On Wed, 10 Jun 1998, Chaipi Wijnbergen wrote:

> Hello PicList members,
>
> I need to build a device that would be able to measure an analog signal that
> range from -10V to +10V.
>
> Can anyone please suggest a circuit that will enable a PIC 16C77 to sample
> this voltage ?
>
> Thanks Chaipi

Chaipi,

I think that this could be best done by having + and - 5 V
supplies running a rail-to-rail op-amp in unity gain inverting voltage
adder configuration. The input signal would go thru a divide by four
attenuator and into the adder. There would also be a 2.5 V signal feeding
the other input to the adder. A +/- 10V signal, divided by four and added
to 2.5, yields 0 to 5V, which could be fed to the pic. The signal
reaching the pic would be inverted, so the software would have to invert
it back. THe whole circuit would involve only 10 parts. Hre is a lame
ascii art drawing of the circuit:


X-------+
        |             +-------------------10k---------------+
        15k           |                                     |
        +---10k-------+-----[op amp inv input               |
        5k            |                           output----+-2.2k---+-PIC
        |             |                                              |
       GND            |   +-[op amp non inv input                    -
                      |   |                                          ^
           5v         |  5k                                          |
           |          |   |                                         GND
          10k         | GND
           +-----10k--+
          10k
           |
          GND

Note the protection diode on the pic input. THis is to supplement the PIC
internal diode if the op amp should output below 0 V. If the voltages
very close (within 60mv or so) to  +10 and -10v matter, run the op amp
off +/- 5.5V supplies. Otherwise, you can run it off +/- 5v. You might
also want to use a potentiometer for the 2.5V voltage divider, since you
can compensate for slight errors in the circuit by adjusting it.

Good luck,

Sean


>
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> ! Chaipi Wijnbergen                                                        !
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