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AD621 drift with temperature

I'm using an AD621 in amp to read a pressure transducer. The transducer is excited with -10Volts, the 621 is powered with +/-15Volts. The gain of the 621 is set to 100 ( pin 1 and pin 8 connected). The output of the 621 is being fed to an opamp to provide gain, and then feeds a microcontroller to digitize the voltage. I'm using a linear regulator for the -10V and zener diodes for the +/- 15V.

The problem I'm having is a zero drift with temperature. I have built a resistor bridge with the same resistance of the transcuder for testing. While I heat the AD621 and associated circuitry I don't heat the bridge, the bridge remains at abmient temperature. From ambient to 125C the output of the 621 drifts from -.113 to              -.095. The -10V is stable, it changes around 20mV through the heat range, the +/-15V changes more but that change should not be reflected in the output voltage (I don't think).

Any ideas on what might be causing the drift? Any comments are appreciated.

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