> I'm planing to use the A2D functionality of the 12F675 to convert
> 0-10V out to a 0-1024 range, 10 bit A2D conversion. Everything
> on the 12F675 looks good for that. My question is whether I have
> to supply a 10V for Vref/GP1/AN1 or is there a way to scale VDD
> internally such that I can get a 0-10V mapping on the A2D conversion
> that is performed on AN0?

Hi Laura, 10V is way too high for Vref and exceeds the voltage rating
of the pin (Vdd + 0.3V). If you take a look  at Section 7.1.3 of the
12F675 d/s, there are two options for a voltage reference. Vdd or the
Vref pin. So you need to either (a) run the PIC on a reference voltage,
eg 4.096V or (b) have 5V for Vdd and use a reference voltage on the
Vref pin. You'll also need to scale down the 0-10V you want to measure
to get it in the pin's spec wrt Vdd. For best resolution, the higher Vref
the better

Say 4.096V is the reference. Therefore use a voltage divider to reduce
the 10V (max) to 4.096V for full-scale A2D reading. As the A2D is
10-bit, each bit of the A2D represents (4mV*divisor) of your 0-10V

If you assume that your 0-10V is actually 0-10.24V (but never reaches
10.24V, only 10V) then you could use a 5.12V reference (with Vdd
equal to at least this but not exceeding max rating). This would simplify
maths by allowing a direct integer multiplication. ie 10V is resistively
divided by 2 -> 0-5V. Vref is 5.12V. Therefore each bit represents
(5120/1024)*2 = 10mV. The highest result you'd get from the A2D
is h03E8 (= d1000), which represents 1000*10mV = 10V

Don't forget the impedance of the A2D input when choosing the
resistive divider. If your 10V to measure is high impedance you can
use suitable high value resistors then buffer the resulting voltage it with
an op-amp to get a lower impedance signal for the A2D input

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist