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HELP: +/- Variable Voltage from uC

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sparky961:
I tried to be succinct in the subject of this post.  That's pretty much it, I'm trying to think of the easiest or best way to have software control of signal ranging from +10V to -10V.  The software and digital hardware parts I have no problem with, but any analogue circuits involving transistors are a weakness of mine.  Suggestions for simple and effective solutions, or pointing me in the right direction would be greatly appreciated.

At my disposal and seeming to have promise are the following:
- Powerful uC with multiple 5V PWM outputs
- Plenty of digital I/O pins
- Two separate reference voltage signals, +10 and -10
- Unregulated power supply between 20 and 60VDC

I was thinking maybe using two transistors, NPN and PNP between the +10 and -10 "rails" could be the solution, but wasn't able to get past that point.  the generated voltage is a control signal and doesn't need much current.

-Sparky

sparky961:
Hmm... or maybe an OpAmp?  Not sure why I didn't think of that before posting the first question.  I'm going to look into that.

Bluechip:
You need to be a bit more specific.
 
If you use the digital outputs into an op-amp adder [ summing ] circuit, you'll get just about any output you want, except it will be inverted.
 
So, you just need to invert it again with a unity gain inverting buffer.
 
Some stuff in these, particularly the latter.
 
Dave BC
 
http://www.bing.com/images/search?q=op+amp+adder+circuit&qpvt=op+amp+adder+circuit&FORM=IGRE
 
http://www.ti.com/lit/an/slod006b/slod006b.pdf
 
 

sparky961:
Thanks for your input Dave.  The best solution would  limit the number of pins and discrete components, so I'm interested to see what else there would be that performs the same function with fewer components.  I'd like to see about 8 bits of resolution in the output signal.

Whether the output is inverted or not doesn't matter, as I can design for the polarity in firmware.

To simplify, what I think I need is a circuit that converts 0 -> +5V into a proportional -10 -> +10V signal, so a gain of 4? and offset of.... ?  Ok, that's where I start to get lost. :)

Bluechip:
You might like to look at one of these ..
 
http://www.ti.com/lit/ds/symlink/dac0808.pdf
 
Or two ??
 
One of the diagrams has a negative Vref, I'm not sure if this implies you can get negative voltages out of them or not.
 
I have used this ic. but never negative, always with a +ve output.
 
But if I undestand the thing, it will drive to more than 10V .. Last time I faffed with one, it was to +5V .. not recently either ..  :scratch:
 
Dave BC

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