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Question on peak detector circuit

David Pfaltzgraff
Expert 2425 points
Other Parts Discussed in Thread: LMP7715, TINA-TI, OPA192, OPA365, LMC6482

I Built up the circuit as shown, but am having problems with the input section of U11. It seems to keep blowing!

What have I done wrong and how can I get it working?

Note that I changed R17 to 10K and still didn't get what I expected.

Thanks

  • 0
    TI__Genius 15975 points
    Hi David,

    It seems that you have not attached the circuit. Is it possible to re-post the attachment?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Expert 2425 points

    Slowly learning how to use the system. Here's the schematic...

  • 0 in reply to David Pfaltzgraff
    Intellectual 310 points

    U11 doesn't seem to get feedback until after U12. Why not design each stage as a working subsystem. U11 should purely be a peak detector and U12 should purely be a buffer. Right now they are mixed and unless I simulate it can only speculate that this will be unstable.

    Try simulating with a similar op amp in LTSpice (I guess TINA-TI if you like it but I find LTSpice much easier to use). Just use a basic op amp in there, no need to use the LMP7715 to validate the schematic above, I don't see why any op amp wouldn't be able to do this.

  • 0 in reply to Bryce Salmi
    TI__Guru 62085 points

    Hello David,

    When using an op amp in an "ideal diode" application, the op amp becomes open-loop for conditions where the diode is not conducting.  This forces the output to saturate until the output returns to a condition that allows the diode to forward bias, closing the feedback path again.  

    The LMP7715 features diode clamps between the inputs and has a maximum differential input signal of +/-0.3V.  When the op-amp saturates the virtual short between the two inputs is lost and the inputs are pulled apart.  In your application the inputs are likely getting pulled apart by more than 0.3V which damages the device.  Switch to an amplifier without input diode clamps, such as the OPA192 and the circuit should work.

    Here's the Abs max table for the LMP7715, notice a "Vin Differential" absolute maximum value of +/-0.3V.

  • 0 in reply to Collin Wells
    Intellectual 310 points
    Would you mitigate pulling them apart by directly attaching R17 to the non-inverting input of the peak detector U11 and providing current limiting resistance so that when the ideal diode function pulls the inputs of U11 apart you prevent a damaging amount of current from flowing?
  • 0 in reply to Bryce Salmi
    Expert 2425 points
    Hi Bryce,

    In looking for an OpAmp that would meet the differential signal requirement, I came across the OPA365. If you go to the bottom of the first sheet of the data sheet, you'll see the circuit that I wanted to use. The data sheet shows a small cap to the negative input. I'm guessing that this speeds up the circuit a little, but it's essentially the same.

    I'll need to wait until I get parts in, but the little I was able to test, it appears that the hold-up time for this circuit was much higher than for the case of the simple peak detector followed by a voltage follower. In either case, now that I think the problem is solved, the hold-up time should be more than enough.

    Just another comment: I found that the reverse leakage specs on the MBR0520 were not very good. So, I'll be going to a diode with a lower leakage specification.

    Thanks for your input,
    Dave
  • 0 in reply to David Pfaltzgraff
    Intellectual 310 points
    The OPA365 looks like it should work.

    They are optimizing for higher speeds with the capacitor to the inverting input. Check out this app note (hopefully no one minds non-ti ap notes, couldn't find a TI one quickly, let me know if this is an issue!)
    cds.linear.com/.../dn61f.pdf



    As for the MBR0520 yeah, that diode is overkill at 0.5A forward current rating. the power doides will have much higher reverse leakage in general.
  • 0 in reply to Bryce Salmi
    TI__Guru 62085 points

    Hi David,

    Glad you found a solution.  The OPA365 is a good choice.  Consider the IN4148 for the diode, it doesn't have the lowest forward drop, but does feature low (~25nA) reverse currents at reverse voltages of 25V. 

    Bryce,

    To answer your question from earlier, you're correct and series resistances could be added to prevent damage to the amplifier.  However, the amplifier is placed into an undesirable state any time that the absolute maximum ratings are exceeded and the circuit performance would likely be much worse than a circuit with an amplifier that didn't contain the diodes.  Thanks for helping!

  • 0 in reply to David Pfaltzgraff
    TI__Mastermind 49966 points

    Hi David,

    As Collin pointed out, the LMV7715 does have diodes across the inputs.

    The peak detector circuit in the LMC6482 datasheet uses a second diode that keeps the first amplifier from going fully open loop. This should help.

    Also, try using a BAT54 Schottky diodes instead, less voltage drop.

    Regards,