Hello all,

I thought I'd try to stimulate some more discussion on this as the=20
previous thread wandered and then died.  ;-)

After finding out that the datasheet says not to exceed Vdd - 1.5V=20
(3.5V) as a comparator input, my upper voltage tracking is working=20
perfectly (whithin 1mV).  Some experimentation shows that (this=20
particular) PIC will work well at up to 4.0V applied to the=20
comparator's input.  Anything over that, and the comparator starts=20
acting strange causing me to get ADC readings that vary by more than=20
one LSB.  By keeping it less than 4.0V I get consistant readings that=20
vary by only one LSB.

As far as feeding the comparator <300mV (even though the data sheet=20
seems to say that's fine), the comparator's output is quite simply=20
wrong.  :-O  300mV to the + input and 0V to the - input does *not*=20
result in a high comparator output, even though the + input is at a=20
higher voltage than the - input.  Increasing the voltage to above 300mV=20
results in the comparator correctly identifying the fact that 300mV is=20
larger than 0V and setting the output bit high.  The cap voltage then=20
immediately rises to match the pot voltage.

No offense to Roman, but he seemed to be getting it backwards.  ;-)  The=20
PIC output has no trouble charging the cap rail-to-rail (within a few=20
mV anyway).  The comparator *does* seem to be the problem when operated=20
within 300mV of ground or within 1V of Vdd.

Here is some ugly ascii art of the schematic.  Obviously it has been=20
simplified for my convenience, but it shows all the components of the=20
feedback loop.  The feedback loop/ADC works perfectly as long as you=20
don't push the comparator inputs near the rails.


+5V     |-------------
|       V            |            16F628
|____/\/\/\/\___     |         __________________
                |    |___RA2__| Comp +   Comp - |___RA1___
       10K      V             |                 |        |
               Gnd     __RA3__| Charge Pin      |        |
                       |      |                 |        |
                       |      |_________________|        |
                       |                                 |
                       |                _________________|
                       |                |
                       |____/\/\/\/\____|___|(______
                                                   |
                              4.7K         1uF     V
                                                  Gnd

Thanks for any advice anyone may have.

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