piclist 2002\09\28\062615a >
Thread: A/D vs. Counting Pulses - Design Question
face BY : Russell McMahon email (remove spam text)

> Amplify whatever's "left-over" after the filter and rail-to-rail (or
> even clip) the output from 0v to 5v with another op-amp circuit if need
> be and add clamping diodes (oh wait the PIC's inputs are clamped... just
> gotta watch the current, right?),

**** WRONG !!! ****

I'm afraid.
This starts a holy war every time it is raised and you can explore any
number of versions of this in the archives. So -

I'll just give you the absolute facts :-)
People will come along and say all sorts of contrary things but the
following is the good oil and if you depart from it you may verily have

- PIC clamp diodes are there to provide protection.
- PIC clamp diodes are not intended by the PIC designers to EVER conduct
during in-spec operation.
- The spec sheet doth verily give information about how much current said
clamp diodes will handle and about how high a pin may be taken outside the
supply rails but this data is in the section relating to absolute maximum
values and verily doth not relate to normal operation.
- Spec sheet also gives pin acceptable voltage range during normal opertaion
and this range is such that clamp diodes do not conduct.
- People with itching ears who want to save a cent or so and some PCB real
estate will read the former pin voltage ranges and apply them to the latter
situation and great shall be their sorrow sometimes.

- IF you allow the clamp diodes to conduct during normal operation the PIC
shall often behave still as intended. But sometimes it will be seen to
behave in most wierd and unpredictable ways. And, worst of all, it shall
sometimes APPEAR to be behaving normally but shall truly be behaving badly
and you may never know but your results may be trash and worth only
spreading on walkways in lieu of expended rock salt. Or worse.

- Problems that do occur when thou strayest from the true path in such
manner do not have to relate to the pin that offendeth or even to the
function of the processor related to the pin that offendeth.

- Even though the great Microchip their very selves doth on occasion allow
to be published official application notes containing circuits which allow
clamp diodes to conduct during normal operation such application notes must
not be taken as holy writ for in the day thou runnest after and emulatest
them thou shalt surely maybe die (or thy circuit or application in thy

Here endeth the lesson.

Note well how many abominators shall cry out against it in their ignorance.



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