>I though of using a I2C capable digital potmeter, but they all
>seem to max out at 0-5v.

Check out some of the Maxim ones. Look for MAX5436/7/8/9, selection of 50k
or 100k, and +/-15V with 3/3.3/5V control logic, SPI/Microwire interface.
128 step res, 5ppm/C ratiometric temp drift, 35ppm/C end to end temp drift.

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