Question about Schmitt Triggers - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: August 23, 2021, 03:16:50 pm
content: I recently saw the video on Schmitt Triggers, and as I was looking up more information, I had a question to ask about them.At its core, a Schmitt trigger works on positive feedback, introducing a little hysteresis in order to (in a nutshell) 'widen' a threshold voltage for comparators in order to reduce noise, reduce actions upon slow transition times, etc. I think I kind of get it based on the typical VIN/VOUT curve as shown in the link below. Ideally, -T and -T would be the same at a fixed voltage reference point in a world where there is no need for such a device; by widening the gap between the two thresholds, VIN has a bit of leeway for a lack of a better term in order to transition between M and -M. So, using the graph as an example, rather than VOUT transitioning when VIN is 0, VIN would have to cross something like 0.5 volts or -0.5 volts in order for VOUT to transition. Otherwise, it stays fixed at where it was before. (If this is totally wrong, please let me know)The Schmitt trigger seems to be applicable in digital logic circuits and/or ADCs as a buffer. I can see why it's used as a debouncer. The one thing I'm a bit confused about is how it is applied. For instance, the example explanation page showcases the LM193 comparator using positive feedback to introduce hysteresis, thus acting as a Schmitt trigger. My question is: is this just showing how to add some hysteresis to a circuit via positive feedback to function as a Schmitt trigger (if so, is this method only for comparators), or is this just showing how a Schmitt trigger works and this will never work in the real world? How is this different than the buffer ICs that function as a Schmitt trigger?https://components101.com/articles/schmitt-trigger-introduction-working-applicationshttps://www.ti.com/lit/ds/symlink/lm193.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1629703940445&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Flm193

author: Benta
author_rank: Super Contributor
timestamp: August 23, 2021, 03:32:53 pm
content: It has a bit to do with terminology.Over time "Schmitt trigger" has evolved to be a term for special logic inputs with hysteresis.In analog circuits it's more common to use the term "comparator with hysteresis" to emphasize that the trigger levels are important.But the basic function is the same.

author: TimFox
author_rank: Super Contributor
timestamp: August 23, 2021, 03:40:21 pm
content: A Schmitt trigger built with a comparator can have very well-defined input levels due to relatively-low offset voltage and well-defined output levels driving the positive feedback network. Any circuit with non-inverting input to output (like a logic gate) can be used to make a Schmitt trigger with an appropriate positive feedback network, but the exact input levels may not be as accurate (but accurate enough for binary logic levels). It depends on the application.

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2021, 07:03:32 pm
content: Quote from: LoveLaika on August 23, 2021, 03:16:50 pmLogic Schmitt triggers are doing the same thing internally, in one way or another, as the common comparator examples; they are using positive feedback to alter the input logic threshold based on the output state.

author: Renate
author_rank: Super Contributor
timestamp: August 23, 2021, 09:21:17 pm
content: You can use positive feedback to make a Schmitt trigger.You can use two comparators with two different thresholds to make the same kind of action.A 555 IC is basically two comparators set at 1/3 and 2/3 thresholds.(Yes, it can do more complicated things too if you don't connect pin 2 & pin 6 together.)It's basically an inverting Schmitt-(like) trigger.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: August 25, 2021, 01:47:26 pm
content: Thanks. I think that helps clarifies things from both a digital and analog perspective.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: August 25, 2021, 01:52:53 pm
content: Thanks. It seems that ICs, specifically logic ICs, can incorporate Schmitt triggers at their inputs. That's all well and dandy, but as a Schmitt trigger can be considered as its own separate circuit element, is it more accurate to incorporate a pre-built one rather than one you build your own of? It's one thing to build one from a comparator, but I think I saw somewhere that you can also see a comparator as a series of MOSFETS somehow. Of course, it depends on the application, but what's the trade off?

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2021, 02:01:38 pm
content: Building your own has the advantage in that you can tailor the switching levels to your application, and take advantage of variable levels of DC and AC hysteresis. Very few integrated Schmitt triggers allow external adjustment but there are some which do.In logic gates, Schmitt trigger functionality does not come free. Including it sacrifices speed, power, and area.

author: Benta
author_rank: Super Contributor
timestamp: August 25, 2021, 03:52:47 pm
content: Logic Schmitt trigger inputs are very imprecise. The trigger levels have huge tolerances and can not really be used in the analog sense. They're there to compensate for slow rise/fall times on the input.A home built Schmitt trigger (or "comparator with hysteresis" can be built to almost any precision.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: August 25, 2021, 05:08:48 pm
content: Quote from: Benta on August 25, 2021, 03:52:47 pmThanks. So, if I got this right, we've established how a Schmitt trigger works (it uses hysteresis to buffer the input for a lack of a better term). In the analog world, it's a comparator using positive feedback, making it more suitable for analog applications. In the digital world (AKA logic), it's the same functionality but it most likely uses a different topology to achieve the same goals. Looking at the SN74LVC1G17DBVT, it uses CMOS technology to act as a Schmitt trigger. Now, to me, it seems bad to use SN74LVC1G17DBVT in an analog circuit, as it's not built for that, so naturally, you would build your own Schmitt trigger using a comparator and making it very precise.Is this an adequate summary? David Hess also mentioned how Schmitt trigger functionality comes at a cost of speed, area, and power. Is that referring to ICs with internal Schmitt functionality (like theSN74AHCT14N)? If so, what makes Schmitt trigger ICs like the SN74LVC1G17DBVT handy? I suppose you could use them in non-crucial circuits where speed doesn't matter, but aren't they tailored to allow for functionality without sacrificing anything else?

author: Benta
author_rank: Super Contributor
timestamp: August 25, 2021, 06:04:38 pm
content: Quote from: LoveLaika on August 25, 2021, 05:08:48 pmYes, pretty much. Concerning speed, area and power, that's not really your concern, that's an issue for the logic chip designers. We're talking about the logic input circuitry here.What makes a 74LVC17 handy? I already mentioned that. It's to deal with slow rise and fall times on logic inputs. Examples:- convert 50/60 Hz sine to square wave.- Debounce a switch input with RC delay.- relaxation RC oscillators- and, and, and...

author: Bassman59
author_rank: Super Contributor
timestamp: August 25, 2021, 08:22:36 pm
content: Quote from: LoveLaika on August 25, 2021, 05:08:48 pmWhat makes the Schmitt trigger useful in a logic gate like the 1G17 (or a comparator, which is generally used like a logic gate) is that its different logic-level thresholds prevent the output from "chattering" as the slow input signal transitions. Think this through.A typical CMOS logic gate has its thresholds at 33% and 66% of the supply rail. When the input signal is below Vil (33%) the output goes low. When the input signal is above Vih (66%) the output goes high. When the input signal is between the thresholds, the output is literally undefined. What can happen when the input is in this range for "too long" is that the output "chatters" between high and low. Now a fast input transition goes through this unknown zone quickly so the output doesn't chatter. But feed the input with a slow sine wave and you have problems.The Schmitt input has Vih and Vil thresholds, too. The difference is that once the input signal rises above Vih, the output goes to logic high the output will stay there even if the input goes below Vih. The input has to go below Vil for the output to change (to low). Similarly, once the input signal goes below Vil, it must go above Vih before the output switches to high. And this means that the input signal can dwell in the middle of the range between the thresholds and the output will not change. No chatter!

author: TimFox
author_rank: Super Contributor
timestamp: August 25, 2021, 08:52:49 pm
content: Exactly. With a reasonable gap between the Schmitt trigger’s rising and falling thresholds, the amount of input noise required to “nuisance trip” the output increases dramatically.