A schottkey to ground with the cathode on the signal.  This will limit
negative voltages to 0.2V.  I've used 1N5817s to good effect.  Your
source is such low impedance that the diode leakage shouldn't affect the
signal.  I'd add an RC network for this circuit.

John Dammeyer


Wireless CAN with the CANRF module.
http://www.autoartisans.com/documents/canrf_prod_announcement.pdf
Automation Artisans Inc.
Ph. 1 250 544 4950


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jinx
> Sent: Monday, July 08, 2002 9:43 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: Analogue pin protection
>
>
> I've got this signal
>
> http://home.clear.net.nz/pages/joecolquitt/lv_an.html
>
> going into AN0 of a 16F877. It works but the signal must
> have some negative-going components (or possibly a
> switch glitch) as the port occassionally latches. How can
> I best protect the pin against signals going below Vss-0.6V
>
> According to 20.11 of the Mid-range Manual
>
> "Since the analogue pins are connected to a digital output,
> they have reverse biased doides to Vdd and Vss. The
> analogue input therefore must be between Vdd and Vss. If
> the input voltage deviates from this range by more than 0.6V
> in either direction, one of the diodes is forward biased and
> latch-up may occur"
>
> 'kin oath it occurs, mate, 'kin oath
>
> --
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>
>
>

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