RE: [AVR-Chat] Re: ADC Vref
2007-12-14 by Tim Mitchell
Graham Davies wrote: > --- 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. > Ah yes. Good point. I remember now that the reason we did it was because we were using a low vref, but the voltage to be measured was under vcc. I'll get my coat... -- Tim Mitchell tim@sabretechnology.co.uk http://www.sabretechnology.co.uk Sabre Technology (Hull) Ltd, 3a Newlands Science Park, Hull HU6 7TQ Registered in England and Wales no.3131504 t:01482 801003 f:01482 801078