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Peak Detector using the LMP7715

JustDavid
Prodigy 30 points
Other Parts Discussed in Thread: LMP7715

I built up a peak detector circuit using the LMP7715. However, it's not working as expected.

Before I get into the circuit itself, my question is whether or not this choice is reasonable.

I need rail-to-rail input and output, so this helps limit the choice of amplifiers. Then, since it must drive a capacitive load through the diode, that determines some aspects of the output. I went with the LMP7715 because it was already used elsewhere in the design.


So, is this a reasonable choice, or should I immediately go elsewhere?

The application needs to capture the peak of a pulse of a ion detector. Measuring the value of the peak will determine the energy of the ion.

Thanks.

  • 0
    Intellectual 310 points
    I don't see why it wouldn't work at first thought. What isn't working about it?
  • 0 in reply to Bryce Salmi
    Expert 2425 points

    I wasn't sure the schematic came through and was waiting for someone to tell me how to get it there! So, if you're seeing it... (Please pardon any confusion in the user name. My first post was under an older user name and I haven't figured out how to purge it from my system.)

    What I'm doing to test it is take the calibration signal from my scope, which is a 3V square wave at 1KHz through a 33K ohm resistor to the input. With the circuit as shown, the input somehow "loads down" the scope output and obviously is not a high input impedance. I'll try to describe it...

    Starting at the top of the first cycle, it looks OK. Then when the input starts dropping, it drops to about 2 volts and then shows the classical exponential decay until the start of the next cycle when it goes to the top.

    It looks as though somehow the output of the circuit is feeding back into this node. I've gone so far as to lift the pin from the PCB to make sure there's nothing else on the board loading the circuit.


    To diagnose the problem, I removed R17 and connected the storage cap to the negative input. After replacing the part, it worked. But when I hooked it back into the configuration where the output of the second amp was the feedback path, it failed again. (Right now I'm out of spare parts!)

    I'm open to suggestions - even if it is to use the simplified version. (I should have parts here by the middle of next week.)

    Thanks

  • 0 in reply to Bryce Salmi
    Expert 2425 points
    Sorry for the confusion. For the "correct thread", see e2e.ti.com/.../442526

    The previous reply here was for that thread.

    Thanks.