Why does Schmitt Triggers Have 2 Series Inverters

WBahn

Joined Mar 31, 2012
26,398
What makes you think that it "usually" has two series inverters following it?

Many (most?) applications do have follow on inverters. Some only have one, in which case it might be to simply avoid inverting the logic.

There can be all kinds of reasons why a Schmitt trigger might be followed by a pair of inverters. One common reason would be that the Schmitt trigger device might have limited drive capability and so it is followed by a pair of strong inverters to increase the drive strength. Another possibility is that the edge speed at the output of the Schmitt trigger might be too slow, particularly for a slowly moving input signal, and so the inverters are their to sharpen the edges. Yet another reason might be to isolate the impact of the load on the hysteretic behavior of the Schmitt trigger.

The devils are in the details.
 

Thread Starter

samus aran

Joined Nov 22, 2020
9
Thanks for the answer. I am a power supply engineer and I seldomly encounter Schmitt triggers. I am not familiar with its detailed applications. However, in one of our applications there is a "crowbar" circuit that makes use of a Schmitt trigger.
 

dl324

Joined Mar 30, 2015
12,273
Why does the Schmitt trigger logic symbol usually has 2 series inverters?
It doesn't. More context is needed.

Sometimes they show that configuration in datasheets, e.g. TI CD40106, to indicate that the input is unbuffered and the output is. Sometimes it could be to show delay tuning.
 

crutschow

Joined Mar 14, 2008
26,073
in one of our applications there is a "crowbar" circuit that makes use of a Schmitt trigger.
Okay, but the two series inverters are not part of the Schmitt trigger, so it's certainly not "usual".
The device in front of the two inverters (below) is an inverting Schmitt trigger.
That symbol is the Schmitt trigger (hysteresis) symbol.
The two inverters may be just output buffers for the trigger.

1606109397645.png
 

Thread Starter

samus aran

Joined Nov 22, 2020
9
Okay, but the two series inverters are not part of the Schmitt trigger, so it's certainly not "usual".
The device in front of the two inverters (below) is an inverting Schmitt trigger.
That symbol is the Schmitt trigger (hysteresis) symbol.
The two inverters may be just output buffers for the trigger.

View attachment 223128
Yes, I know that it is the symbol of the Schmitt Trigger. What I meant is the logic symbol on the data sheet. They "usually" draw it with a series inverter. Anyway, thanks for the answer.
 

andrewmm

Joined Feb 25, 2011
917
can you give us an example of the data sheet your referring to please ?

I'm wondering are you referring to to the "how it works inside the chip" type pictures that some manufacturers provide ?
If so, then they are showing that inside the chip they have a Schmitt function that inverts, then they buffer / invert twice, which as you say is "redundant" . Its just the chip manufacturer marketing / drawing department being "accurate" , they probable have a couple of FET followers in the pack, which are each inverters, so the drawing dept. drew that..

BTW: If its the "internal working " drawing, keep in mind, that its an artist impression. A lot of the details is missed off.
 

WBahn

Joined Mar 31, 2012
26,398
Notice that they don't include the two follow on inverters in any other figure in that datasheet. You would need to ask them what, if any, significance they were trying to convey in that one figure. Perhaps that figure merely matches the on-chip configuration best and that's what they were trying to convey.

I would not use this one figure in this one datasheet as the basis for claiming that Schmitt trigger inputs are "usually" shown with two inverters following a Schmitt tigger inverter.

As counter examples. consider the following:

https://www.ti.com/lit/ds/symlink/s...https%3A%2F%2Fwww.ti.com%2Fproduct%2FSN74LS14

https://www.ti.com/lit/an/scea046/s...21046&ref_url=https%3A%2F%2Fwww.google.com%2F
 
Last edited:

ericgibbs

Joined Jan 29, 2010
11,667
hi samus,
As we cannot see the rest of the circuit you are querying, another possible use of two Schmitt inverters in series is to introduce a delay in a signal.
eg: Say delaying the edge of a Clock pulse of a latch input, in order to allow for the setup time of the data being latched.
E
 

andrewmm

Joined Feb 25, 2011
917
As that's in the logical diagram of the data sheet,
its trying to show the internal , contains a schmit trigger, then two followers,
but as it shows to the outside world, its just an inverting schmit trigger,

Just take it as its another way of looking atteh chip.

If it was shown on a schematic , as two inverters following a Schmitt, then , I agree it could be a design idea to add some delay, bu t in the is case, its just inside the symbol, so its the artist at the chip manufacturer trying to show more info and confusing in th e process.
 

dl324

Joined Mar 30, 2015
12,273
I couldn't find any die photos of a Schmitt inverter, so will use CD4049 as an example.

Here's the schematic of one inverter from a 1980 RCA CMOS databook:
clipimage.jpg
They show a single inverter with input protection diodes.

Here's a die photo from a teardown I found years ago somewhere on the internet:
clipimage.jpg
You can see that the input coming from the lower left pad goes through the protection diodes, then 3 inverters before going to the output pad. The first inverter is small to minimize loading on the driver. Then it goes through larger inverter and then the largest one for the output.

Some datasheets might have chosen to show the additional inverters for output buffering.

That's my best guess as to why some datasheets show a Schmitt inverter with two inverters buffering the output.

If you notice asymmetry in the sizes of the NMOS/PMOS transistors, that's due to the fact that until the early 2000's, PMOS drive was about half of NMOS, so the PMOS transistors needed to be larger.
 

AnalogKid

Joined Aug 1, 2013
8,834
Why does the Schmitt trigger logic symbol usually has 2 series inverters?
That is a concept block diagram of one particular manufacturer's approach to a Schmitt gate. As such, it is not a reference design for the internal guts of a working circuit.

AND - there are three inverters in series - note the bubble on the output of the Schmitt stage - so the overall effect is an inverter with hysteresis on the input.



ak
 

Thread Starter

samus aran

Joined Nov 22, 2020
9
Thanks for all your inputs. Now, I get the general idea why.
Just a correction, the circuit application was a charge pump, not a crowbar.
 
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