[sdiy] Re: 7400 / 4000 series ICs--compatible?

[MBEDTOM at aol.com]

In a message dated 8/27/2005 2:56:32 PM Central Standard Time, 
alvitar at comcast.net writes:


> I'm not familiar with that specific part, but the differences
> between 40xx, 74HCxx, and 74HCTxx parts are primarily related
> to supply voltage and input thresholds.
> 
> 40xx:    Vdd ranges from +5V to +15V
> 74HCxx:  Vdd +5V, CMOS compatible input levels
> 74HCTxx: Vdd +5V, TTL compatible input levels
> 
> 

Within the 4046 devices alone, there are significant differences amongst 
manufacturers.  Some have a built-in preamplifier while others do not.  That 
radically changes the apparent sensitivity to the phase comparator inputs.  The 
4046s that I have (Motorola brand) DO have the preamp in them and have a lock 
sensitivity of about 80 millivolts P-P.  Other brands did not have this preamp 
and did not work in my application.  As that variation did not even involve 
74xxxxxx series parts, I would make certain to study the datasheet for the chip 
from the manufacturer of the chip you actually buy.

I would suggest you get datasheets from all manufacturers from whom you might 
potentially buy, and compare them spec by spec.  That might just change your 
choice of manufacturer ... it did for me!

Peace.
Tom Farrand
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