6
\$\begingroup\$

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

\$\endgroup\$
8
  • \$\begingroup\$ By that same reasoning, why not ask the same qustion for the [Digital]/diffschmitt: why does it have tree inputs? \$\endgroup\$ Jul 5, 2022 at 10:25
  • \$\begingroup\$ @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? \$\endgroup\$
    – lalala
    Jul 5, 2022 at 10:34
  • \$\begingroup\$ 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. \$\endgroup\$ Jul 5, 2022 at 10:48
  • 1
    \$\begingroup\$ Bottom one is ground connection. \$\endgroup\$
    – winny
    Jul 5, 2022 at 10:53
  • \$\begingroup\$ I read the docs, but this does not mean I know the ground pin. The docs are very silent about the schematic. \$\endgroup\$
    – lalala
    Jul 5, 2022 at 10:53

2 Answers 2

4
\$\begingroup\$

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

\$\endgroup\$
2
  • 1
    \$\begingroup\$ "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. \$\endgroup\$ Jul 5, 2022 at 15:01
  • 2
    \$\begingroup\$ @aconcernedcitizen Good find! \$\endgroup\$
    – winny
    Jul 5, 2022 at 15:17
6
\$\begingroup\$

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.

\$\endgroup\$
8
  • 1
    \$\begingroup\$ 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. \$\endgroup\$
    – lalala
    Jul 5, 2022 at 9:24
  • 1
    \$\begingroup\$ @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. \$\endgroup\$ Jul 5, 2022 at 9:27
  • \$\begingroup\$ 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. \$\endgroup\$
    – lalala
    Jul 5, 2022 at 9:31
  • \$\begingroup\$ @lalala Can you show that example? \$\endgroup\$ Jul 5, 2022 at 10:15
  • 1
    \$\begingroup\$ @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). \$\endgroup\$ Jul 5, 2022 at 10:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.