I need to generate an analog signal ranging between -10V and 10V using a 3V MCU. I'm wondering what the best approach is going to be. My plan is to implement the DAC using pwm and a lowpass filter.

Does it make more sense to use an h bridge to pwm the +-10V rail directly, or generate a 0-3V analog signal and use opamps to shift/scale the output voltage?

 

This is a loaded question - there's so many ways to skin the cat... and without knowing all of your requirements it's hard to make good suggestions. I'd suggest going to linear's website and looking through their A/D converters.

uC's are usually very versatile and you should use them as much as possible. If you need power out of your +/-10V then I'd use the uC to set a DAC and output to a opamp with a power output stage. Sometimes an RC filter is all you need on the output.

Knock yourself out!
http://www.linear.com/parametric/Digital-to-Analog_Converters_%28DAC%29#!cols_1049,1031,1050,1054,2234,1056,1057,1367,1059,2275!s_2275,0!gtd_

Also - most DACs work on the SPI or I2C bus and don't take up your GPIO.

 

This is driving some high impedance inputs to a fast steering mirror...so power consumption is negligible. I'm also trying to avoid using an external DAC, as my timing requirements are pretty tight and I'm worried the overhead involved with the serial bus might make it tough keeping things in sync.

Could a simple differential amplifier with a bipolar power supply do this? I'd use a 1.5V reference on the inverting input then tweak the gain to hit +-10V.

 

The diff amp should work... but I think you'll find that the uC over a serial bus will be much faster and accurate than an integrator. I recently made my uC output a 100kHz PWM and was getting ~3ms settling times with significant voltage ripple over an RC integrator.

It all depends on what you need...

 

Can you float the mirror common connection? If so they you could use PWM to a bridge driving the mirror with a 20V supply to get ±10V across the mirror. It avoids the level shifting required to drive a plus and minus voltage bridge.