74LVT transition times: How low can you go?

I just remembered this...

Lose the XOR for your purposes.

This has delay plus snap action, so the edges don't get perverted like with typical RC delays.

...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've been delaying a response, because I beieve somebody has their head up, and I have come to the conclusion that it is me. I was thinking of old fashioned 4000 series CMOS, where the transistors are enhancement mode, with thresholds of something like 8V. At a Vcc of 12 V no input voltage can turn both units on, but as you reduce Vcc you come to the condition where both are on at some input level. I really am not familiar with the characteristics of the devices forming modern chips, so my opinion becomes relatively worthless.

--
 
 
 
 "A man who is right every time is not likely to do very much."
                           -- Francis Crick, co-discover of DNA
 "There is nothing more amazing than stupidity in action."
                                             -- Thomas Matthews

Threshold is much lower on CD4000 since you can operate them down to 5V VCC, some of them even at 3V although they become like molasses down there.

The problem is the opposite. Many people including me use them for analog purposes on occasion. If you bias an inverter that isn't a Schmitt at VCC/2 or via a feedback resistor and then run it at 12-15V VCC you will get an incredible cross current, to the point where the chip gets way too hot. Down at 3-5V things are more manageable but you still have both devices partially on. Just try it out. You can make quite good amplifiers for next to nothing in cost.

--
Regards, Joerg

http://www.analogconsultants.com

8V thresholds would have to be before my time ? ;)

For the 4000 series devices we use today, have a look at

In Fig 4., you'll see thresholds in the 1.5-1.75V region. HCMOS drops that to around 1V, and LVC drops that to around 0.6V

On the CPLD (ATF1502BE) we measured, the thesholds were

0.63V for NFET ~1uA, and -0.57V for the PFET ~1uA

-jg

yup .. and if you current feed them (supply thru a largish resistor) you can make nice low power (single digit uA region) 32KHz oscillators, from HEF4069UB.

-jg

Them's were the tube days. Of course, most of those are depletion mode devices. SCNR.

And the lower it goes the more leakage has to be dealt with. Probably Jim has pulled his hair at times about that. I couldn't because my hair is already gone :-(

--
Regards, Joerg

http://www.analogconsultants.com

The schematic solutions using 40xx and 74xx as the analog parts were quite popular and even published in the textbooks about 30 years ago. It kind of worked, but it never worked for very good. The linearity and the gain is quite poor. Changing to different logic series required components change if it worked at all. The CMOS devices operating in the linear mode were quite easy to fall into the thyristor latchup. So I would not recommend using the tricks like that for production.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

What many of those publications failed to mention is to take a very close look at the cross current specs (not all parts are spec'd for that) and set VCC accordingly. Or current limit/source the supply. Else, yes, it could go kablouie on you.

There are several popular products that use this trick and were in production for many years, if not decades. One example is the Datong RF clipper. It's the blue box in the first picture:

I've got one. Stunning performance.

--
Regards, Joerg

http://www.analogconsultants.com

I am not sure what you mean by old fashioned 4000 series CMOS, but at least the "McMos Handbook" second edition (1974) from Motorola at page

3-15 display the Idd for MC14011AL NAND gate for Vdd 10 and 15 volts.

At Vdd = 10 V, the drain current starts to rise at 1.5 V input voltage reaching a peak of about 2 mA at 4,5 V and falling back to zero at 8 V.

At Vdd=15 V, the drain current starts to rise at 2 V reaching a peak of 5-6.5 mA at 6,5 V and falling back to zero at 13 V.

Clearly both transistors are on even with Vdd=10 V for a long period of time.

About the question could the device self destruct due to too much heat, taking the peak current of 6.5 mA and the 15 V supply voltage, this is only 100 mW and with 4-6 elements in a package, we are talking about 400-600 mW, which is quite a lot for a 14-16 pin DIP package, but at least not destroy the package immediately.

Paul

Damn deserters! ;-) OTOH, I was completely gray by my 22nd birthday. A jerk that I used to work with loved to tease about it, but I kindly pointed out that he would never have to worry, because he was already half bald, and my age. His favorite line was, "Your hair dye can't handle the job", so I bought one of those temporary kits and used it. It was an almost perfect match for my hair when I was in high school. The shocked looks when i walked into work? ;-) Three days later it had washed out, without a comment. I caught them all together and told them I know i am 100% gray, but I don't care. You've seen how I used to look, but you'll never see it again. I'm on the inside, can't see my hair without looking into a mirror, and really don't care what color it is so laugh all you want.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

I have the same amount of hair now as I did as a teenager.

It's just that it's moved down to my nose, ears and eyebrows ;-)

...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.

When I stop coughing up chunks of phlegm and shaking, (over 10 days now) I'm going to try to get a decent picture of myself. A new Fuji Finepix S5200 digital camera was donated to my Veteran's website project, in trade for help launching a small business website. The woman wanted some pictures of her shop for her site. I told her that my camera wasn't very good. She asked what camera I wanted to use, then she bought the camera so I could take her photos. Then told me to use it for the Veterans project. :-)

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

My first white hairs were noticed at age 14, i was thouroghly salt and pepper by 17. I have thinned on top but it is still somewhat salt and pepper decades later. (way heavy on the salt though)

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

I was on antibiotics for abscessed teeth for over a month, a couple years ago. Towards the end of the treatment, some color returned. It lasted only till it grew out and was replaced with more gray hair. It was weird having gray hair with brown roots, followed by brown hair with gray roots. Some research I did years ago suggested bone disease as a cause for premature gray hair, but I couldn't access the referred medical journals to do more study.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

My grey hair is a known genetic attribute, my mum's hair was nearly white by age 18, and her mum's hair was white by 14 but thick and wavy into her

90's.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller