AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Tim Mitchell" <tim@...> wrote:
 
> Why couldn't you use a N-channel
> fet at the bottom end of the
> voltage divider, with fairly
> high resistors for the divider?

The microcontroller analog inputs are now permanently connected to the 
supplies to be measured via the upper resistors in the divider.  When 
your N-channel FET is off, that's their only connection.  If the inputs 
have protection diodes, current will flow, so we don't achieve our 
objective.  If the inputs don't have protection diodes, then they will 
rise to the supply voltage, which is above their maximum rating.

Graham.