At 02:52 PM 5/30/97 +0000, you wrote:
>On Tue, 27 May 1997, Pioneer Microsystems wrote:
>>
>> To achieve your goal of bipolar, I have a dirty trick you can try to get
>> around a 'special' DAC.  Use a unipolar device, and then add a sign amp on
>> the other end.  Take a 'good' IE. OP07 amp, set it up like a classic diff
>> amp with four resistors, tie the two incoming resistors together to the
>> signal, and replace the resistor that goes from the + pin to ground with an
>> open collector to ground instead.
>
>I don't understand the last step, what to you mean ?
>
>Bye,
>Antonio Almeida
>

Ah, let's see if I can do something in text..
                                             R1
    ------------                 ----------|===|---
    |          |          R2     |  | \           |
    |  DAC     |--------|===|-------|-  \         |
    | 0-5VOUT  |      |             |     >-------------------> +/-5vdc out
    |          |      --|===|-------|+  /  OP07
    ------------          R3     |  | /
                                 |
                                 --------------------
                                                    |C
                                                 --------
                                      10K      B |      |
                  DIGITAL -----------|===|-------| NPN  |
                  POLARITY                       |      |
                  BIT                            --------
                                                    |E
                                                    |
                                                   GND

I'm starting to feel guilty that I didn't build this before I opened my
mouth, but the idea is that if the + pin is grounded, the op amp must make
the - pin zero, and thus drives the negative oposite of the DAC output.
(R1=R2, opamp gets +/-12V).  If the NPN is off, then the + is driven with
the DAC output voltage, and in order for the op-amp to drive the + pin with
the same voltage, the output must be driven at the same voltage. (Is that
an echo?).  The output is thus driven at the positive equal to the DAC.
Therefore the polarity bit controls the sign of the output.  If you must
transit from + value to - value smoothly, you might pass through 0 volts
first.

I have an example of another circuit that accepts a +/- voltage, and
generates essentially sign (bit) and magnitude (positive volts) and then
drives a 12 bit A/D input.  The example I have managed to use a 741 with
good results.  Imagine what you could do today!  If you loose more than an
hours sleep trying to picture that circuit, I will pull that info out for
you also.

Christopher Eddy, PE
Pioneer Microsystems, Inc.
Pittsburgh, PA (no, it is not dirty)