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Electronic Integrator

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I have a voltage signal (represents velocity of a beam) and I want to integrate it to get the displacement.

the frequency range is from 20 to 60 KHz

I used the standard integration circuit.... opamp with resistor and a capacitor at the feedback.

I used a sinusoidal zero mean input, and the peak to peak output was always correct but there was always a dc shift (sometimes up and sometimes down)
up means that the maximum of the sine wave is 15 and when I decrease the amplitude of the input the input of the output decreases but keeping the maximum at 15 (not exactly 15 but I mean at upper maximum of the range of the opamp output)

the output is consistent for a given amplitude and frequency ( I power off every thing and ground every thing and I get the same exact results)

I tried a wide range of opamps,
I tried also putting a large resistance across the capacitor( this made the output mean constant but not at zero)
I tried balancing input current but it didn't improve anything

thanks alot


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Whats the 'purity' of the sinewave input ie: the form factor.

I assume that you are integrating the positive and negative half cycles.?

Do you have a circuit that you could post.?


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For an integrator, the voltage should rise ONLY when the mean voltage is zeero. If a sine vawe is not pure, that means the mean voltage isn't seero, and it will cause the voltage over the cap to vary - as you won't.

You can partly solve the problem by add an resistor in parralell with the cap, but it create a new problem - the cap will discharge over time as you've already discovered, and the circuit then becomes useless for the original purpose.

Need to ask you this question: What are you gonna do with the output from the integrator? If you are feeding the integrator with a sine wave with a frequence that vary, you'll get a frequency out that vary.

I think you cannot use this approach to actually measure distance as I thing you're planning to.
What you need is a square wave to feed your integrator where the duty cycle changes with the speed. Only then I think you actually can use an integrator output as a measure of distance. If the voltage out will be linear compared with distance is to find out.

Actually I think you have an pretty interesting project here :)
Good luck.


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I put a analog switch across the capacitor. At the start of a test short out the capacitor to get zeroed out. Open up the switch and integrate for a time. Then reset for the next test.


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I put a analog switch across the capacitor. At the start of a test short out the capacitor to get zeroed out. Open up the switch and integrate for a time. Then reset for the next test.
It depend. We don't know how long time the integrator is meant to collect charges. Also we don't know the RC values on the integrator.
A small RC value can change rapidly, but I think for this procject the RC value should be quite large.

Also a switsh over the cap won't help if the input voltage has a unwanted dc-value.

However if ASeleim know that the signal will cause the cap to charge more in one direction under some circumstances (ex above a certain speed), adding a switch ower the cap may be a solution.
But then again: It will only offer a solution if the integrator is chargin the cap during a short time. Any long time it take may cause the cap to take unwanted charge.
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