Missing Schmitt Gates??

FFF ?

There are a bunch of families and vendors of tiny single- double- and triple-gate SMD products. It's dizzying. I'm skeptical that so many are needed and economically viable, so I too would be cautious about depending on the exact characteristics of any given part.

OTOH, there are so many pin-compatible substitutions that a generic requirement shouldn't be that difficult to replace--if not outright

Fairchild, OnSemi, TI, NXP, and Toshiba all make parts, off the top of my head.

Then there's always the roll-your-own approach if you can tolerate the effects of feedback delay. Many times you just want an oscillator, or to keep the thing from staying linear & wasting power.

Cheers, James Arthur

Form - Fit - Function.

Purchasing speak for a real drop-in replacement.

That's why I am cautious with this stuff.

But I haven't found anything that looks as nice and versatile as the Fairchild parts you mentioned.

And sometimes I want exactly that, linear behavior. Like when using CD4000 logic as an amplifier. When others see that you can almost hear their stomachs turn.

SN74AUP1G57 is darn close...

TI's Little Logic(tm) offerings overview here:

Selection guide here:

NXP makes them too:

Cheers, James Arthur

Nobody ever made any because it wasn't worth it.

Hope This Helps! Rich

On Fri, 30 Nov 2007 11:52:10 -0800, John Larkin wrote: ...

I guess that would depend on your definition of "feedback". ;-)

Cheers! Rich

I'll take that as a sign that I'm doing the wrong thing. :)

D from BC

Thanks! I didn't know this stuff was so popular.

krw snipped-for-privacy@att.bizzzz posted to sci.electronics.design:

You might not.

D from BC snipped-for-privacy@comic.com posted to sci.electronics.design:

So look up the original discrete circuit.

Heck no! It just means that you need to apply a little bit of cleverness to your design. ;-)

Cheers! Rich

We love these things. Since most logic is migrating to fpga's, once in a while you need a single gate or flipflop. The tinies can often run from 5 volts (which fpga's can't any more) and some are blindingly fast, like flipflops with 1 ns prop delays or buffers with 600 ps rise/fall times. And - you'll like this - they're CHEAP!

We often put tiny logic schmitts right at an fpga to buffer incoming clocks, especially when there's any doubt about signal integrity. Or make clock trees. Works miracles.

Tiny schmitts can do analog-like stuff, too, like being the basis for a boost regulator, or acting as a frequency-counter front end.

Relay and led drivers, too.

John

By now you've probably used one as a crystal oscillator too, right?

I thought schmidts were no good for that - you need to bias them into linear operation, right?

No. I don't design XOs any more. It's too easy to buy a packaged surface-mount thing that just works, usually within a couple of ppm, right out of the tube.

Incidentally, Schmitts make rotten XO's, at least the couple of times I tried it.

A lot of the Tiny gates will pull huge currents if their inputs are much off the rails. I have thermal images to prove it.

John

Yes, which I've done, although apparently from what you and John are saying this wasn't such a great thing to do. :-)

The tiny schmitts don't draw a lot of shoot-through current, so they make decent rc oscillators and discriminators and stuff. The non-schmitt tiny parts can get really hot if operated in their linear range.

John

If I got this right....???

XO oscillators work ideally with one magic phase. Kinda like a phase shift oscillator. The energy within the XO has to be allowed to rock back and forth sinusoidally (linearly) and oscillation sustained by linear regenerative feedback. Nonlinear elements like clipping, xover and nonlinear transfer in the feedback path mess up the frequency stability. I don't see how all these linear requirements can be achieved with digital parts meant for digital applications. A crystal needs a good linear amp. A shaper cct. then makes sq wave. This can be a Schmitt.

D from BC

Yes, although it's often not as bad as you'd first think.

Because the digital parts don't "know" that they were only meant for digital applications. :-)

Everything is linear if you look closely enough...

I am being a little obtuse here -- the kind of oscillator I was thinking of was your canoncial microcontroller/FPGA clock that doesn't need to be particularly accurate -- it's common to use 50 or even 100ppm rocks in such systems; this is a completely different league of oscillator than those you build for, e.g., fancy RF applications where you're after 2.5ppm or better.

---Joel

I was never able to get the Schmitts to oscillate anywhere near the supposed crystal frequency.

John