piclist 1999\10\18\215932a >
Thread: AD and PIC interfacing problem
picon face BY : Thomas Brandon email (remove spam text)

I'm not exacxtly an electronic's master so this may be way of the mark, but
if you were to drop the supplies (both ground and +5V (you could not drop
the +5V as 4V is within supply specs)) of the PIC 0.5 -1V (I think I saw an
couple of messages about dropping .6V with a zener diode, if so that'd do
the trick) then the -0.5V - +0.5V relative to the sensors ground would
actually be around +0.1V - +1.1V (you could also raise the ground of the
sensor 0.5V, assuming it could handle a -0.5V supply drop). Don't know how
accurate the zener would be, so extra calibration may be neccesary but this
was quite possibly on the cards anyway. Now you at least have a unipolar
signal which increases your range of components to choose from (you may need
some protection to clamp the input above 0V as I don't think your ADC will
like a -ve input).

In terms of scaling the input to full +5V range this may not be neccesary
depending on the required accuracy. For instance, the 10bit AD of the 16F87x
series will theoretically give a proper 8bit range with an FSR of only
1.25V, however this is of course using the 8 LSB's of the ADC so all the ADC
noise will be present (as opposed to scaling 5V FSR to 8bit by taking 8
MSBs). But if 7bit was adequate you could probably get that out of only 1V
FSR (with scaling).

Of course you'd still need an op amp to buffer the high impedance sensor,
but it could be a single supply model, and could simply be used in unity
gain configuration.

{Original Message removed}
<00c801bf19d5$ec8f5c90$fb255e81@tb> 7bit

See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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Subject (change) AD and PIC interfacing problem

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