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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

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  • \$\begingroup\$ Your circuit looks like it should be OK. Double check the implementation. \$\endgroup\$
    – Andy aka
    Mar 27 at 16:25
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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.

enter image description here

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    \$\begingroup\$ Increase R3 to 330 and add a 220 in series with the sensor? Increasing Vcc too. Maybe he is powering the Arduino with 12V and could use this for the op. amps. \$\endgroup\$
    – devnull
    Mar 27 at 17:39
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    \$\begingroup\$ @vangelo Sure. A single resistor on the ground side would also work and wouldn't multiply the error from resistor (mis)matching. I don't think Vcc is an issue, and keeping it 5V or less is useful because you don't need to add clamping complexity. \$\endgroup\$ Mar 27 at 17:43
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    \$\begingroup\$ Yes, indeed. But maybe a 2% sistematic error is not much of a concern considering the input voltage offset of the op. amps. ;) Good catch BTW. OnSemi datasheet doesn't provide this graph. \$\endgroup\$
    – devnull
    Mar 27 at 17:47
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    \$\begingroup\$ Thank you for your response, this is really helpful. And thanks @vangelo, I shall try that configuration \$\endgroup\$
    – TGS
    Mar 27 at 22:16
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    \$\begingroup\$ @vangelo Thank you for helping me so much, it's reassuring to know that the circuit is correct \$\endgroup\$
    – TGS
    Mar 31 at 15:19
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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.

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  • \$\begingroup\$ Was this meant to be a set of comments on David Mikeska's answer...? \$\endgroup\$ Mar 28 at 0:39
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    \$\begingroup\$ @ThreePhaseEel - Yes. Sorry about that. I've edited to make it clear. Thanks for pointing it out. \$\endgroup\$ Mar 28 at 16:42
  • \$\begingroup\$ Prove that it is unity stable \$\endgroup\$ Apr 12 at 9:55
  • \$\begingroup\$ @DavidMikeska - Will the manufacturer's data sheet do? ti.com/lit/ds/snosc16d/snosc16d.pdf "Internally Frequency Compensated for Unity Gain" Also, note Fig 8, which shows a follower driving a 50 pF load with ringing but not oscillation. \$\endgroup\$ Apr 13 at 0:50
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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).

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    \$\begingroup\$ The o/p from OA1 is 495mV and from OA2 is 489mV, same at the final i/p's. I'm using an LM324. Thank you for your help, I will try and modify the circuit with your advice \$\endgroup\$
    – TGS
    Mar 27 at 22:22
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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: -

enter image description here

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..)

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  • \$\begingroup\$ I just noticed the output voltage issue. its a bad op amp. \$\endgroup\$ Apr 12 at 10:10
  • \$\begingroup\$ @DavidMikeska glad you found it dude. Are you TGS by the way? \$\endgroup\$
    – Andy aka
    Apr 12 at 10:19

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