Hi Steven,
Without understanding your accuracy requirements, and assuming you are
planning to use the on-chip A2D on a PIC, here are some design
considerations:

1) Whether you should use an op-amp or just a straight voltage divider
is probably best decided by the impedance of the voltage source you
are measuring.  Microchip recommends no more than 10K ohms of input
resistance into the A2D line on an '870, for example.  If this level
of current draw is compatible with your voltage source and accuracy
expectations, a simple voltage divider can be used.  If not, the
isolation provided by a high input impedance op amp may be a better
choice.

2) The voltage divider must ensure that Vref isn't exceeded on the A2D
input.  There is a small voltage guardband (.3V on '870) but you need
to make sure you really understand the maximum input voltage when you
create the divider.  As an example, if Vref is programmed to the 5V
Vdd supplied to the PIC chip, then the divider could be implemented
with a 1K ohm (precision) resistor on the ground leg and a 2K ohm
(precision) resistor on the source leg of the divider respectively
using your specified 15V maximum Vin.

3) A2D accuracy will be established not only by the A2D and divider
precision but also by the quality of your Vref.  If you've chosen the
PIC Vdd as your Vref make sure Vdd is regulated within your accuracy
expectations.

4) If you're measuring voltage in a noisy environment and you are
concerned that the 15V max might be exceeded, a 15V zener diode (and
current limiting resistor) can be used to clamp the voltage divider
maximum Vin at 15V.  At 7 US cents quantity one from Mouser, a 15V
zener can provide affordable protection for the A2D input on the PIC
_assuming_ you have designed your divider to not exceed Vref with a
15V input.

5) Remember to take into account the divider ratio when scaling the
A2D output in software.

There's a good description of A2D operation and programming in the PIC
Midrange Intro manual on the Microchip web site.  You'll find a number
of detailed app notes there as well.

                Cheers,
                *Lew*


Subject: [EE]: 0 to 15V on PIC A/D line.


> 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.
> Steven Kosmerchock
> steevn.geo@yahoo.com
>

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics