Jim Thompson wrote:
On Fri, 24 Jun 2011 10:38:16 -0400, Phil Hobbs
wrote:
Spehro Pefhany wrote:
On Thu, 23 Jun 2011 22:55:08 -0700, the renowned Tim Wescott
wrote:
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.
Best regards,
Spehro Pefhany
Not so good for an oscillator, it's true.
Not so horrible for charge dispensing, though, if you're using good
quality current sources--all you need is for one of the thresholds to
stay reasonably still.
Cheers
Phil Hobbs
I'm just looking to debounce an edge that has slow recurrence, so a
few ms time constant should do for my purposes.
(I'd use a 555, but I don't have the space for another IC :-)
...Jim Thompson
My usual rule of thumb is that monostables are fine if you don't care if
their delay varies by a factor of 3 either way. If you're using the
positive AC feedback method, the timing will be okay on the leading edge
anyway. Just an RC lag plus a Schmitt won't be nearly as good.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics Electro-optics Photonics Analog Electronics
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net