So, what circuit and over voltage protection would you use for an A/D input
that measures signal in the range 0-20V where the signal can sometime go
beyond the dynamic range to let's say 40V ?

(12F675, Vdd = 3V)

Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dwayne Reid
> Sent: Thursday, May 30, 2002 6:13 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Analog input clamp voltage
>
>
> At 04:42 PM 5/30/02 -0700, Tal (Zapta) wrote:
> >Hello,
> >
> >Is it OK to use the internal clamp diode of the analog input of
> 12F675 for
> >over voltage protection ?
> >
> >That is, what happens if the input is connected through a reasonable
> >resistor (e.g. 10K) to a
> >voltage of let's say 20V ?
>
> Probably NOT.
>
> Try it and see what happens to the OTHER analog inputs.  I
> suspect that you
> will see that those other analog inputs get messed up pretty
> badly whenever
> ANY analog pin has a high enough voltage to allow current to flow through
> the PIC's substrate.
>
> This is definitely the case with the 16c71 and 16c73 chips.  I assume that
> all Microchip parts are affected by these substrate currents.
>
> I won't do this on any of my designs.
>
> dwayne
>
> --
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
>
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