Noise issues

HI. Im designing a temperature sensor circuit to measure the heated effect of a silicon photo diode. For the temperature sensors i've used two thermistors, attaching one of them to the backside of a silicon photo diode and the other as a reference sensor on a piece of copper. By illuminating the diode with a laser, the change in resistance in the thermistor will provide a voltage drop different from the reference thermistor. To amplify the difference i've used an instrumentation amplifier (AD620) wtih a gain about 225. But im having noise issues. Any idea where it origins from? The circuit is inside an aluminium box. And input voltage is 5V.

Thanks

And input voltage is 5V.

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It needs a positive and negative supply.

Where is your gain resistor? There wont be too much gain without one! Also you need to ground the reference pin...

Sorry. My sketch was a bit unclear. But i do use a positive and negative supply. The gain resistor is 220 ohms and the ref pin is grounded.

Can you take a snapshot!! Are you using a scope or just a multimeter.. How much noise is in the circuit... Pots are notorious for noise, try a multiturn trim pot...

Without seeing the noise it's hard to comment!

Does the noise have a main's frequency component or other specific frequency, or is it hash (white noise).