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stretching ns analog pulses for digital conversion

Shyam
Expert 1960 points

Problem statement

in one of my design i need to stretch narrow ns pulses coming from photo diode after TIA stage, if i have to measure the pulse amplitude i have to stretch it at least by a factor of 100, in this search i have found a approach in which the circuit was using a capacitor after its feedback path, but i have not understood how exactly the circuit works, but when i tried to replicate it, no pulse stretch was happening, here below is the circuit and kindly suggest few modifications to the circuit or suggest an approach for the analog pulse stretch

Opamp is a high slewrate 3000V/us and high BW of 100M as BW = 0.35/risetime and slewrate = 2* pi * BW * Vpp, so for my 10ns input the opamp would do the job.

pulse output is just zero !

  • 0
    TI__Genius 13935 points
    Hi Daram,

    I'm sorry we missed this post. I think you are looking for a sample and hold circuit. If you do a search you should find some options.

    The circuit you showed won't work for a number of reasons. First, the feedback resistors are too large for a high speed circuit. Second, the gain is way too low, the signal applied to the diode will always be less than the forward conduction voltage. Gain = Rf/Rg. In many peak detector circuits the gain is equal to one.

    You'll also want a way to reset your capacitor voltage between peaks, this will be covered in a sample and hold circuit (would not be handled by a peak detector).



    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Expert 1960 points

    thanks for the reply

    my objective is detecting the peak of pulses ranging from 50nA - 100mA of current from photo diode, so before my peak detection i should have a TIA or a resistor based I-V conversion, then i should detect the peak of the pulse so that i can find its amplitude from a ADC stage  and further pulse width know from a different circuit should help me to calculate the pulse power = pulse amplitude / pulse width

    the pulses are 10ns wide and repeating at a rate of 22us, so this is making the power calculation difficult

    i am totally confused whether a peak detector will be able to do the job or i need a completely different approach to handle this

    please help

    thanks

  • 0 in reply to Shyam
    TI__Genius 13935 points
    Hello Daram,

    Here is a reference design with a TIA and a high speed ADC. WIth a system like this you may not need to stretch the pulses.
    www.ti.com/lit/ug/tiduaz1/tiduaz1.pdf

    TI does offer stand alone sample and hold circuits as well:

    App note:
    www.ti.com/.../tidu155a.pdf

    Here are our sample and hold products.
    www.ti.com/.../sample-and-hold-products.page

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Expert 1960 points
    in the above example the time to trigger is initiated by the software itself, but in my case the pulse is an input pulse and i have no idea when the pulse will come, it may come at any instant, so above reference is not at all valid in case of pulse power measurement, its a sample hold mechanism where time to sample is know
  • 0 in reply to Shyam
    TI__Genius 13935 points

    Hi Daram,

    Have you searched E2E for this topic? I found two threads that look helpful. 

    Regards,

    Loren