detect pulse of 140ns and 50mv.

Discussion in 'General Electronics Chat' started by hiren.dave, Sep 4, 2012.

  1. hiren.dave

    Thread Starter New Member

    Jun 22, 2012
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    Hi,

    i want to detect a pulse with 140ns and 50mV amplitude. i want to design such a circuit when pulse comes pic microcontroller should get an interrupt. how can i do that?

    thanks in advance...
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    From where is this fast low amplitude pulse coming?
    You will have to amplify it in some way.
    And perhaps stretch the pulse to have enough time for the microcontroller to see it.

    Bertus

    PS Don't use your email-adresses as user name.
     
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  3. crutschow

    Expert

    Mar 14, 2008
    13,018
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    You could use a high speed comparator such as one of these with the signal applied to the plus input to amplify the pulse. You will need to bias the negative input at perhaps 20mV with a resistor divider from the plus supply to set the pulse detect level.

    To stretch the pulse you connect a capacitor from the output to the plus comparator input with a resistor from the source to the plus input. The length of the pulse stretch past the fall of the input pulse will be approximately equal to the RC time-constant.

    What is the frequency of these pulses?
     
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  4. ramancini8

    Member

    Jul 18, 2012
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    118
    Good advice, but with a 40mV signal I suspect that noise, dc level shift, and signal amplitude changes will make this a hard problem to solve reliably. Also, you should establish the signal-to-noise ratio before trying to design a circuit. This could involve 3 or 4 stages to be done correct. You need to supply a lot more information.
     
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  5. hiren.dave

    Thread Starter New Member

    Jun 22, 2012
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    hi,
    the source is photodiode.
    currently i am using lm 324 / lmc6482in ic of opamp for amplification and HCF4098b pulse stretcher that can stretch a pulse. but i am not getting a pulse. is these IC are good enough ? Should i buy another one? please suggest me...
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic?

    The used opamp is way to slow.
    The slew rate of the LMC6482 is 0.9 V/μSec.
    You will need an opamp with at least 75 V/μSec if you want amplify to a pulse of 5 Volts.
    A fast comparator would be a better choise as crutschow suggested earlier.

    Bertus
     
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  7. hiren.dave

    Thread Starter New Member

    Jun 22, 2012
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    hi,

    i am giving my circuit below. in which photodiode output is given to amplifier than pulse stretcher with led.




    now, i have changed my opamp to MC33274AP (on semiconductor) and pulse stretcher is CD4098BE(texas).
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When using the MC33274AP the output puls will only be 0.7 Volts, due to the low slew rate.
    The 4098 will not react on such a low pulse.
    You need a faster opamp than that.
    The 75 V/μSec I have shown is for a 5 Volts pulse.
    The microcontroller will also work on 5 Volts.

    Bertus

    PS Don't use your email-adresses as user name.
     
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  9. hiren.dave

    Thread Starter New Member

    Jun 22, 2012
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    my friend, i can't understand. please explain me with details.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The MC33274AP has a slew rate of 10 V/μSec.
    So a 1 volt rise will take 0.1 μSec.
    Your pulse is 140 nSec, this is the full pulse, I take half of it, as I do not know the rise time.
    So the rise time is 70 nSec. 100 nSec is 1 Volts, so 70 nSec is 70% of that, so 0.7 Volts.

    Bertus

    PS Don't use your email-adresses as user name.
     
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  11. ramancini8

    Member

    Jul 18, 2012
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    I would look into a current feedback op amp like the HA5020 from Intersil; it has an 800V/uS slew rate and 300MHz BW. This should be fast enough for your amplifier. Use a gain of about 20 to get a 1.0V pulse, and follow this with a fast comparator.
     
  12. hiren.dave

    Thread Starter New Member

    Jun 22, 2012
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    0
    hi friends,

    Finally i got the pulse using MC33274AP opamp and cd4098 pulse stretcher. thanks to all for being with me.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    I assume this thread is a continuation of your last thread:
    detect pulse of 140ns and 50mv.

    Now you want to amplify the 23 mV pulse to about 1 V.
    The slew rate must be higher tan 1 V / 70 nSec = 14.3 V / μSec.
    The GBW must be higher than (1 / 280 μSec) * 47 = 168 Mhz.

    Bertus
     
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