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Input/Output Impedance Clipping Voltages

  1. Oct 20, 2011 #1
    Hey guys,

    I’m trying to do a simple experiment where both my DAQ (Data AcQuisition) card and my microcontroller trigger off the same signal.
    The trigger source is from the Agilent 33210A which is a 50 Ohm impedance output.
    From that source, I have connected it to both the AlazarTech DAQ card (50 Ohm Impedance) and my microcontroller (9S12E128) (Set to High impedance)
    I’ve noticed that my microcontroller is not triggering properly. When I disconnect the DAQ card, it works fine.

    I am suspecting that the low impedance on the DAQ card is clipping off the high voltages.
    I’m wondering if you guys have any ideas on how to fix this. I wouldn’t want (But if needed then I will) to introduce opamps or other circuits because it will create delays in the trigger signal.
     
  2. jcsd
  3. Oct 20, 2011 #2

    berkeman

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    Can you look at the waveforms with an oscilloscope?
     
  4. Oct 20, 2011 #3
    Yes I have. I sent my oscilloscope to high impedance, and I notice that when I plug the trigger line into my DAQ card the trigger signal gets reduced SIGNIFICATLY.

    It drops down from about 4.11 V (peak to peak) to 2.77 V.
     
  5. Oct 20, 2011 #4

    berkeman

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    Yes, that is how things work with a 50 Ohm source driving a 50 Ohm load. The open circuit voltage of the voltage source is reduced to half when it is terminated into a matching impedance.

    You would need to find out what the Vih spec is for the logic you are driving, and set the signal source to > 2x that voltage.
     
  6. Oct 20, 2011 #5
    What is "vih"? I suppose you are referring to the logical HIGH and logical LOW values that the micro controller uses?

    Also, I can't change the signal source, the Agilent function generator is fixed. (can't change the impedance, nor the voltage)
    I can't change the impedance of the DAQ card either.
     
  7. Oct 20, 2011 #6
    Are you living in Newark Ca? I see from your HAM radio stuff below by neither link work. I live in Sunnyvale.

    Back to the question, Depend of the speed of the trigger signal, if the rate is slow, maybe is safer to remove the 50ohm termination from the DAQ so the signal don't get loaded down. I just afraid by accident the DAQ is removed and the Microcontroller will be cooked by to over voltage.
     
  8. Oct 20, 2011 #7
    We wrote at the same time. Look at the Agil for another output to drive the controller separately. Then you can safely raise the voltage output.
     
  9. Oct 20, 2011 #8

    berkeman

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    [hijack] Yep! PM me if you want more info. [/hijack]
     
  10. Oct 20, 2011 #9
    I'm using a 500MHz aquisition rate on my DAQ.

    berkeman, I found this on my microcontroler

    V_CC 5.0
    V_OH 4.2
    V_IH 3.25
    V_IL 1.75
    V_OL 0.5
    Gnd 0

    **V_OH 4.2, not 5.2
     
    Last edited: Oct 20, 2011
  11. Oct 20, 2011 #10

    berkeman

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    Yes

    Why can't you set the 33210A amplitude and offset? It is able to put out 10Vpp into 50 Ohms:

    http://www.home.agilent.com/agilent...y=1000032746:epsg:dow&cname=AGILENT_EDITORIAL

    .
     
  12. Oct 20, 2011 #11

    berkeman

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    Yeah, setting the 33210A to 2.77Vpp (with offset voltage half of that) will not meet those CMOS logic levels.

    If you're stuck with the 33210A setting for some reason, you could use a high speed comparator as the buffer between the signal generator and your logic input...
     
  13. Oct 20, 2011 #12
    You are looking at the output. Yes the output can go up to 10v p-p. I'm using the sync output as the output is used to output the function I want the Acquisition card to sync to.


    The Agilant fn Generator has:
    BNC Output (Used)
    BNC Sync (Trigger out-What I am having difficulties with)
    BNC Trigger In (Also used)
    BNC Modulation In - for other purposes.
     
  14. Oct 20, 2011 #13

    berkeman

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    The Trigger Output is not meant to drive a 50 Ohm load. It is meant to drive logic inputs. The datasheet that I linked to lists it as having a "5V Zero-Going Pulse".
     
  15. Oct 20, 2011 #14
    Trigger output is used to trigger other systems. I'm using it to trigger my DAQ card (Below) and the microcontroler.

    TRIG IN (External Trigger) Input
    Input impedance 50 Ω
    Coupling DC only
    Bandwidth (-3dB) DC - 250 MHz
    Input range ±3 V
    DC accuracy ±10% of full scale input
    Input protection ±8V (DC + peak AC without external
    attenuation)
    http://www.alazartech.com/products/ATS9350_v_1_1b.pdf [Broken]
     
    Last edited by a moderator: May 5, 2017
  16. Oct 20, 2011 #15

    berkeman

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    If the trigger input is always 50 Ohms and not configurable, then you will need a beefy buffer to drive it. You could use a 74AC244-type gate to do it (or a single gate version).
     
    Last edited by a moderator: May 5, 2017
  17. Oct 20, 2011 #16
    Ok thanks for your help!

    This is what I feared is that I'd require a amp. Hopefully I can find one with response times in the 500+GHz range also or else I'll need to decrease my acquisition frequency.
     
  18. Oct 20, 2011 #17

    berkeman

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    In the which-what frequency range?!
     
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