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Strange AD595 behavior


I'm using a K-type thermocouple and an AD595 to measure temperatures in the range +20 C to -100 C.  The system that I'm measuring has rapid temperature changes, covering the entire temperature range in as little as 2 seconds.

I'm powering the AD595 with +/- 12V (from two lead acid batteries), my thermocouple has an ungrounded junction and I've made the ground connection as described in the AD595 spec sheet.  The output of the AD595 goes directly into a NI USB data acquisition card.  The batteries are also powering 2 pressure transducers.

The problem that I have is that once the temperature drops below 0 C, I get very large oscillations on the output.  I've atached a screenshot of some of the results.  The frequency of oscillations is larger than my sampling frequency (10 kHz).  I have no idea what's causing it or how to fix it.  If it weren't for the fact that this only occurs at negative temperatures I would just assume I need some low pass filtering and be on my way, but I'm confused by this situation.

Any help that anyone can provide would be very much appreciated!

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