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I'm doing some circuit design in order to translate a load cell input into the pedal input of a Logitech G27. What I want to achieve involves having a balanced state output as close to 5V from the amplifier as possible (the original pedal outputs 4.9V, using a potentiometer as a voltage divider to measure the travel), and at full load on the load cell have a lower voltage (original reads 1.4V at full load).

Load cell amplifier circuit

Circuit simulation: here. Sorry, I haven't translated it to CircuitLab, hopefully this will suffice. Based on a design from here, albeit the design there is using an INA122 (similar to the INA126, but with different internal resistances which alter the gain calculation).

What I currently have is a straight amplifier circuit, using an INA126 IC. The load cell is a half-bridge model, with 1000 ohm resistance at no load on both sides. At max compression, the resistance changes to roughly 1002/998 ohm on respective arms. This resistance difference is converted using a Wheatstone bridge by the INA126 (represented in the simulation as a single op-amp, since the simulation only has singular op-amps; the INA126 is a two op-amp instrumentation amplifier with an external gain resistor, setting the gain to roughly 1000; and having internal feedback connections [datasheet]).

In terms of voltages available, I only have one 5V source, and no negative voltage supplies available.

Is it possible to reconfigure this circuit to output on a 5v-0v scale, rather than 0v-5v, without adding any more op-amps or ICs? Or will another rail-to-rail op-amp be required, purely to invert the voltage on this scale?

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  • \$\begingroup\$ Your op-amp has no feedback. How is it that it is not acting as a comparator? \$\endgroup\$
    – TimWescott
    Jan 2, 2019 at 15:43
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    \$\begingroup\$ When you have a differential amplifier, you can invert the signal by simply swapping the inputs. \$\endgroup\$
    – Dave Tweed
    Jan 2, 2019 at 15:44
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    \$\begingroup\$ @TimWescott: I think we can assume that the amplifier shown is the INA, with an implicit gain-setting resistor. \$\endgroup\$
    – Dave Tweed
    Jan 2, 2019 at 15:46
  • \$\begingroup\$ @DaveTweed: in the simulation he links to, the device is called out as an op-amp, not an instrumentation amp. If it were an instrumentation amp, and it were rail-rail, then a first-cut solution would be to connect the reference to 5V -- I'm not sure if normal instrumentation amplifiers are that versatile, though. \$\endgroup\$
    – TimWescott
    Jan 2, 2019 at 18:11
  • \$\begingroup\$ I've added a little more detail in the question. @TimWescott: the op-amp in the simulation is used as a representation of the full INA126 chip, which is a two op-amp instrument amplifier. It's not quite rail-to-rail, though. \$\endgroup\$
    – DragRedSim
    Jan 3, 2019 at 6:11

1 Answer 1

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You should be able to do this.

  • Swap the input pins.

  • Connect the REF pin to +5 V.

(caveat: I haven't tried such high voltages on the REF pin myself, and I'm not specifically familiar with the INA126)

In order to be sure of getting 5 V output with the INA126, you need a positive supply voltage of at least 5.9 V, and to be sure of getting a 0 V output you need a negative supply voltage below -0.95 V. If your load currents are very low, you may get away with smaller margins between the output voltages and the supply voltages.

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  • \$\begingroup\$ This looks like a good line to follow. As stated in the post, the voltages I'm trying to reach range from 1.4V to 4.9V, so the 0V requiring a negative supply voltage is a non-issue. However, the top end of the voltage range looks like an issue, since I can't add any other voltage sources which would be higher voltage. \$\endgroup\$
    – DragRedSim
    Jan 3, 2019 at 6:28
  • \$\begingroup\$ @redion1992 if the load pulls up to 5 V rather than down to 0 V, you might get outputs closer to 5 V. But you'll have to experiment with the INA126 to find out how close. \$\endgroup\$
    – The Photon
    Jan 3, 2019 at 15:44
  • \$\begingroup\$ Well, I managed to get the top end of the voltage (at no load) to reach 3.3V, and that worked fine with the G27 as a nil reading. Thanks for your help on this! \$\endgroup\$
    – DragRedSim
    Jan 27, 2019 at 7:35

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