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'[PIC]: Analog input clamp voltage'
2002\05\30@194528 by Tal (Zapta)

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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 ?

Thanks,

Tal

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2002\05\30@202619 by M. Adam Davis

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That clamping diode is meant only to protect the chip.  IMHO it should
not be used as a part of the circuit (clarity, changing chips, etc).

But I believe the current capability of the diode is part of the data
sheet for the pic, so you coud certianly take advantage of it with no
ill effects.  I don't know the current capability offhand, but I imagine
your 2mA would be ok.

-Adam

Tal (Zapta) wrote:

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2002\05\30@204613 by Tal (Zapta)

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Thanks,

The absolute maximal rating is +/-20ma.

Tal

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2002\05\30@211454 by Dwayne Reid

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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

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2002\05\31@070558 by Olin Lathrop

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> Is it OK to use the internal clamp diode of the analog input of 12F675 for
> over voltage protection ?

As long as you stay within the current limits stated in the spec sheet.


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2002\05\31@073136 by Olin Lathrop

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> The absolute maximal rating is +/-20ma.

Which means you want to stay well below that in operation.  A good way to
think of absolue maximum ratings during design is as "guaranteed to fail at"
ratings.

When I first read your statement, I was thinking you were off by an order of
magnitude and I was getting ready to send a "Yo, bonehead ..." message.  I
grabbed the 16F87x data sheet so I could site the exact page number, etc,
and found that you were right instead.  I'm pretty sure I remember MUCH
lower specs for the protection diodes from somewhere, so check the data
sheet for the particular PIC you are using.  I'm not going to look thru all
the other data sheets right now, but does anyone else remember lower specs
and remember where they applied?


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2002\05\31@074019 by Olin Lathrop

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> 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.

You are absolutely right.  I have encountered exactly what you describe.  I
had several analog inputs fed thru 10K resistors to a 16F876.  The other
ends of the 10K could go as high as 8V, causeing as much as 300uA thru the
protection diodes, which was well within specs.  The chip ran fine and
didn't get damaged, but all the other analog inputs converted to bogus
values when one of them was doing this.  I actually suspect that this had
more to do with messing up the internal Vref than causing any problem due to
the protection diodes conducting.  I split the 10K into two 5K resistors
with a diode to +5V in the middle.  That fixed the problem.


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2002\05\31@074859 by Alan B. Pearce

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>I split the 10K into two 5K resistors
>with a diode to +5V in the middle.
>That fixed the problem.

When doing this I like the idea of using a schotky diode to ensure the
internal diode does not conduct on the basis the diode inside the chip is an
ordinary silicon diode.

When doing this it may be possible to have the diode right at the pin
instead of having the second resistor.

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2002\05\31@075047 by Tal Dayan

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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 removed}

2002\05\31@082845 by Olin Lathrop

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> When doing this I like the idea of using a schotky diode to ensure the
> internal diode does not conduct on the basis the diode inside the chip is
an
> ordinary silicon diode.
>
> When doing this it may be possible to have the diode right at the pin
> instead of having the second resistor.

I agree with both points, but I didn't happen to have a schotky diode around
and this was not a commercial product.


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2002\05\31@124338 by Harold M Hallikainen

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       There's been a lot of discussion about use of the internal clamp diodes
on the list. Several people discourage it. I have used them with 16c74
and 18c452 with a transformer secondary (something like 12VRMS) through a
current limit resistor to the RB0/INT pin to detect zero cross of the AC
line. It has served me well.
       Reading further on the list, I now see you're doing this on an ANALOG
input. As pointed out by Dwayne Reid, this does seem to have an effect on
other analog inputs. I don't recall right now what the effect was, but I
think it was more of an analog multiplexor leakage problem than anything
to do with the clamp diodes. Putting too high a voltage on an analog
input does seem to mess up other analog inputs. I noticed this a couple
years ago, so I don't recall the details.

Harold


On Thu, 30 May 2002 16:42:34 -0700 "Tal (Zapta)" <RemoveMEtalspam_OUTspamKILLspamZAPTA.COM> writes:
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2002\05\31@135439 by Tal (Zapta)

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Thanks for the info.

Two questions if I may:

1. We are using only one analog input and even if there are other inputs in
the chip, we don't use them as analog inputs. Does it means that it is OK to
count on the internal diode ?

2. Is there a way to get from Microchip an official clarification for this
issue ?

Thanks,

Tal

> {Original Message removed}

2002\05\31@145603 by uter van ooijen & floortje hanneman

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> Is it OK to use the internal clamp diode of the analog input of 12F675 for
> over voltage protection ?

It will probably work when you try it, but expect some funny things when you
build such a unit in large series.

> That is, what happens if the input is connected through a reasonable
> resistor (e.g. 10K) to a voltage of let's say 20V ?

Then you are operating the chip outside its 'normal operating paranmeters',
which roughly means that you are in the gray area: all *might* be well. but
Microchip will not stand for such use.

Wouter van Ooijen
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2002\05\31@145611 by uter van ooijen & floortje hanneman

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> The absolute maximal rating is +/-20ma.

But note what 'absolute maxima' means: the chip is guaranteed to survive,
but not guaranteed to work 'normally' while you exceed the 'operation
parameters'. IIRC there is no current through the protection diodes
specifies in the 'operation parameters'.

Wouter van Ooijen
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2002\05\31@150529 by Tal (Zapta)

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Ok, got the message, ;-)

Will look for a cleaner solution.

Thanks,

Tal

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'[PIC]: Analog input clamp voltage'
2002\06\03@031014 by Russell McMahon
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> > Is it OK to use the internal clamp diode of the analog input of 12F675
for
> > over voltage protection ?
>
> As long as you stay within the current limits stated in the spec sheet.

I agree, subject to the caution that you should not expect your PIC to
*necessarily* function correctly while the clamp diodes are conducting. This
condition is covered by the "absolute maximum ratings" portion of the
specification and NOT by the normal operating conditions portion. While
normal operation MAY in fact occur this is not certain and you have no
come-back whatsoever if you PIC crashes or performs in any possible manner
whatsoever under these conditions.



           Russell McMahon

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2002\06\03@031632 by Russell McMahon

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> 2. Is there a way to get from Microchip an official clarification for this

It's already clarified in the spec sheet.

Non destruction is guaranteed up to the specified limit.
Correct operation is not guaranteed when you exceed the voltages specified
for normal operation (which are lower than the point where significant
internal diode conduction occurs). (About 0.3 volt outside supply rail
AFAIR.


       RM

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