Help with Precision Rectifier Circuit

Discussion in 'The Projects Forum' started by YWK, Jan 29, 2015.

  1. YWK

    Thread Starter New Member

    Jan 29, 2015
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    Hi,
    Been working on precision rectifier circuit to rectify a sine wave +-4V generated using an Arduino & corresponding circuits (to represent an Hall Effect Current Sensor). The problem with the circuit below is that i am getting a full wave rectified signal but unequal amplitudes for diff half cycles. I did a simulation with LTSpice and it gave me the signal i wanted. Perhaps its the component i am using or?

    Op-Amp : TL072CN -> +-12V supply
    Diode: 1N5817
    Resistors: 1% Tolerance (don't think its precision resistors?)
    R1 = 2 47kohm (in series)
    R2 - R5 = 100 kohm an001-f6a.PNG
    Circuit from (http://sound.westhost.com/appnotes/an001.htm)
    Will post a picture of circuit i soldered when i get back.

    Any help would be greatly appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
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    It's the gain loop on the inverting input of the second op-amp. Gain = 1+ Rf/Rc but in this case, Rc is (2) resistors in series to get back to the virtual ground.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  4. YWK

    Thread Starter New Member

    Jan 29, 2015
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    It seems like the 1N5817 Schottky diode caused it, i am guessing. I followed the quote within the TI datasheet posted about using 1N4148 silicon diode, and something to do with larger leakage currents that comes with the Schottky diode. I replaced the 1N5817 diode with the recommended 1N4148 diode and the circuit is giving me a full-wave rectified signal with equal amplitude. Can't seem to understand why....perhaps my Schottky diode was faulty? Or perhaps it has something to do with the nature of the Schottky diode itself which made the circuit produce a unequal amplitude fullwave rectified signal?

    I am quite happy that it is all working now and thx for the help posted. If i had time, i would be most interested in finding out why but if anyone out there has perhaps a clue on why the diode choice made a difference, pls do advice.

    Once again, thanks for the advice given. I am a newcomer here and would definitely be coming back for more help from the community for any projects (university/hobby).
     
  5. YWK

    Thread Starter New Member

    Jan 29, 2015
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    Also, i think it was important to note that when I measured resistance the individual R2 & R3 resistor when it was connected in the circuit with the 1N5817 Schottky diode, I didnt get 100kohm, when i clearly used a 100kohm resistor for R2 & R3 (measured before soldering). I got something like ~60kohm for both which i found odd.

    After changing the diodes, R2 & R3 gave me 100 kohm on the multimeter. Just wanted to note that.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Schottky diodes have significantly higher reverse leakage as compared to PN junction diodes and that likely is the cause of the asymmetry.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,856
    Be careful when you measure high resistance diodes. If you hold them in your fingers, your skin will be measured in parallel and can make it appear like your resistance is much lower than reality. Best is to clamp the ohm meter leads to the resistor and keep your hands off when reading the value.
     
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