Typically, the input to the A/D on a PIC needs to be less than the
reference voltage. If you use the PIC supply (normally 5V, then you need to
scale your input voltage from 0-15V to 1-5. Depending on the load you can
put on the signal you are measuring, you may be able to get a way with a
voltage divider alone, without an op-amp. Typically, you want the A/D to
see no more than 10kohms source impedance. If your signal source has a low
output impedance, you can just use a 3:1 voltage divider, with, for
example, a 20k resistor from the signal to the PIC input pin, and a 10k
from the pin to ground. I would use 1% metal film resistors if you need any
sort of accuracy and stability.

If the signal can't drive a 30k load, as outlined above, then you need to
use an op amp. What op amp and how it is configured depends on how close to
0 volts you need to measure. If you don't need to get below a few 10s of
millivolts, then a rail to rail op amp can be powered from the same 5v
supply as the PIC. If you need to get right down to zero volts, then you
will need to provide a -ve power supply voltage for the op amp as well.
This can be done using a chip such as the ICL 7662 voltage converter, and a
couple of 10uf capacitors.

In a recent project, I needed to measure a 12 volt (nominal) signal, but it
could range as high as 15 volts. The A/D converter had a 4.096 volt
reference, and I needed to use an op amp because of source considerations.
I used a National LM6464 (it's a quad, but there are dual versions) rail to
rail, low offset amp, and a 4:1 voltage divider. Worked like a charm. But I
DIDN'T need to get close to 0 volts, so I didn't need to use a -ve supply.

Larry


At 08:25 AM 3/24/2003 -0800, you wrote:
>Friends,
>I am doing a project where I will need to monitor an A/D voltage that can
>range from 0 to 15V. I have never tried this before but was thinking of
>doing something like a voltage divider going into an op-amp. Have any of
>you tried this before? I would love to hear from someone that has done
>this, what op-amps, and gotchas ... etc. Or if anybody has done it a
>better way I would love to hear from you.
>This is my first post since leaving awhile back, so I am using YAHOO. If
>it is in a format (HTML) other than plain text I apologize. I looked but
>couldn't find a way to control it.
>Thanks in advance for your time.
>Steven Kosmerchock
>steevn.geo@yahoo.com
>
>
>
>
>
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Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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