Thus spake Martin Larran (fuglisi@UNSA.EDU.AR):

> for the A/D. As I use a symethrical(12,-12) PS for the Op. Amp. of the
> sensors,
> if something goes wrong with some sensor(like wire breaks), the Op.Amp
> will
> saturate blowing the A/D. So which is the best way to prevent this?

What about just using a series resistor for current limiting and a pair
of clamp diodes to GND and VCC? The series resistor should not affect
the A/D if it is much lower in value than the input impedance of the A/D,
and similarly the diodes should not affect anything if their leakage
is low. I would presume the PIC has protection diodes on the input
anyway, but external diodes won't hurt.

I would probably go for something like a 1K series resistor and a
pair of 1N914's. That should limit the current to 7mA, with minimal
effect on the A/D acuracy - Microchip recommend a maximum 10K source
impedance for inputs to the A/D, but take this into account when calculating
the minimum sample time.

To be really safe, you could go with two resistors, like so:

                                   VCC
                                    |
                                    |
                                  ----
                                   /|\
                                  / | \
                       1K           |           220
      From Op amp ----/\/\/\/\---------------/\/\/\/\/\/--- to A/D
                                    |
                                  ----
                                   /|\
                                  / | \
                                    |
                                   GND


This will ensure that if the A/D input does have clamp diodes, and they
have a lower saturation voltage than the 1N914's that they do not
take the bulk of the current, as they would if simply paralleled with the
external diodes.

Cheers, Clyde

--
Clyde Smith-Stubbs    | HI-TECH Software,       | Voice: +61 7 3354 2411
clyde@htsoft.com      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.htsoft.com | QLD, 4051, AUSTRALIA.   |
---------------------------------------------------------------------------
Download a FREE beta version of our new ANSI C compiler for the PIC
microcontroller! Point your WWW browser at http://www.htsoft.com/