Author Topic: EEVblog #941 - The Amazing Schmitt Trigger  (Read 11594 times)

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Offline EEVblog

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EEVblog #941 - The Amazing Schmitt Trigger
« on: November 11, 2016, 11:11:05 pm »
Fundamentals Friday
What is a Schmitt trigger and how does it work?
What is hysteresis?
And how do they fix two common problems in electronics, namely slow slew rate signals on CMOS digital chip inputs casuing metastability, and noise on comparator inputs.
The issues are demonstrated first on the breadboard, then the whiteboard explanation, and then the hysteresis fix is added and demonstrated.
And how are Schmitt Triggers physically implemented in CMOS logic?

« Last Edit: November 12, 2016, 02:53:55 am by EEVblog »
 
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Online tggzzz

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #1 on: November 11, 2016, 11:47:22 pm »
Low slew rate and metastability are completely unrelated issues. Schmitt triggers cannot reduce metastability.

Low slew rate causes outputs to "oscillate" up and down whenever the input is near the threshold voltage, because the input is an invalid input voltage.

Metastability occurs when a flip-flop's input transitions at exactly the wrong time (in the metastability window), and the flip-flop "can't decide which way to go". Metastability can happen when both the clock and input signal are correct well behaved logic signals, but there is no defined phase relationship between them. In a properly designed system that should only occur in synchronisers.

Metastability is rare and difficult to demonstrate, so much so that it took the best part of a decade for it to be accepted. The reason for the rarity is that the metastability window is very very small, much much less than the maximum clock frequency.

OTOH, Low slew rate problems will probably occur on every transition.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #2 on: November 12, 2016, 12:13:42 am »
Metastability occurs when a flip-flop's input transitions at exactly the wrong time (in the metastability window), and the flip-flop "can't decide which way to go". Metastability can happen when both the clock and input signal are correct well behaved logic signals, but there is no defined phase relationship between them. In a properly designed system that should only occur in synchronisers.

Yes, and I've explained that in another video somewhere I'm sure. Obviously it's not metastability in terms of setup and hold times for clocked system, that's a different thing entirely. I should have made that clear.
Actually, I think I might edit the video again to make this clear.
« Last Edit: November 12, 2016, 12:21:40 am by EEVblog »
 

Offline Tom45

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #3 on: November 12, 2016, 12:17:51 am »
What is that riding on the top of the square wave display at about the 5 minute mark? Appears on the high and low flat of the square wave, but only after the trigger.
 

Offline vitormhenrique

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #4 on: November 12, 2016, 12:40:27 am »
video is not working here... marked as private video...
 :-//
 

Offline lazarusr

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #5 on: November 12, 2016, 12:59:49 am »
video is not working here... marked as private video...
 :-//
Ditto.
 

Online Andy Watson

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #6 on: November 12, 2016, 01:10:56 am »
video is not working here... marked as private video...
 :-//
Ditto.
It was working, but now it isn't. I suspect Dave has taken it down to make some edits.
 

Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #7 on: November 12, 2016, 01:18:57 am »
A separate video on metastability  maybe?

Maybe.
I could have sworn I've talked about it somewhere...
 

Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #8 on: November 12, 2016, 01:19:34 am »
It was working, but now it isn't. I suspect Dave has taken it down to make some edits.

Yep, some edits to tidy it up.
 

Online nctnico

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #9 on: November 12, 2016, 02:31:17 am »
It was working, but now it isn't. I suspect Dave has taken it down to make some edits.
Yep, some edits to tidy it up.
Well then I'll watch it after taking a nap first.
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Offline ez24

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #10 on: November 12, 2016, 02:49:05 am »
Blocked for me too
YouTube and Website Electronic Resources ------>  https://www.eevblog.com/forum/other-blog-specific/a/msg1341166/#msg1341166
 

Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #11 on: November 12, 2016, 02:54:11 am »
It's back up.
 

Offline Jaunedeau

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #12 on: November 12, 2016, 06:13:59 am »
Haha, this night was the first time I used a schmitt trigger (to clean the output from a phototransistor).

It did not work well, I still hade some 1-10 wuick state switch before the signal stabilise (74hc14 from TI).

I then made a very simple circuit with a potentiometer, the schmitt trigger, and a led, set the input to 1.95V and... tada, the led is blinking !!

Lesson learnt : dont trust circuits you bought on aliexpress :):)
 

Offline blueskull

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #13 on: November 12, 2016, 06:22:52 am »
Lesson learnt : dont trust circuits you bought on aliexpress :):)

No one makes a cent by selling fake 7414. They are already down to cent range even form TI, let along from many cheap but legit Chinese companies. The price difference from a non Schmidt version to a Schmidt version is absolute zero.
Before questioning parts, first question your decoupling. If your naked eyes can see it blinks, it is definitely not "IC speed", therefore think more on external circuit.
 

Offline hedley

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #14 on: November 12, 2016, 06:37:00 am »
Dave - Thanks  again . From your early days fundamentals Friday has been the highlight of your channel for me . In South Africa there are +- 52 Fridays a year so please keep them coming . Worked examples with circuit , datasheet and scope measurements are unique to your channel and really add insight to viewers.I even forced my spousal unit to watch the 34063 episode I think #110 which got me hooked.
 

Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #15 on: November 12, 2016, 06:51:49 am »
I even forced my spousal unit to watch the 34063 episode

Don't do that!  :o
 

Offline bktemp

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #16 on: November 12, 2016, 06:52:38 am »
Nice cheating on the practical demonstration  ;D
Does it make any difference if you add capacitors across Vcc&Vss?
You can see a small glitch in the middle of the ramp, this is probably the point where the gates are switching. As you probably know, CMOS gates can draw a significant current (compared to the idle current) while switching. Whitout any capacitors on the supply rail, the voltage will drop and this will shift the input threshold voltage causing the input gate to toggle while the input voltage is near the threshold voltage. Adding a capacitor may reduce help with a slow clock (I'm saying it the correct way to do it, always use a fast clock egde, but even then you need a good decoupling of the supply voltage).
 

Offline Jaunedeau

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #17 on: November 12, 2016, 07:36:58 am »
No one makes a cent by selling fake 7414.
No, but by selling parts that has been wrongly marked because or a mistake, yes.

They are already down to cent range even form TI, let along from many cheap but legit Chinese companies.
Btw, if I can send you a picture of the IC, with a badly printed TI logo, you'd know there is a broblem :)

Before questioning parts, first question your decoupling. If your naked eyes can see it blinks, it is definitely not "IC speed", therefore think more on external circuit.

I did, and when my circuit came down a 3.7V lithium battery, one 74hc14, one resistor, one LED, and one potentiometer, when my (admittedly cheap) multimeter showed 0V on AC voltage on the input, 1.9V DC voltage on input, and the led was randomly blinking, my conclusion is : this part HAS a problem. They were sold by 10 pieces and none of them have any hysteresis.
 

Offline blueskull

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #18 on: November 12, 2016, 08:00:41 am »
Btw, if I can send you a picture of the IC, with a badly printed TI logo, you'd know there is a broblem :)

That's a horrible one... I also buy from AliExpress or Taobao, but I never buy parts that are too cheap to be genuine.
A misprinted logo usually means 100% fake. That's interesting.

I did, and when my circuit came down a 3.7V lithium battery, one 74hc14, one resistor, one LED, and one potentiometer, when my (admittedly cheap) multimeter showed 0V on AC voltage on the input, 1.9V DC voltage on input, and the led was randomly blinking, my conclusion is : this part HAS a problem. They were sold by 10 pieces and none of them have any hysteresis.

Maybe a cap directly across your IC will help?
Also, if the issue is caused by local digital transient current, you potentiometer's output may not show that because of capacitive loading effect. You need a scope to exactly pin point where signal/power quality starts to go bad.
 

Offline EEVblog

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #19 on: November 12, 2016, 08:32:42 am »
Nice cheating on the practical demonstration  ;D
Does it make any difference if you add capacitors across Vcc&Vss?


A little, but still does it.
 

Offline jonovid

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #20 on: November 12, 2016, 09:46:20 am »
like the video  :-+ but you did not show 555 timer as schmitt trigger or transistor logic trigger circuit only FETs
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Offline sibeen

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #21 on: November 12, 2016, 01:18:16 pm »
Nice video, but, are you sure that the Schmitt trigger symbol comes about as you stated.

I was taught, many moons ago, that it comes from the hysterisis loop (B-H curve) exhibited by magnetic material.
 

Offline rch

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #22 on: November 12, 2016, 01:47:13 pm »
Nice video, but, are you sure that the Schmitt trigger symbol comes about as you stated.

I was taught, many moons ago, that it comes from the hysterisis loop (B-H curve) exhibited by magnetic material.

I would think your teacher was mistaken. Magnetisation hysteresis may well have been historically one of the first examples plotted like that, but it was a long time ago.  It may also be one of the first examples taught in school, but the hysteresis of heater thermostats was also a popular example.  (I expect it is used to show the advantage of  PID now, with much more maths!)

By the time a symbol for Schmitt triggers was needed it was a pretty commonplace engineering idea, so I am pretty sure the logo was based on the hysteresis loop of Schmitt trigger!

 

Offline Brumby

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #23 on: November 12, 2016, 02:28:59 pm »
Nice video, but, are you sure that the Schmitt trigger symbol comes about as you stated.

I was taught, many moons ago, that it comes from the hysterisis loop (B-H curve) exhibited by magnetic material.

Hysteresis is a descriptive term - the graphical representation of which on an X-Y graph gives the shape we all know.
 * You can find it with magnetic materials - and this is the most commonly known example.
 * Plotting the function of the Scmitt trigger also gives it.

... and there could be numerous other examples.


Hysteresis is not defined by the B-H curve.
« Last Edit: November 12, 2016, 02:32:19 pm by Brumby »
 

Offline bills

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Re: EEVblog #941 - The Amazing Schmitt Trigger
« Reply #24 on: November 29, 2016, 03:25:22 am »
Thanks Dave even a dumb a-- like me got it, I have been trying to understand this for some time now.
bill
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