Spehro Pefhany wrote:
On Thu, 23 Jun 2011 22:55:08 -0700, the renowned Tim Wescott
On 06/23/2011 04:50 PM, Jim Thompson wrote:
Anyone measured the actual hysteresis of a 74HC14? The data sheet
claims almost a volt. Is that a reliable number? I don't have any on
hand, so I thought I'd ask before ordering some.

Read the data sheet -- I think you'll find that proper engineering term
is "untrustworthy".

TI lists the high-going switch point as ranging between 1.7 and 3.15
volts, while the low-going switch point ranges between 0.9 and 2.2 volts
-- hopefully without the 3.15 and 2.2 happening on the same part. That
makes sense for noise immunity, but it doesn't quite fly for any but the
coarsest of analog uses to which you may wish to put it.

Yes, it's not very useful. As well as the postive- and negative-going
threshold values, the Fairchild datasheet gives typical values for the
hysteresis at various Vdd values, as well as min/max.

http://www.fairchildsemi.com/ds/MM/MM74HC14.pdf

At Vdd = 4.5V & 25C typical is 900mV, min/max is 400/1400mV at 25C,
and the guaranteed values over -40~85C are the same.

From interpolating, you'd expect the values at Vdd=5.0V to be about
33mV greater.

NXP's data sheet shows hysteresis value with the same limits, but
substantially different typical values (980mV at Vdd=4.5V):

http://www.nxp.com/documents/data_sheet/74HC_HCT14.pdf

So, if I did the back-of-envelope calculation right, an 'HC14 RC
oscillator could have a period anywhere from 0.356 tau to 1.287 tau at
4.5V Vdd, assuming 50% duty cycle, with typical at 0.89 tau for the
NXP part and 0.81 tau for the Fairchild. Pretty sloppy.

But, do they at least guarantee that the high threshold is more positive
than the low threshold in a given part? ;-)

Thanks,
Rich