A positive peak detector circuit

  1. FOIWATER

    FOIWATER 391
    Gold Member

    I am building a positive peak detector circuit using AD829 opamps.

    I have attached my current circuit I built in multisim.

    I used a "superdiode" configuration on the charging circuit so that I simultaneously get the charging across the capacitor, no discharging during the falling edge, and also no attenuation due to the diode.

    The MOSFET is used to short the capacitor during the negative half cycle of the input wave. In this way, the positive peak detector is self resetting.

    I used another AD829 set up to rail when the supply goes negative to gate the transistor and short the cap.

    Well - it all works pretty good (See the attached waveforms) Except there are certain specifications this circuit has to be able to maintain, a voltage input of from 0.2V to 2.5V over a frequency range of 10Khz to 500Khz.

    I have attached some pictures showing the input vs. output waveforms for different combinations of frequency and voltage

    My main question is:
    1)Why is the capacitor discharging on the falling edge? WHy is it not able to hold it's charge? The diode is in reverse bias. I even removed the mosfet from the circuit, it really cannot hold the charge but this only seems to be the case at low frequencies (It's reactance?)
    2) Why is the ascension linear for high frequency and high voltage? (the linear charging path of the cap ie it does not follow the source) (Vcc is too low?)

    I am having issues with the specs because it seems I need to change Vcc for certain configs whilst I need to change cap values for others.

    I am having issues is there any theory that can be given to perhaps point me in the right direction?

    ANY consideration appreciated.
     

    Attached Files:

  2. jcsd
  3. AD1 - clearly you have some leakage current (diode leaking, op amp input bias current) in the circuit or maybe oscop in multisim have some input resistance.

    AD2 - This look like a typical slew rate limitation. The AD829 has a max Short-Circuit Current close to 32mA.
    SR_max < 32mA/2nF = 16V/μs Try add emitter falower circuit at the AD829 output.
     
  4. FOIWATER

    FOIWATER 391
    Gold Member

    Jony,

    An emitter follower using a BJT, for current gain?
     
  5. FOIWATER

    FOIWATER 391
    Gold Member

    Nice thanks friend,

    I'll keep at it and post back..
     
  6. I suspect you are not bringing the gate negative enough with respect to the source of the mosfet. Most likely it will need to be several volts negative depending on the particular mosfet you’re using.

    In your second plot, it appears the capacitor value is decaying from 400 mV to 300 mV in 25 uS. For a 2 nF capacitor that means that the discharge resistance must be about 3600 ohms. The only place a value in that range could come from is the mosfet. You need to use a dual supply for your second opamp too and make sure the output goes negative.
     
  7. FOIWATER

    FOIWATER 391
    Gold Member

    That's actually a NMOSFET in this program, I think you were assuming it's a PMOSFET?

    I'll look into that as well Thanks Skeptic

    PS: With the mosfet removed from the circuit, I receive the same droop
     
  8. Looks like leakage currents. Calculate the current required to produce the slope you see across a .002uF capacitor. Compare that to the specs for the 1N4148 and the amp.

    Measure the currents, both through the diode and into the amplifier.

    As an experiment, if you double the capacitor you will halve the long term droop.

    There are other secondary effects you will need to consider if you want high accuracy, but first let's fully understand the droop caused by leakage. Notice how the droop increases as the reverse voltage across the diode increases.
     
  9. FOIWATER

    FOIWATER 391
    Gold Member

    OK meBigGuy, yes I noticed that as well...

    Jony - putting the emitter follower on the output offers great results at low frequencies, but removes the benifits previously obtained for high frequencies.

    Not sure why it is doing that,

    See attached Waveforms
     

    Attached Files:

    Last edited: Sep 29, 2013
  10. FOIWATER

    FOIWATER 391
    Gold Member

    Here's another circuit idea I got from a textbook,

    But same story here.

    The 0.2v/2.5v 10kHz-500kHz is killing me....

    I don't even think it's possible at this point with this type or arrangement.

    The capacitors are either too big to charge or too little to hold a charge and not droop.

    The extremes of frequencies require one or the other..

    See attatched

    The idea is simply to add an extra opamp which brings the output voltage back to the anode of the second diode. Making the voltage across it zero volts and preventing leakage current (As mentioned by meBigGuy)

    But, to no avail.
     

    Attached Files:

    Last edited: Sep 29, 2013
  11. 1. Your reset circuit is brutal. You need to short the capacitor without shorting the opamp output to ground.

    2. .002 uF causes slew problems and you haven't fixed that. If you fix 1, you can make D1 a darlington. I'm not sure what issues you had with that previously.
     
  12. FOIWATER

    FOIWATER 391
    Gold Member

    I'm trying here.

    Adding the emitter follower to the output output (not grounding the opamp output) just complicated matters for me.
     
  13. I meant that if you fixed the reset circuit you could add the darlington. Otherwise shorting the darlington to ground would be extra brutal.

    You still have the .002 slew rate problem, correct?
     
  14. Can you post your simulation file?
     
  15. FOIWATER

    FOIWATER 391
    Gold Member

    The multisim file?
    Which results Jony
     
  16. Yes, multisim schematics file.
     
  17. Do you still have the .002uF slew rate issue? How do you plan to address that?
    Just post the same waveforms and schematic you posted initially (updated for the new circuit) and include the diode current when the reset occurs.

    We can address the reset problem so we can address the slew rate problem.
     
  18. FOIWATER

    FOIWATER 391
    Gold Member

    Yeah meBigGuy I still have that issue, I am unsure how to address it.
    I did the calculation for my maximum slew rate required for this circuit with an input waveform of 2.5sin(2pift)
    and I only got about 7V/us for the highest frequency (500kHz)
    But the AD829 is good for up to 50v/us so obviously there is an issue with the circuit.
    I can't post the multisim file here its an invalid file type..
    The previous post is still about where I am at. I have tried adding resistor capacitor arrangement with the 0.002uf cap, and also tried tampering with the opamp hysteresis by adding capacitors to the other inputs of the AD829 but the range is still getting me.

    About the reset circuit, I am unsure how to do it any other way,

    About the output of the opamp being grounded, should I include a emitter follower?

    About the AD829 being used as a comparator, should I replace it with a comparator?
     
  19. FOIWATER

    FOIWATER 391
    Gold Member

    meBigGuy, do you have any suggestions for how I should monitor diode current during reset..
     
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