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I've made a full-wave precision rectifier with TL072, for a vu-meter, and the first 3 leds are ON without signal, because I have +15mV offset at rectifier output and the levels for leds are: led1=7mV, led2=10mV, led3=14mV, led4=20mV...

I want to make this rectifier with TL071 so I can adjust the offset. But which opamp should I adjust to null the offset of the whole rectifier ? Or should I adjust both ?

enter image description here

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I'm afraid you've missed the point of the circuit. The second op amp does not need any diodes. You can adjust the offset with one adjustment of the second op amp.

There is one slight issue which you might not have considered, though. Unless R8 and R9 are exactly of a 1:2 ratio, the output will have slightly different gains for signals of different polarity. Since you appear to be looking at overall amplitude of an AC signal, this probably is not a concern.

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  • \$\begingroup\$ You are right. I've simulated the circuit without diodes on the second opamp and it works. But this circuit, with diodes, is recomended in LM3915 datasheet. Why they put them if it works without ? They didn't know ? It's strange... \$\endgroup\$ – Marus Nebunu Feb 11 '15 at 21:31
  • \$\begingroup\$ Ah. You're confusing circuits. What you have shown is figure 20, a precision peak detector. Look instead at figure 19, to get the precision full-wave rectifier. \$\endgroup\$ – WhatRoughBeast Feb 11 '15 at 21:44
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Both. Each will contribute some, and they add.

By the way, there are other full-wave topologies that only use one opamp. That should be easier to adjust. Here's one of them, from a quick Google Images search:

http://www.tradeofic.com/Circuit/15653-Single_op_amp_full_wave_rectifier.html

Output impedance of that one is different for each input polarity, but that might be okay if you're driving high-impedance comparators.

If you're making a VU meter for visual display, you probably don't need precision either, but I think you still need full-wave for an accurate reading in all cases.

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