Output from TD100 with instrumentation amplifier not as expected

Question

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:

^{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

Answer 1

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.

Answer 2

In response to David Mikeska's answer, and I do so because it's too long for a comment:

I would think you would need to transpose your Wheatstone bridge's offset voltage across a resistance to induce a current across this resistance to produce the input signal.

Sorry, but that makes no sense at all. The two measurement points are both voltages, produced by (essentially) a pair of voltage dividers. You seem to be under the impression that a bridge is a current source. It's not.

Try putting a 220K resistor across the output of the bridge.

All that will do is to slightly reduce the bridge voltage, as well as introducing a slight nonlinearity.

Also keep in mind that most op amps are not stable in gain at unity (1) or less, and proper selection is needed for use at unity gain.

Just no. Most op amps these days are unity-gain stable. There are some which are not, so your final admonition is accurate, but it's not the sort of issue you seem to make it. For that matter, the LM324 is unity-gain stable.

Answer 3

What is the output voltage of the first two opamps and the voltage at the inputs of the finals stage?

With only a 5V supply it is easy for the intermediate stages to exceed their output range or common mode input range.

Are you actually using LM324 devices?

The maximum output is only about 3.5V with the minimum about 0.7V if required to deliver any signicant current twith a 5V supply, as explained by @Spehro.

The common mode input voltage is also limited; it can go down to ground but only up to within 1.5V of the supply rail (~3.5v with a 5v supply).

Answer 4

I think it's worth answering with some form of reassurance that the circuit (as shown) should operate as expected. Here's a simulation using micro-cap 12: -

So, given that the OP reports the output to be greater than 3 volts, it's highly likely that the problem is an implementation issue (faulty op-amp, wrong value resistor, bad connection, wrong connection, wrong op-amp, failing supply, lack of supply decoupling etc..)