Rectifier Help

Discussion in 'The Projects Forum' started by anthonyadams, Feb 22, 2014.

  1. anthonyadams

    Thread Starter New Member

    Feb 22, 2014
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    Hello,

    I'm building a "precision rectifier" using an op amp and some diodes. The schematic is attached. On a breadboard, this circuit behaves as expected, producing a clean, rectified sine wave. When I transfer the circuit to a protoboard, I get the signal shown in the attached scope capture. One side of the sine looks clean and the other is noisy. I've done this twice now, with the same results! Any ideas?

    One possible culprit... I'm driving the input with an iPhone signal generator app, because I don't have a proper function generator. Maybe it's not up to the task? It seemed to work fine when the circuit was on the breadboard. I've rung the circuit out multiple times with a multimeter and I'm still not sure what's going on here.
     
    Last edited: Feb 22, 2014
  2. crutschow

    Expert

    Mar 14, 2008
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    The circuit appears to be oscillating. Are your input and output signals well separated on the protoboard?

    I don't see the schematic, only the 'scope waveform.
     
  3. anthonyadams

    Thread Starter New Member

    Feb 22, 2014
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    Thanks for the reply, Zapper. I've edited the post to include the schematic.

    The components were much more spread out on the breadboard than the protoboard, so you may be on to something.
     
  4. anthonyadams

    Thread Starter New Member

    Feb 22, 2014
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    Here are some photos of the protoboard, if it helps at all.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I don't see any power bypass caps. Add 0.01 to 0.1uF or so at the power pins: + to - is usually all you'll need (but since you didn't draw the power connections....).
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    Your construction technique is an open invitation for all kinds of noise, oscillations and instability.
     
  7. anthonyadams

    Thread Starter New Member

    Feb 22, 2014
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    Haha. Fair enough. I'm a software guy. I'll try to shorten up the wires.
     
  8. anthonyadams

    Thread Starter New Member

    Feb 22, 2014
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    Thanks for the tip. I had bypass caps on my first board but they didn't seem to help. I'll add the caps to my second board and see if things improve.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    One 0.1μF cap directly from the + pin to ground and one cap from - pin to ground is best.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    3,244
    That and avoid output signal wires being close to any input signal wires.
     
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