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Very unpredictable results with -AD volts

 
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Eugeneo



Joined: 30 Aug 2005
Posts: 155
Location: Calgary, AB

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Very unpredictable results with -AD volts
PostPosted: Tue Sep 06, 2005 1:40 am     Reply with quote
This was so strange I have to share it with the rest of you. Just take a 16FXXX series processor setup an A/D input pin in 10 bit mode. Then put a small negative voltage with respect to ground. The whole chip starts interrupting and crashing.

I always thought there were protection schottky diodes to ground and VCC.
languer



Joined: 09 Jan 2004
Posts: 144
Location: USA

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PostPosted: Tue Sep 06, 2005 2:09 am     Reply with quote
There are, but you are probably going beyond the maximum ratings. Just out of curiosity, what is the series resistance between this negative supply an the PIC input? Twisted Evil
Ttelmah
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PostPosted: Tue Sep 06, 2005 2:33 am     Reply with quote
Yes.
The clamp diodes are only 'specified' for voltages above Vdd, not below Vss. The 'input low voltage', is specified as Vss _minimum_. There is also the seperate danger of CMOS latch-up. Though modern gates are designed to make this unlikely, if a -ve voltage is present on the input, before power is applied to the chip, the potential for this does exist...

Best Wishes
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