----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, February 09, 2004 6:46 AM
Subject: Re: [PICLIST] [PIC]: Minimum A/D VREF Voltage For F818? (LT1790
2.048V source?)


> James Nick Sears wrote:
> > Conveniently enough I can't find an LT1790 2.048V reference anywhere
> > but digikey and they have a backorder.  Estimated ship date 2/4/2004.
> > Hmmm... I only really need one unit finished for the initial
> > installation (the probes will be buried in an enormous compost pile
> > as it is turned over the course of a month or so) so I am considering
> > just going with the one that works at
> > 1.25V first but I really need to get these chips asap.
>
> The "power of 2" voltage references are pretty silly in my opinion when
> you've got even a minimum of computational power available (like a PIC).
I
> would stick with the lower price and better availability of more native
band
> gap references.  It shouldn't matter much what the value is, as long as
it's
> within the useable range and well specified.  I use the LM385 often in
> situations like this.

I need a pin-compatible replacement to my LT1790-1.25 for my PCB though.
Also they shipped today. :)

>
> Note also that the reference need not be driving a Vref pin.  You can set
> the A/D range from Vss to Vdd and make the reference another A/D input.
I'm
> doing something like that on a project now where I want to measure the
> battery voltage that is also running the PIC.

I am half asleep here but doesn't that still have the same cost in precision
(if you aren't amplifying the signal up to full-scale Vss to Vdd) except for
tolerating a varying Vdd?  It has to - say in the general case when you are
measuring a 0 - (a)V signal with Vss = 0V, and Vref = (r)V on a general
analog pin (of course max(a) < max(r) < max(Vdd)).  When you sample the
signal with a Vdd reference you get (2^10) * (a/Vdd) possible levels ranging
from 0 to (a / Vdd) (expressed as a fraction from 0 to 1).  Multiplying this
by (Vdd / r) scales these values from 0 to a/r over any Vdd but there are
still only (2^10) * (a/Vdd) possible values, which wastes LogBase2(Vdd /
Vref) bits of resolution (about 1 in my case).  Right?  It is a cool trick
that I hadn't though of though to keep it accurate over varying Vdd.

Thanks,
Nick

Nick

>
>
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