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precision rectifier


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Hello everyone,
please, see the attached figure1....
it is a full wave precision rectifier..... the opamps i m using is LM324.... all the resistors are 330 ohms .... i found it in an article..... calculations went fine but when i m checking it's output on oscilloscope,giving a 5 volt peak to peak sine wave input, the oscilloscope is displaying a waveform like figure 2.... 
please tell me what might be the problem? ......
Thanx for any help
MNA

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The LM324 is about the slowest opamp ever made a long time ago.
The 1N4002 diodes are also very slow.
The 330 ohm resistors have a very low value.
The opamps are not biased.

I have used a similar circuit with pretty fast opamps and diodes.
Its resistors have a reasonable value.
The opamps are biased at half the supply voltage but a negative supply could be added if you want the opamp outputs to go to 0V.

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Simulation also show no obvious problems using the part values in his diagram.

The output minimum sink current of an LM324 is only 10mA which is only 6.6V across the 330 ohm feedback resistor. The simulator uses the typical current value of 20mA.

With a very low input frequency that the old LM324 can handle and a very low source impedance, I think the output will not be symmetrical:
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The output minimum sink current of an LM324 is only 10mA which is only 6.6V across the 330 ohm feedback resistor. The simulator uses the typical current value of 20mA.

With a very low input frequency that the old LM324 can handle and a very low source impedance, I think the output will not be symmetrical:


Audioguru, my calculations resulted symmetrical (see the fig).... m i doing wrong?
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Start a process of fault finding. You can't just rely on the calculations.

Check circuit connections.
Increase  the resistance values to see any improvement.
Change the opamp or use another type.
Make sure your AC signal you feeding in does not have any DC offset. This will cause what you are seeing. This can happen when you are using a function generator with the offset control not zero.

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I think that his problem is due to resistor tolerances. Doing the math on his circuit shows that using 1% resistors the peaks can differ by as much as 10%

One of the resistances (R3) should be made variable to adjust for proper balance.

Using 5% resistors, peaks can differ by 50%

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Start a process of fault finding. You can't just rely on the calculations.

Check circuit connections.
Increase  the resistance values to see any improvement.
Change the opamp or use another type.
Make sure your AC signal you feeding in does not have any DC offset. This will cause what you are seeing. This can happen when you are using a function generator with the offset control not zero.


Thanx AN920 and audioguru for yr suggestions....
I checked the input AC signal from the function generator. the DC offset was zero.... so i gave the input to the circuit and it still gave me the same output.... i again disconnected the input and checked the input again, it was then giving some DC offset aroung 0.7-1V..... I balanced the offset with the offset knob, then the output of the rectifier was good..... but after sometime without doing any changes to the input it again showed some DC..... so is there something wrong with the function generator since it's offset is varying from time to time?.......


A diode conducts in only one direction with a voltage drop of about 0.7V.
You shouldn't do calculations with "ideal" diodes.

still the output at pin 7 will be 2 volts.....  R4//(R3+R5) isn't it?

Thanx

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the generator's output impedance is 50 ohms..... changing the resistance values  on the simulator doesn't result in those waveforms which i m getting from the output of the rectifier..... but giving a 1 V dc offset inthe simulation do result intaht waveform..... so it's infact the dc offset which is causing problems.... but why the function generator is changing the dc offset from time to time...i couldn't figure out......

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You don't need much offset to mess up the result.


sorry but i didn't get it?


Have you tried making R3 variable and adjust for balance?

No i haven't tried it yet but i think it has nothing to do with the waveform which is a result of some dc offset....
But i will make it variable because of the 1% tolerance factor....
i have checked the resistor values with the multimeter and then with those values i simulated the circuit...it is still not making such a big difference......


shouldn't the generator give 0 dc offset when the offset knob is closed/not pulled out(default)?..... why is it not giving 0 dc offset?.....
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I am trying to say that very little DC offset on the input will make the peaks uneven.

Read your generators instruction manual to find out where is the cal position. Some generators will have a internal adjustment to calibrate the DC offset zero position. That may be out of calibration. More expensive digital synthesized generators will be auto cal'd by the uP and offset will be very close (within uV's) to zero.

Just checked my digital generator and the offset is 11uV. I think this will be equal to 1LSB of the internal DAC

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  • 2 weeks later...

As AN920 is indicating the generators output does not appear stabile. It seems to drift over temp/time. Will the real world input also drift? That will cause a constant problem. What MIGHT be required is to sample the difference in peaks and use that as a correction voltage to control the offset circuit. This will keep the output centered.

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