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I am researching about instrument amplifier circuit by use op-amp is LM358.

enter image description here

In this circuit gain is 1 by Rgain=R2=20k, R1=R3=10k.

Testing is define frequency range 1 Hz-10 kHz but when this circuit is tested on 10 kHz sine wave input, see figure below, vout is not correct same sinewave input.

enter image description here

In datasheet of LM358 bandwidth maximum at 1 MHz or 100 kHz. I have a question that in my range research LM358 can not working in instrument amplifier?

But when I am testing LM358 to be buffer input sinewave 10 kHz it is working correctly.

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    \$\begingroup\$ Correct, LM358 is a terrible choice. You are seeing gain and phase errors because it has significant distortion and low slew rate and bandwidth. Note that despite an overall gain of 1, you are using a gain of 2 on the first two amps, which leaves them with less ability to correct errors by feedback. \$\endgroup\$ Apr 11, 2023 at 6:41
  • \$\begingroup\$ I replace CA3140 instead LM358 it is good output .Because it high slew rate and bandwidth than LM358 right ? \$\endgroup\$
    – EricSphere
    Apr 12, 2023 at 3:18
  • \$\begingroup\$ The LM358 is never used for audio because it is noisy, produces crossover distortion and its slew rate is so low it causes problems above only 2kHz at high levels. \$\endgroup\$
    – Audioguru
    Apr 12, 2023 at 14:38
  • \$\begingroup\$ My instrument amplifier is used for measurement voltage drop across load (R,C circuit). \$\endgroup\$
    – EricSphere
    Apr 13, 2023 at 5:50

1 Answer 1

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This circuit is working just at "max slew rate".

enter image description here

If you lower Vgen (V4 and V5) or "fr" a little (2x), it should be "good".

EDIT: Tried also with an LT6203 at 100 kHz. It is ok.

enter image description here

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  • \$\begingroup\$ I think CA3140 can be fix this problem .How are you thinking ? \$\endgroup\$
    – EricSphere
    Apr 12, 2023 at 3:20
  • \$\begingroup\$ Sorry. Don't have CA3140 in my databank sheets. Will try with a rail-to-rail opamp LT6203. It is ok. \$\endgroup\$
    – Antonio51
    Apr 12, 2023 at 6:46
  • \$\begingroup\$ Thank you so much for testing LT6203 but It can not find in Thailand . \$\endgroup\$
    – EricSphere
    Apr 13, 2023 at 5:54
  • \$\begingroup\$ Ok. Search rail-to-rail opamp. I think it will fit. With TL082c, ok until 200 kHz. \$\endgroup\$
    – Antonio51
    Apr 13, 2023 at 7:47

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