Voltage Fluctuations in Arduino-opamp-thermistor circuit

[ktizzle]

I'm working on a board that takes measurements of voltage off a thermistor through an arduino and amplified by an opamp (really an instrumentation amp AD623), also there are two potentiometers (picture of schematic attached).

I've been receiving some very strange voltage fluctuations that I can't seem to isolate to any particular component. I've tested both the voltage source and the thermistor independently of the circuit and they are both stable.

Attached is a graph, and on the bottom of the pic you can see the fluctuations over time with no change implemented in the system. At times they look almost oscillatory, but then stabilize at a random voltage before jumping around again. example, would go from 0.01-0.06 volts, jump around 0.3-0.4 volts, and occasionally go as high as 1-2 volts (With no change to the thermistor!)

I was wondering if anyone had any ideas for what is causing the noise/fluctuations. Also, I've heard of a floating ground (not sure what that is), and I was wondering if that was an issue here.

 

[DragonPetter]

Have you read through the application information for the opamp?

http://www.analog.com/static/imported-files/data_sheets/AD623.pdf

Have you tried testing your code/hardware on a test input voltage, rather than from the thermocouple?