Jan said:
the CD4093 has a Schmidt Trigger on the input.
Useful in noisy environments
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