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I have designed a differential amplifier circuit in NI MultiSim using the LTC2050, with an expected gain of 15, connected to a strain gauge in a full bridge configuration. However, the gain I am getting is 1. I am unsure why this is.

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I am quite certain this is a property of the op-amp I have overlooked as testing with another op-amp ADA4091 yields the expected gain of 15.

In terms of the LTC2050, I have looked into rail-to-rail operations, the upper and lower power limit, slew rate, but could not determine what factor may be causing this. Please let me know if more information is required.

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  • \$\begingroup\$ What's the frequency of your input signal? It could be you're running into the LTC2050's somewhat low gain-bandwidth product. \$\endgroup\$
    – Hearth
    Dec 13, 2023 at 15:53
  • \$\begingroup\$ 100Hz. Can you explain what gain-bandwidth is in the op-amp context? \$\endgroup\$ Dec 13, 2023 at 15:55
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    \$\begingroup\$ Gain-bandwidth product is exactly what it sounds like--conventional voltage-feedback op amps have a maximum gain that is inversely proportional to frequency, resulting in a fixed product of gain and bandwidth. Since the gain-bandwidth product of the LTC2050 is 3 MHz, for instance (which is pretty low), with your input of 100 Hz you have a maximum possible gain of 30,000 (3 MHz / 100 Hz). In other words, this isn't your problem. \$\endgroup\$
    – Hearth
    Dec 13, 2023 at 15:57
  • \$\begingroup\$ Have you made this circuit in reality, or are you only working off of the simulation? \$\endgroup\$
    – Hearth
    Dec 13, 2023 at 16:02
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    \$\begingroup\$ @HFOrangefish I'm not familiar with MultiSim, can you tell me what the differential voltage on the inputs is? I simulated it in LTspice with 166 mV pk-pk on the input and I get 5 V pk-pk (centered on 2.5 V) on the output. – \$\endgroup\$
    – GodJihyo
    Dec 13, 2023 at 17:02

2 Answers 2

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It would depend if the 2.5V RMS AC voltages at the inverting and non-inverting inputs are 180 degrees out of phase or not. Given your 5V single supply and your 2.5V rms inputs, I can't imagine you'd want a 53.1V peak output (which would be the intended result with 2.5v rms inputs with opposing phase at a gain of 15). I do see peak-to-peak voltages in the 10's of millivolts on the same net labels as the 2.5V RMS, as well as a .01 volt discrepancy between DC voltages. Not sure what's going on with all that. Your nets are labelled "Right" and "Left", is this an audio error amplifier of sorts? One more thing to consider— it does matter what your AC voltages are referenced to. Just trying to jog your memory here to get you to catch something you missed. Hope it helped.

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'Green' components have no models, so your simulations with them are completely fictitiousenter image description here

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