I'm reviewing a design that is using a 74HC14 schmitt trigger with the input biased to 1.2 VCC and a small signal AC coupled in. the AC signal is about
400mVp-p.VCC VCC | | R | Signal----||---+--- Schmitt trigger ---> digital out C R | | GND GND
Since it is a schmitt trigger it has ~ 50mV of hysteresis but is there cause for concern by leaving the input biased in the middle like that?
oops....Make that 1/2 VCC not 1.2.
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The current it draws might be high in that mode. In volume, the trip point will move around, so your resistor scheme might not be centered. Probably there is a tempco issue with the trip point.
I'm not a fan of hammering with a screw driver, but your mileage may vary.
I've dealt with system designers that take your datasheet and black out anything that is a typical, then they do their design.
I'm currently using an inverter (single-stage, no hysteresis) as an analog comparator, offset-zeroed _every_ measurement cycle... works great ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | 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
The gate will draw significant quiescent current, a mA or so, if that's a concern. And a big pulse will take out the gate--no current limiting. Otherwise, it's fine.
Cheers, James Arthur
The way to do it....
VCC | | Signal----||---+-----74HCU04---+-- digital out C | | | | GND | | | +-----R---------+Add more inverters after to square up if needed, though 400mV P-P is pretty robust.
...Jim Thompson
--
| James E.Thompson, P.E. | 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
This is more fun...
| +---------|-----------+
John
According to
Cheers! Rich
It won't cause extra *input* current, it causes extra Idd.
James Arthur
Oscillator!!
Cheers, James Arthur
Yep. Ground the signal input and you can get an output anyway ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | 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
Hey, and a few years ago you told me not to do that ...
Anyhow, keep VCC well below rated because these things can draw an enormous cross current in this mode. If things don't have to be fast I use CD4000 logic at just a few volts. Stays nice and cool. A 74HC at 5V might get quite toasty.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Ahm, a Schmitt usually has a "non-Schmitted" first device pair and that does draw cross current. Not a lot but on a battery operated design this current can be painful. There is a Philips/NXP family spec sheet about that.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Not terribly toasty if 74HCxxx, but 74ACxx biased like that will burn your finger.
I killed a few early 'AC parts that way--they can pull something close to 100mA. I wondered where that 85mA was going in my 2mA ckt. Then I put my finger on the answer.
Cheers, James Arthur
It's an HC_U_, note the "U", it'll draw only a few mA. A plain 74HC04 will sing like Blago when the US Attorney finally grabs him by the balls ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | 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
T'is why I invested around ten bucks into a Harborfreight IR sensing thermometer. It'll pay itself by having to use less burn gel.
I've seen HCU's draw more than a few mA. Not that smoke comes out, of course, but in batt-gear you can't have that. And not in med-gear.
I really like the CD4007UBE but the TSSOP version is out of stock a lot. Not a good sign. I wish such a panacea chip came out for all logic families. But I guess most young lads wouldn't know what to do with it anymore and the market has shriveled up.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Well I have a board I built in the late 80's in front of me now with a MC74AC00N with one gate used as an inverting amplifier with a 47K feedback resistor and a 1nF input coupling capacitor which I used to recover the crappy 4 MHz base drive clock signal (a bit over half a volt peak to peak) from the Sinclair ZX Spectrum ULA and eventually derive a TTL level clock for interfacing various Z80 peripherals. The other gates were used normally. It never ran more than mildly warm and I wouldn't have cooked up such a wild scheme if I hadn't seen some prior art and reckoned it would work within my propagation delay budget. Presumably not all 'AC was created equal.
you suggest to replace the edge detector with an oscillator ?
that's not going to work well for some sorts of signal.
I've seen it used as input from VR speed/crank sensors. One gate running as an RC oscillator and lowpass filtered used to set the bias via resistors to the other inputs. So the bias was always centered in the trip point window (assuming gates in a package track) A resistor on each gate input limited current for high input voltages, so it worked from a few to a few hundred volts
-Lasse
Works well for sine waves, and other shapes as well, _if_ ~50% duty cycle.
...Jim Thompson
--
| James E.Thompson, P.E. | 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
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