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I'm a spanish engineer student doing my end-of-degree project. I need to remove the offset of a strain gauges bridge and i'm trying to make the calibration automatic, using a DAC (digital to analog converter) that will introduce a positive or negative voltage in the REF pin of a instrumentation amplifier (i'm using INA118, but INA126 is also avaiable).

I don't want a manual calibration via potentiomenter. I've seen in the INA118's datasheet a similar circuit: Figure 27. Optional Trimming of Output Offset Voltage ( http://www.ti.com/lit/ds/symlink/ina118.pdf ).

I've seen other questions like "What is the reference pin on an instrumentation amplifier used for?" where they say "Yes, you can do a voltage shift through InAmp's REF pin", but it didn't helped.

The problem is that when i gave 6 negative volts to the REF pin in a +-2,5 supplied INA118 it burned... So i need to know what are the limit voltage in the REF pin???? With -2 volts in REF there was no problem (the limit is the supply rails maybe?)

Thanks all!

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  • \$\begingroup\$ You don't need dual power supply at all, if you need to measure strain gauge bridge. Rather a differential measuring instrumentation opamp. \$\endgroup\$ – Marko Buršič May 6 '16 at 15:30
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They do not list anything on the datasheet about the REF pin, although it clearly lacks the 'overvoltage protection' shown on the (other) input pins.

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Yes, you should keep it within the supply rails, or at most a few hundred mV beyond the rails. Since you need to drive it with a low impedance it would be best to feed it with a buffer that is also powered by the same supplies.

That's for protection- for operation see the datasheet figures 3-6 and think of the supplies as being relative to REF. You need to prevent the internal nodes at the outputs of A1 and A2 from saturating as well as the output of A3, and to keep the inputs of A1, A2, A3 within their common mode range- so the region of proper operation is a different 3D volume (based on the voltages on the inputs and REF on each axis) for each pair of power supply voltages.

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The REF pin should not be allowed to go outside the range of the power supplies. Doing so will cause problems and it certainly won't function correctly just as any input going outside of the power supply range.

If you have so much gain that an offset on the inputs theoretically would produce an output voltage beyond the rails then adjusting the REF pin WILL NOT bring it back.

Using a DAC is fine - that's what I do on many jobs when an auto-zero is needed. Just make sure that the DAC output cannot go beyond the power rails of the instrumentation amplifier.

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  • \$\begingroup\$ Disagree. That's the ref pin on the difference amp, and the common mode input range on the difference amp is generally twice as large as the rails because of the voltage divider. You need to keep V+ on A3 between the rails. \$\endgroup\$ – Scott Seidman May 6 '16 at 16:00
  • \$\begingroup\$ I don't know if the OP is single-siding this, but if he is, -6 is probably bad regardless. \$\endgroup\$ – Scott Seidman May 6 '16 at 16:01
  • \$\begingroup\$ @ScottSeidman I'm not talking about the common mode voltage. The common mode voltage has nothing to do with what I'm saying. \$\endgroup\$ – Andy aka May 6 '16 at 16:17
  • \$\begingroup\$ It sorta does. Vref can go beyond the rails so long as V+ on A3 doesn't, and the voltage on the A3 inputs can't exceed the common mode range for that component (though that isn't clearly specced) \$\endgroup\$ – Scott Seidman May 6 '16 at 16:19
  • \$\begingroup\$ @ScottSeidman yes I know what you mean but the data sheets for both devices in the question do not specify max limits on the REF input and you have to play safe in my book. \$\endgroup\$ – Andy aka May 6 '16 at 17:10

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