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I am simulating a difference amplifier circuit using LT1215 Opamp. I'm using LT1215 since I need supply voltage around 3V. The input voltage for both the terminals of the opamp is 200 μV and 100 μV. The output should have been around 100 μV but I'm getting output 6.24 mV to 6.2mV. The closed loop gain of the opamp is 1.

Difference amplifier

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  • \$\begingroup\$ A corresponding circuit diagram would certainly help to give an answer. \$\endgroup\$ – LvW Nov 30 '15 at 7:43
  • \$\begingroup\$ I suppose, you are applying single-supply operation, correct? \$\endgroup\$ – LvW Nov 30 '15 at 11:02
  • \$\begingroup\$ Yeah. I'm using single 3v supply \$\endgroup\$ – Alok Nov 30 '15 at 11:24
  • \$\begingroup\$ You asked this question yesterday. Didn't you like the answer? \$\endgroup\$ – Brian Drummond Dec 1 '15 at 17:25
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In the circuit shown, you are simply out of luck. Assuming the two inputs are at different frequencies, your expected output range is +/- 300 uV. With a minus power input of zero volts, you can never get less than 0 volts out, so the zero to -300 uV portion of your output range is simply impossible. Even the with a different op amp, the positive portion of your range is not likely to work, either. Op amps with rail-to-rail outputs do have some limits, and most don't do well when operating within a few millivolts of the rails. Since you want sub-millivolt outputs, I'm afraid you're doomed to disappointment.

What you need to do (whether you like it or not) is to provide something like -3 volts to the minus power input.

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  • \$\begingroup\$ Or bias the inputs to 1.5V. \$\endgroup\$ – Brian Drummond Dec 1 '15 at 17:23
  • \$\begingroup\$ I did provide with -3 volts but the output has peak to peak from 840uv to 480uv. \$\endgroup\$ – Alok Dec 3 '15 at 4:06
  • \$\begingroup\$ @Alok - The fact that the signal is not centered on zero is caused by input offset - see data sheet. If the two sines are of different frequencies, you need to look over a long period to find the peak. A few cycles won't do. \$\endgroup\$ – WhatRoughBeast Dec 3 '15 at 18:49
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Two things. Firstly, the op-amp's input offset voltage is about 125 uV so this represents a massive error compared to your signals. Secondly, the output typically will not go below 5 mV with no load and 30 mV with a 1 mA sink.

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  • \$\begingroup\$ Is there an alternative opamp which does this work? \$\endgroup\$ – Alok Nov 30 '15 at 9:50
  • \$\begingroup\$ For your application, I only know what you have written and, this is about 10% of the information needed to make a reasonable guess at an alternative op-amp. Maybe if you post a circuit diagram and a description of what the inputs are and where the output feeds it will help. \$\endgroup\$ – Andy aka Nov 30 '15 at 9:59

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