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Old 4th February 2013, 03:36 PM   #1
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Default Need help with LT spice op amp connections

My input signal is 0.7 volts and the amp amplifies by a factor of 10.
I decide the output signal by 10 and then feed it to inverting end of op amp
and the original signal at non inverting end of op amp.
I want to get difference between the 2 signals but i am getting a square wave as op amp seems to be running in high gain.
How can i set this op amp to unity gain so i get only the difference of 2 signals

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Old 4th February 2013, 03:52 PM   #2
Mooly is online now Mooly  United Kingdom
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You can't do it as you have shown. The opamp will give a squarewave because the difference between the the two signals is huge (in opamp terms... even if it were only a fraction of a millivolt difference)

I'd have to think about how you would do it... something like an adjustable gain amp differential amp.

Have a look at fig 3 here for ideas. You might need more than one opamp to match and process the signals first.

High_Voltage_Diffamp: Analog Dialogue: Analog Devices
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Old 4th February 2013, 06:36 PM   #3
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This helped.
I ended up using 3 op amps

Thank you Mooly
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Old 4th February 2013, 06:42 PM   #4
Mooly is online now Mooly  United Kingdom
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Well done, its not an easy problem to crack
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Old 4th February 2013, 07:39 PM   #5
jcx is offline jcx  United States
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if you just want the diff measurement then you should look at waveform math - in the waveform viewer you can enter equations for trace definitions, multiply, divide, add, subtract any of the node voltages or terminal currents


if you want even more calculating power you can use B sources which create waveforms in the sim that are defined by equations

Spice simulation

or look over a sim that is only BV equations and .func

We can not hear group-delay under 100Hz?

Last edited by jcx; 4th February 2013 at 07:44 PM.
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Old 4th February 2013, 08:17 PM   #6
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Quote:
Originally Posted by jcx View Post
if you just want the diff measurement then you should look at waveform math - in the waveform viewer you can enter equations for trace definitions, multiply, divide, add, subtract any of the node voltages or terminal currents


if you want even more calculating power you can use B sources which create waveforms in the sim that are defined by equations

Spice simulation

or look over a sim that is only BV equations and .func

We can not hear group-delay under 100Hz?
Yes i was able to add subtract/add the waveforms from the output > add trace and then my custom expression.
Will need look more into the B sources

Thanks JCX
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