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LTspice has a Schmitt component (schmitt, or schmittbuf) which has two inputs. The upper input seems to be the signal input. What does the other input do?

I read the docs on it, but I am slightly confused. Can somebody tell me what the two inputs of this component are supposed to do?

I know what a Schmitt-trigger with one input and one output does, also the one input two output (one inverted) is clear.

circuit example

lalala
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  • By that same reasoning, why not ask the same qustion for the `[Digital]/diffschmitt`: why does it have tree inputs? – a concerned citizen Jul 05 '22 at 10:25
  • @aconcernedcitizen absolutely, I would have asked. I just happened to start with schmitt and wanted to understand it. I havent gone through the whole library yet. Counterquestion: why didnt you look at schmitt when I mentioned it in the question? – lalala Jul 05 '22 at 10:34
  • You said: "*which has two inputs*". Since you said you have read through the documentation and knew about the ground pin, so I presumed you meant the differential input one, and the `Schmitt` you were referring to was the netlist name. It would have been less time consuming if you had posted a picture to show what you meant, from the beginning, instead of leaving it to the imagination. There's a reason (or more) why most people here will make comments in the lines of "schematic, or it didn't happen". Maybe I should have said it, too. – a concerned citizen Jul 05 '22 at 10:48
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    Bottom one is ground connection. – winny Jul 05 '22 at 10:53
  • I read the docs, but this does not mean I know the ground pin. The docs are very silent about the schematic. – lalala Jul 05 '22 at 10:53
  • @winny Thank. Make it an answer and I will accept it – lalala Jul 05 '22 at 10:54
  • A-devices are generally undocumented or poorly documented. The majority of them are not meant to be used outside of LT/Analog's internal models. – Hearth Jul 05 '22 at 14:27
  • @Hearth lets say you wouldnt want to use an A device then. What would you use instead for schmitt? – lalala Jul 06 '22 at 12:18

2 Answers2

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All A-devices have 8 pins: 5 inputs, two outputs, and one ground. Some exceptions apply, but that's their basis. What you see is a symbol, a visual representation to aid you in using it in the schematic editor. That means that both inputs are signal inputs. It's just that the inverting input will be, well, inverted, so do take that into account when setting vh. And it's provided as it is to help people implement comparators with, or without hysteresis (see, for example, this or this). Or simply having a differential input.

In particular for the Schmitt trigger, under the hood, the netlist involves only two inputs (the first two), the other three are silently ignored. It doesn't make sense to have a three-way difference.

I'll leave you with an encouragement: no device will blow up in SPICE, so whenever you have such questions (e.g. "how to use it?", or "will it work?"), just grab the darn thing and run it through hell an back. See what you get. Who knows? Maybe you may even find the answer for yourself. At the very least, you'll prove to be a person with curiosity, quite neede (and, some would go as far as saying mandatory) in this field.

a concerned citizen
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    Thanks for the answer. The links show diffschmitt and not schmitt. While I agree one could 'try it out and guess', I think a basic question like that to a simulator should also have a basic answer. – lalala Jul 05 '22 at 09:24
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    @lalala They are all the same `Schmitt` under the hood, that's why I added the netlist part. Add a `[Digital]/diffschmitt` and a `[Digital]/schmitt` in a blank schematic, no need for any connections, then check the netlist (*View > SPICE Netlist*). You'll see both listed as `Schmitt`. The names that you see in the component browser are just that, names. What matters is the netlist name. And their pinout have the same base: 5 inputs, 2 outputs, 1 ground. – a concerned citizen Jul 05 '22 at 09:27
  • I checked an example, your answer is not true. The second input (in the gui) makes it act like a ground offset, so it effects input and output. – lalala Jul 05 '22 at 09:31
  • @lalala Can you show that example? – a concerned citizen Jul 05 '22 at 10:15
  • sure. edited the question – lalala Jul 05 '22 at 10:18
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    @lalala That's not an input, that's the ground (the 8th pin. As I said, and repeated, check the netlist. Add names, for better viewing, but check the netlist. You'll see that the connection to the bottom pin appears as the 8th pin. Then, if you'll aslo check the manual under *LTspice > Circuit Elements > A. ...*, you'll see which pins are what (the very first paragraph). And the reason the 8th pin is visible is for those cases where a floating ground is needed, because, as you'll read in the help, when not connected, it defaults to net `0` (zero, or `GND`). – a concerned citizen Jul 05 '22 at 10:22
  • Thanks. Netlist says A1 N001 N002 N002 N002 N002 N002 P001 N002 SCHMITT, will try to change the names then post again – lalala Jul 05 '22 at 10:27
  • Yes: A1 Source Goff Goff Goff Goff Goff out Goff SCHMITT with Goff the at the ground. The first paragraph says: "Current is sourced or sunk from the complementary outputs, terminals 6 and 7, and returned through device common, terminal 8." Thanks. Now it is clear that 'device common' is a word for 'element ground' – lalala Jul 05 '22 at 10:32
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The bottom connection is ground.

Digital circuits in LTspice are special and comes with limitations since they appear ideal, can source and sink infinite current without consuming anything and will only have the parasitic you give them. Upside of this is extreme speed advantage.

I have not found any official documentation stating that the bottom connection is ground, but if you edit the symbol or look in the netlist, you will see it's called COM, for common.

enter image description here

winny
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  • "*I have not found any official documentation stating that the bottom connection is ground*" -- quote from the manual, *LTspice > Circuit Elements > A. ...*, 2nd §: "*Current is sourced or sunk from the complementary outputs, terminals 6 and 7, and returned through device common, terminal 8.*". The picture you're showing has `com` as the label of the pin, becuase it doesn't make sense to say `gnd` since it's meant to be used either grounded or floating, e.g. a reference for the other 7 pins. Like a VCVS, for ex.: just because the `-` pin is grounded usually, it doesn't mean it's the ground. – a concerned citizen Jul 05 '22 at 15:01
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    @aconcernedcitizen Good find! – winny Jul 05 '22 at 15:17