# How they differ….

Discussion in 'Electronic Basics' started by Animesh Maurya, May 12, 2004.

1. ### Animesh MauryaGuest

Hi all,

How do NAND schmitt trigger CD4093 differ from usual NAND gate CD4011,
thought they both perform the same function?

Thanks

2. ### Jan PompeGuest

the CD4093 has a Schmidt Trigger on the input.

Useful in noisy environments

3. ### Geir KlemetsenGuest

If a gate have a schmittrigger input, it means that it's able to handle

4. ### Guest

I rephrase the question: What is a schmidt-trigger(ST)?

A ST is a device that have a built in memory of one previous event (it is not a memory like a sram or eprom but it is a bistable (two-state) device).

It works like this. The ST have a switching threshold window as below, also called hysteresis.

(use fix width font to see figure)

VDD--------------------------------
------
/ \ /\
Vtu-----/--------\--/--\---------
/ \/ \
Vtl---/------------------\--------
/ \
/ \
VSS---------------------------------

If the input voltage is below Vtl the ST will interpretate it as a '0'.
If the input voltage is above Vtu the ST will interpretate it as a '1'.
If the input voltage is between Vtl and Vtu ST will keep the last value.

In this example it would be the sequence "010"

If the NAND do not have a ST it will only have one threshold voltage Vt, see below.

VDD--------------------------------
------
/ \ /\
/ \ / \
Vt ----/----------\/----\----------
/ \
/ \
/ \
VSS---------------------------------

In this example the sequence would be "01010"

So why and when to use ST-inputs?

In the previous post above it is stated to use it to avoid false values due to noise.

Another very important use is to speed up the transition time. Eg if the input signal have a long rise time it will affect the risetime on the output of the nand. But if the nand have a ST on the input the ST itself is constructed to have fast rise and
fall times. I.e. the output rise and fall time from the nand will be more or less independent on the input rise and fall time.

Hope this can be to some help

/Pfna

5. ### Bob MastaGuest

The 4093 does indeed perform the same logic as a
"normal" NAND gate like the 4011, and can probably
be used to replace it in almost all logic functions.
The difference is when you have input signals that
don't have sharp transistions, or in fact may be sine
waves or slowly varying voltages. The Schmitt trigger
action of the 4093 means that you still get a sharp
transition on the output, and since there is some
hysteresis you don't get bursts of multiple transistions
on slow, noisy input zero crossings like you would on
a conventional gate. These are great for building all
sorts of oscillators, one-shots, debouncers, etc. But
their inputs will be seeing only fast outputs of other gates,
there is no advantage over 4011.

Hope this helps!

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

6. ### grahamkGuest

Have a look at http://homepage.ntlworld.com/g.knott/elect344.htm

7. ### JeffMGuest

What is a schmidt-trigger? [sic]
The *trip* point and the *release* point are different levels.
Another way to diagram Schmitt trigger hysteresis (loop):

8. ### Animesh MauryaGuest

Thanks to all repliers.
(two-state) device).

According to my preliminary knowledge, bistable device are the basic
building block of sram.

Thanks

Best regards,
Animesh Maurya