Half wave rectifier diodes

Discussion in 'General Electronics Chat' started by sealdogfish, Nov 15, 2011.

  1. sealdogfish

    Thread Starter New Member

    Nov 15, 2011
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    Hi all!

    I'm working on a project, I have a pressure sensor with a -50mv to +50mv output, I have 2 AD8666 op amps set up, one inverting one non inverting, what sort of diodes should I use on the outputs for half wave rectification?
    I've looked all over google and the forums and all they say is diode..
    Gain is 40 btw, I want the output from the non inverting to be 0V to 2V when the pressure sensor is 0 to 50mv and the output from the inverting to be 0 to 2V when sensor is 0 to -50mv

    (Both op amps are set up correctly and working, it's just the half wave rectifiers..)

    Thanks!
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Can you post a schematic of your setup. So we can be 100% sure about your setup
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You should look up "precision rectifier".
    If you don't use a precision rectifier, the forward voltage of the diode will introduce an offset. Precision rectifiers compensate for that voltage drop.

    See this page:
    http://sound.westhost.com/appnotes/an001.htm
    Look at figure 3.
    Note that you will need to use a dual-rail supply (both negative and positive supplies).
     
  4. sealdogfish

    Thread Starter New Member

    Nov 15, 2011
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    Here is the schematic:


    That second ADC1 should be ADC2 btw,
    So what diodes should I use and where to place them? I have read that website but again it does not mention the particular diodes to use
    [​IMG]
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You need to have a negative supply, or else the negative portion of the waveform will be truncated (clipped off) at 0v.

    After that, you can use 1N914 or 1N4148 diodes.

    [eta]
    If you really don't want to have a negative supply, you might use a pull-up resistor on your input signal to get the entire signal above 0v; but you will wind up with some attenuation.
     
    Last edited: Nov 15, 2011
  6. sealdogfish

    Thread Starter New Member

    Nov 15, 2011
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    OK, so all the ground connections should be -3.3V?
     
  7. SgtWookie

    Expert

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    No, just the negative supply for the opamp(s). Everything else stays connected to ground.
     
  8. sealdogfish

    Thread Starter New Member

    Nov 15, 2011
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    Thanks, I'll do that now and see how it works
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Corrupted post
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Here's a full-wave precision rectifier that requires no diodes and no negative voltage supply (it actually won't work properly with a negative supply). It does require a rail-to-rail type op amp.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Carl, I looked at that exact article a few years ago, but the output left a good bit to be desired. There was quite a bit of distortion when it got near ground.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Was that a frequency dependent type of distortion?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    I don't remember; it's been a good while. I just remember that it didn't behave well around the ground rail. I'll have to do some digging to see if I can find it.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I found my simulation of it, and I'm wondering what I saw over a year ago that caused me to not like it?

    Looks pretty good from what I can see. I really didn't change anything.
    V(out) is being overlaid by V(in) on the positive swings; it's there.

    Perhaps I'm getting that one confused with another similar circuit I was simulating at the time. I don't know ... it's late and I'm mighty tired.
     
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