Hi -

I'm new to the PICLIST too, so thanks in advance for the help.

I am building a programmable high voltage power supply. It charges a
capacitor bank that discharges through flash tube (pretty ordinary stuff).
What is a little different is that the output must be confined to
(logarithmically) equal discrete steps. It works by sensing the voltage
output (through a divider) from the power supply compared to a setpoint
(from a look-up table) and altering the duty cycle of the power supply to
meet the setpoint (hexfet type of approach).  There are only 12 steps, and
the accuracy and frequency requirements are such that the internal 8 bit A/D
is adequate to the task of sensing the divider voltage.

Everything works just fine, but here's my problem that I thought would be
simple. The A/D supposedly has two ways of taking a reference voltage
depending on how the register ADCON1 is set. The Vref can either be taken
from Vdd, or taken from whatever external voltage is applied to RA3.
Naturally, I would like to apply a nice, stable Vref rather than taking Vdd
which may (and does) droop internally. Unfortunately, setting the Vref
either way seems to make little, if any difference.

I am using a PIC16C74A. I have checked and re-checked my settings. They are
as follows: RA0 - RA5 tristated (TRISA set), RA0, RA1, RA2, RA4, RA5 set to
analog input, RA3 set to Vref (not Vdd but is seems to make little
difference) via ADCON1, and A/D input channel set to RA0 via ADCON0. I am
addressing the correct bank when setting these registers. The chip is
operating at 20MHz, the A/D clock is set to 32Tosc (1.6uS) . The charging
time is regulated by a software timer and is well above the minimum
recommended. The analog input is buffered by an op-amp so it has a nice, low
impedance.  The Vref is buffered too. Conversion is paced by polling the A/D
complete flag.

To summarize with an example, with Vref set internally to Vdd (5V in my
case) one would expect an A/D response of about128/255 on an input of 2.5V,
and 255/255 for the same input but with Vref selected and 2.5V applied.
Doesn't happen. This is driving me nuts!

Thanks  Paul Howell

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