AMPLAB1:
How are these resistor values calculated?
Did it the quick way, so might not be that accurate (not that it matters a lot).
I assumed VCC is 5.0volt (which it rarely is).
I assumed pin protection can handle ~1mA in case of overload (not documented).
-
If I want to measure ~70volt AC (100volt peak, 200volt peak/peak),
then I pick 100k between amp and analogue pin (the 1mA limit). -
Then calculate the resistor to ground, assuming I want 5volt max on the analogue pin. 5k is about right.
That gives me 5% headroom (105volt peak+).
The resistor from pin to VCC is irrelevant, because there is 5volt on both sides (no current flowing through it). -
I calculate combined resistances (R pin<>amp and R pin<>ground),
so I have 2.5volt in the pin when the amp is idle (0volt, ground) if I use that calculated value from pin to VCC.
(100k * 5k) / (100k + 5k) = 4k76. 4k7 (E12 value) is close enought.
Leo..