Hysteresis of 74HC14

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

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.

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):

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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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

--
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

Its nice for filtering switch inputs and so. I also used it to generate tones for which the frequency didn't matter as long as it was audible.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

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

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

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

Something I used 10 Gezillion years ago...

If you need ms of delay, you can do it nicely (and symmetrically) with a 555 _provided_ that the pulse width is substantially longer than the desired delay. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

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

Thanks, Rich