Capacitor directly connected to an analog input

[lanchon]

Posted on December 03, 2007 at 17:55

Capacitor directly connected to an analog input

[lanchon]

Posted on May 17, 2011 at 12:18

Hi all,

Say you have an analog filter that has a capacitor to ground on its output node. If you want to connect it to an STM32 analog input (they are not 5V tolerant) you need to add a series resistor to limit the injected current (when the STM32 turns off) to the per part/per pin limits on the datasheet.

But my question is, isn't there a practical capacitance limit below which the resistor is not necessary?

If you assume that the worst case power supply turn off time is near zero (can be shorted) then the current injection limit is not very helpful. I guess what is needed is a limit to the injected energy instead, but that's not specified.

So, anyone has a rule of thumb or anything?

Thanks.