Looking for two-input comparator with hysteresis

...and why you *really* don't want to use them as part of your circuit, I presume.

--
  Keith

You are SO-O-O-O funny!

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

OK, why not? I's been a couple of decades since people figured out CMOS latchup and took suitable precautions. Modern HC parts are spec'd to handle numbers like 50 mA without problems.

John

The "suitable precautions" only prevent frying. They don't prevent copious amounts of substrate current flow... substantially more than you inserted, due to PNP action.

The "diodes" (they're actually isolation tubs that, with the adjacent diffusions, can act as bipolar transistors) are only rated for transient ESD events, NOT for clamping slow signals.

But go ahead and use them as clamps... I certainly don't care if your circuits up and fry for "magical" reasons.

I NEVER rely on ESD diodes for clamping... VERY BAD engineering practice.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

The only "bad engineering practice" is doing something that doesn't work. But usually BEP means "something I don't like."

Suppose I set up an HC14 and provide for measuring Icc. Now I force a range of + and - currents into one input. Things that might happen are...

1. Some parasitic transistor is turned on and Icc increases.
2. Some parasitic SCR turns on and the whole chip latches.
3. Some other phenom that I can't predict.
4. One of the above, plus the output level gets wrong.
5. None of the above, the diodes act like diodes.

So which will happen? If you'll make predictions, I'll go into the lab and try it.

John

Hard to predict... but it'll be brand dependent. ESD structures are a black art and are different for every foundry.

Why don't you try it and report back (and specify brand of 'HC14)?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I vote for 1, with Icc increasing somewhat, but less than the input current, for the < ~ +/-100uA we're talking about. No effect on the output.

I used them (on a once-off) as power supply rectifier even :)

74HCsomething sync amplifier to use in-line with a monitor that wanted TTL sync while the computer output

That's a famous CD4000-series feature; remove power, and the logic keeps working from the logic input swings. Of course, the original

4000's were notorious for SCR latchup if you pushed any current through the esd diodes.

John

A broken Vdd trace can turn a CMOS NAND gate into an EX-OR gate. Don't ask...

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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I've always used diodes, but not for any reason I learned in school - for one thing, the data sheet recommends them, and for the other, every piece of "real" equipment I've ever worked on, from military avionics to video games and pinball machines, used diodes.

That is on I/Os that go off-board, so I'm probably not even talking about the same thing, but in this case, I'd use diodes, just because it feels more comfortable.

That's my position, and I'm sticking to it! ;-)

Cheers! Rich

Nah ! All digital circuits are really analogue if you look properly.

Graham

If the clock uses a uC then it already probably has a crystal. Use that instead of 50/60Hz. In fact..... this clock will get the time wrong if it's on the wrong supply.

Graham

Since just about every opto is safety approved, farnkly why bother ?

You're *joking*. On a cheap consumer product every single component that's not actually needed isn't there.

Graham

If it's a 'live chassis' design you don't even need an opto. A cheap 'n cheerful npn transistor and 2 or 3 resistors will do the job. Actually, thinking a bit more, if it's using a line frequency power transformer you don't need an opto either. Use the ac from the transformer secondary.

Graham

Power maybe ? I can't see anyone designing a battery clock that has to be connected to the mains to get its reference frequency !

Graham

You don't even need 2 secondaries. Take the power line frequency reference from the same secondary as used for power. In volume manufacture of this kind of product an extra winding is expensive.

In my example, the same winding as used for power. It would want to be a centre tapped winding though with the centre tap to circuit ground.

Via a potential divider ( to limit the voltage ) and a diode ( to avoid applying reverse volts to the gate ). Add a small cap of suitable size at the schmitt input to Rc low-pass filter the signal ( the R is provided by the potential divider ).

Eh ? Oh 0V . Yes.

Uh ? I think you think wrong.

Yes, via the PD and filter cap.

No. It would need to go to circuit ground ( 0V or what you called lower TTL power rail - i.e the negative or return side )

Yes. It's not quite right.

Indeed.

Graham

I think you mean stop the magic smoke don't you ?

Graham

You're a man after my own heart Joerg !

Graham