At 12:43 PM 7/18/97 -0700, you wrote:
>I want to use an external high prec. Vref with the 16C72.
>Microchip refers all specifications to Vref=5V.
>Since this will be my supply voltage I will have to use a smaller Vref(e.g.
>3.3V).
>How low can I go before I have a 7Bit ADC instead of an 8Bit?
>Has anyone experience or knows where or how I can get information on this?
>
>Thanks,
>     Franky X.
>-----------------------------------------------
>Man is always a victim of his own truth - Camus
>
Franky..

I have a product based upon the '73A that should be almost identical for
this comparison.  I accidentally used a 2.56V reference, and based
everything on it, and then later found out that the chip's REF has a lower
limit of 3.0V, per the book.  We had to solve a problem that MAY be related
to our low reference, that of noise.  This device reads a thermocouple
(amplified) and the ambient temperature on the terminal block.  These
seperate readings are then added.  When you get +-1 bit on each channel,
the result is +-2 bits of drift.  To make matters worse, it comes and goes
at will.  We improved matters dramatically by laying out the grounds in a
very careful manner, and using sleep mode to quiet things down.  Our
reading now bobbles between two values.  I would not expect to pay this
much attention to ground layout unless I were doing a 12 bit A/D.  But we
had to here.  We did try raising the reference to 4V, which did not appear
to change anything.  So we are tempting fate and sticking with the 2.5Vref.