?
 
The need for a relativly low input impeadance is that the ADC will draw current when it does it's sample and hold phase before actually measuring the voltage. if impeadance is too high then the voltage will fall as the sample and hold circuit charges and the resulting read will be low (by a consistent amount you could quantify if you wanted to). If you have a small electrolytic on the input it holds charge. Your ADC starts a read, draws charge from the cap rather than through the resistors and gets a pretty acurate read. The cap then very slowly charges in the waiting period to the next read. As long as you don't pull charge out of the cap faster than the resistors on your sample circuit can feed it in everything works.

So:
storage cap has to be large enough to fully charge the sample and hold circuit without dropping much, going bigger doesn't harm beyond larger caps often having higher leakage (passive drain on battery) and taking longer to full charge.
You can't push the battery monitor resistors higher than where they can charge the cap between reads as per RC math
https://en.wikipedia.org/wiki/RC_circuit
 
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