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Schmitt trigger simulation

Discussion in 'Circuit Simulation & PCB Design' started by lrjaj0000, Sep 8, 2011.

  1. lrjaj0000

    lrjaj0000 New Member

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    how can I schmitt trigger paramter ( U=4v, U(under))=0,8 , Uover=1,6)defenierien
     

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  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    The CD4000.lib which is a download from the User Group has the CD40106 Schmitt inverter

    Its set for the CD trigger thresholds and cannot be modified.
     

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  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Don't use the CD40106 model mentioned in post #2.
    Instead, in your schematic (which I assume shows the Schmitt inverter from the LTS Digital folder) just right click on the Schmitt symbol and edit the Value to read Vhigh=4 Vt=1.2 Vh=.4 Trise=.1u Tfall=.1u

    That will set Vdd to 4V, the lower threshold to 1.2 -.4 = .8V, and the upper threshold to 1.2 + .4 = 1.6V.
    (The Trise and Tfall values are not critical, but are >0 to enable the simulation to run).
     
    Last edited: Sep 12, 2011
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi alec,
    I would say using a 'non standard' set up for a CMOS Schmitt Inverter could cause problems if the OP designs a digital circuit based on those settings and in the real world uses a regular Schmitt IC inverter.

    Of course it may be just a college assignment.??

    Eric
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Hi Eric,
    In what way is the LTS Schmitt 'non-standard'? It is a generic part in the standard Digital library provided by LT with the LTS download. The Help file clearly indicates that its values Vhigh, Vlow, Vt, Vh etc are customisable (I assume to allow for different logic families, e.g. to cover the 3-15V supply range of the CD4000 family); so presumably the simulation correctly takes custom values into account. Why should that cause problems? It is perfectly valid to use 4000 logic at 4V.
     
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi alec,

    I am assuming that the CD40106 thresholds are 'standard' from the CD4000.lib, modelled using manufacturers datasheets.

    I am not saying that LTS model is not standard, I mean that if the OP sets his own thresholds to suit his simulation, when he tries a regular CD40106 IC, his real world circuit will not perform in the same way as as his simulation model.
     
  8. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Ah, I see now what you were getting at Eric. Agreed.
    I assume that in a real 40106 the thresholds are internally set by potential dividers and so are fixed fractions of Vdd. If the OP has Vdd=4V, then .8V and 1.6V seem unrealistically low for the thresholds.
     
  9. lrjaj0000

    lrjaj0000 New Member

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    hi alec_t,
    thank you for your help, can you plaese tell me how you get " the lower threshold to 1.2 -.4 = .8V, and the upper threshold to 1.2 + .4 = 1.6V".

    thanks
     
  10. lrjaj0000

    lrjaj0000 New Member

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    how can I be expected frequency when I opposed from a capacity of 1n and resistance 10kohm
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Do you mean what is the corner frequency or the frequency of a Schmitt oscillator.?
     
  12. lrjaj0000

    lrjaj0000 New Member

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    yes maybe with bode diagram
     
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Do you mean like this image.?
     

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  14. lrjaj0000

    lrjaj0000 New Member

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    i mean like this?? Please
     

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  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Its a pulse generator circuit, frequency approx 141kHz, why do you want to get a Bode plot.??
     

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  16. lrjaj0000

    lrjaj0000 New Member

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    I use RC as a generator and would like to know how big myfrequency if I change c
    f= 1/(2pi*RC)
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    The CMOS version of the Schmitt Trigger was not intended to be used as a precise frequency generator. It's main purpose is to clean up slow moving edges so slower changing signals can be used to clock other circuits more sensitive to edge rise and fall times.
    If you look at a data sheet you'll see the lower and upper thresholds are all over the place, and that makes it a bad choice for a frequency generator that needs a stable repeatable (chip to chip) frequency. Hobby stuff might be ok but for precision stuff go with something else.
     
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Here's a better formula for the frequency F:

    tp=RC*(ln((vL-Vcc)/(vH-Vcc))+ln(vH/vL))
    F=1/tp

    vL is low threshod,
    vH is high threshold.
    Make Vcc slightly lower than power source voltage like 4.8 for a 5v power supply.
     
  20. lrjaj0000

    lrjaj0000 New Member

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    hi,
    can you please tell me the derivation: tp=RC*(ln((vL-Vcc)/(vH-Vcc))+ln(vH/vL))
    F=1/tp

    Please
     
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Al,
    Interestingly is the difference between the observed frequency from the LTS plot of ~141kHz and the ~76kHz from your formula, which is more accurate than the 83kHz derived from the 1/[1.2*RC].

    To get a 141kHz from your formula I had to set the vL=1.9v vH=3.5v and Vss=4.8v
    [of course there other combinations that give the 141kHz]

    Its appears that LTS is using different vL, vH and Vss from its plotted values of Vc.????

    Strange.
     

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