Electronic – arduino – Output from TD100 with instrumentation amplifier not as expected

arduino unoinstrumentation-amplifieroperational-amplifiersensorthermistor

I've been having trouble with my TD100 circuit for a few days now and I cannot understand why the output isn't as I expect.

This is the process:

Order of operations

schematic

simulate this circuit – Schematic created using CircuitLab

I get around 3.105V at the output for room temperature, but I'm expecting ~500mV. The Wheatstone bridge and op-amps share the voltage source from the Arduino Uno. I'm using an LM324 and a 1.1V reference from the Arduino (currently 5V for the unexpected voltages of >3V.)

The Wheatstone bridge outputs the expected ~40.95mV, the instrumentation amplifier has a gain of 10 – so I'm confused why I'm not getting ~409.5mV at the end.

Note: The resistors aren't exactly these values but they are the same to ~10ohms e.g. R5=R7=R9=R11=9.84k

Best Answer

It should (typically) work to have ~410mV out at room temperature. So I suspect an error in the wiring or an incorrect or damaged part.

Note carefully however that the OA1 is called upon to sink Vdiff/5K, so for 3V out it will have to sink about 62uA with an output voltage of about -160mV. Obviously, it won't do that with a single +5V supply.

In fact, it will run out of sink capability well before that, if you look at the datasheet- around 25uA (typically, at room temperature) with a 5V supply. The LM324 behaves well with a grounded load, but it has only a weak current sink to allow the output to approach Vee when it is called upon to sink current.

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