[sdiy] CMOS schmitt trigger question

media at mail1.nai.net media at mail1.nai.net
Thu Dec 27 17:53:44 CET 2001


At 3:46 PM -0500 12/22/01, harry wrote:
>
>> I thought only the inputs needed to be terminated.
>
>That's right. Don't tie the outputs...

I've read in Horowitz & Hill that I should tie the outputs, then again that
book seems less and less accurate as time goes on.

The Harris CMOS Logic Selection Guide 1994, for the CD4000B series it says:

"ALL CMOS inputs should be terminated. An exception can be made in the case
of unbuffered NOR and NAND gates where terminating one of the series inputs
to the proper polarity will not permit current flow caused by a floating
input.  Thus tying low one of the the inputs of an unbuffered NAND gate, or
tying high one of the inputs of an unbuffered NOR gate will satisfy this
requirement."

So besides the previous exception, all the inputs should be tied.  If the
gate is unused, does it matter whether I tie them to Vdd or ground??
Should they ever be tied to Vss??

> >Don't do this with different packages... the matching
> >is not good enough.
>
> Thanks for the tip.

Harris agrees, "4. Paralelling inputs and outputs of gates is recommended
only when gates are within the same package."

>> I have no idea, but what really confuses me here, is that I thought CMOS
>> outputs source current when high, and sink current when low,
>> but according the notes, the output current is listed as a negative
>>value >> when high, and positive value when low.  So I think I must be
>>missing
>> something very important!!
>
>Maybe conventional current flow vs hole flow. Do you like current to flow
>>from positive to negative... or negative to positive ?   Makes no
>>difference either way in this case.  I usually say current is being
>sourced if the load is being driven from the psotitive rail...
>and sunk if it is being sucked down to the negative rail.

OK :)  So if it is high it sources conventional current.

>Gene Zumchak used the convention "sucked" and "blown" in his
>OTA Tutorial (at Rene Schmitz's web site)  I like this because there
>is no confusion.  The industry I work in has a lot of trouble with this
>>point... no one is sure what they are talking about.  I have not got them
>>to agree to "sucked and blown" as a definitve terminology.  ;^)

Oh my!!  What industry is that?? ;)






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