Negative peak detector

Discussion in 'General Electronics Chat' started by cboot, Nov 28, 2009.

  1. cboot

    Thread Starter New Member

    Nov 27, 2009
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    Hi,

    I am trying to make a negative peak detector using a LM324 for a low frequency (e.g., 440Hz), low voltage (500mV max) signal. I'm kinda lost because this op amp is biased at 0V so I think I need some kind of virtual ground, but I'm not very experienced... could someone give me a hand?

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hiya Cboot,
    Welcome to AAC! We're generally a pretty friendly and helpful bunch. Hang around and learn stuff!
    OK, the LM324 is a really ancient, Really SUPER slow operational amplifier. Not good for your application.

    It would help a great deal if you posted your circuit, so that we can see what you're really trying to do.
    We'll give it a good go - but you have to give us a bit more information.

    Circuit diagrams (schematics) are the language that we speak around here.
    They eliminate almost any ambiguities (uncertanties), and almost everyone can understand them, even if English is not their primary language.

    .png format is what I prefer. The images load fast, and you can create them quickly using MS Paint.

    I look forward to helping you with your situation - please post your schematic.

    (Use the "Go Advanced" button below the text box, then "Manage Attachments" button to attach your images.
     
    Last edited: Sep 22, 2011
  3. cboot

    Thread Starter New Member

    Nov 27, 2009
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    0
    Thank you :)

    Hmm I've heard about that. Nevertheless, I wanted to learn how to make a negative peak detector using such a 0V biased op amp, so I could learn how to deal with virtual ground and etc

    Here you go... basically what I have now is a positive peak detector... and what I want to make is a negative peak detector parallel to that.

    What I am trying to do is measure the period of a signal which is polluted with harmonics. I'm using the technique of peak detection to accomplish that. Currently, the output of the positive peak detection is being fed to a voltage divider set to 90% of the peak and then it goes to a comparator which compares the original signal (non-inverting input) with 90% of the peak (inverting input). So the comparator outputs 1 when the original signal is at its peak and 0 otherwise. Now I want to add hysteresis so the output will drop to zero only when the signal reaches 90% of its negative peak... thats why I want a negative peak detector. The hysteresis circuit is no problem for me.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    I don't know why your schematic is a negative. I fixed it.

    The MC3317x and MC3407x opamps have many of the single supply features of the old LM324 but have a much wider bandwidth and no crossover distortion.

    I added an inverting opamp and diode to give the negative peak detection.
    Don't allow its input voltage to go too much negative then the input of the opamp will be negative which might destroy it.
     
  5. cboot

    Thread Starter New Member

    Nov 27, 2009
    7
    0
    Was my schematic wrong?

    Thanks, I'll go buy it when I have a spare time!

    Thanks for your schematic!
    I just didn't get, though... could you explain to me how it works?
     
  6. cboot

    Thread Starter New Member

    Nov 27, 2009
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    About LM324's bandwidth... why would I want a wider bandwidth if LM324 has 1MHz, which should be more than enough for audio applications?

    And about crossover distortion... can you tell me more about it? How does it happen? What's its noise like? High frequencies? If so, How high? I am a computer science student diving into the world of electronics, and I have to say analog circuits are a pain in the a** to learn!
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. Audioguru

    New Member

    Dec 20, 2007
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    The LM324 opamp has full output level up to only 2kHz because its output cannot slew fast enough. The 741 opamp has full output level up to 9kHz. The MC3317x is also low power like the LM324 but has full output level up to 35kHz. The MC3407x and many other opamps have full output level up to 100kHz.

    Look at crossover distortion in google. It is caused when the output transistors in an amplifier do not have enough bias current. It sounds like a buzz especially at certain fairly low levels. It is 3% in the LM324 and LM358.
    TL07x audio opamps have distortion at only 0.003%.
     
  9. cboot

    Thread Starter New Member

    Nov 27, 2009
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    @Bill: yes I've heard of it, but I'm not quite familiar with it yet... the first time I heard of it was a month ago or so, when I looked at the positive peak detector schematic.

    I think I saw 1MHz in a Motorola datasheet... must have read it wrong anyway. I understand that the LM324 is a bad solution for audio... I'll try to find the MC opamp as you said... but for now I'll continue to test using LM324 because I think only wednesday I'll be able to go downtown look for it.

    Thanks! I looked at it in google... but thats a lot of information to process... I'll try to digest it slowly.

    For now, I'm really curious about how to implement the negative peak detector... I'll be glag if you could explain to me briefly the schematic you posted!
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Are the amplitude and frequency constant?
     
  11. cboot

    Thread Starter New Member

    Nov 27, 2009
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    No. The amplitude varies between 100m and 500mV and the frequency is basically in the audible range.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The LM324 barely gets to 1.8kHz at full output level. A low cost TL07x audio opamp and many more get to over 100kHz. Most people who are not deaf can hear to 20khz.
     
  13. cboot

    Thread Starter New Member

    Nov 27, 2009
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    Hmm I see... Thanks for clarifying it. I definitely agree with you. I'm still kinda lost when I look at some datasheet graphs hehe

    But what about the negative peak detector? Could you explain to me the schematic you posted?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    The schematic I posted uses an inverting opamp to invert the negative input and add it to the opamp that has the positive input.
     
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