Re: biasing 74HC14 input to 1/2 VCC for small signals

Jim Thompson wrote:
On Sat, 20 Dec 2008 12:56:42 -0800, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Sat, 20 Dec 2008 08:58:00 -0800, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

IanM wrote:
James Arthur wrote:
Joerg wrote:
Jim Thompson wrote:
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.

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.

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
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.
As long as it's fed with that 4MHz clock it'll be ok. Try it without the clock.
Some of us, well-versed in the art of electronics (the skill, not the
book), know how to avoid that problem as well ;-)

I did it once by running the first stage as an oscillator that just locked onto the signal when one showed up. This produced utter disgust in a design review. It didn't even help when I mentioned that Drake and Barlow-Wadley had done similar things before (they hadn't heard those names).

Test question, what does this do...

C
V2-----SW1----+--||---+--74HCU04----+----OUT
| | |
V1-----SW3----+ +----SW2------+

SWx are analog switches.


It oscillateth :-)

But only if V1 and V2 are sufficiently low impedance.

--
Regards, Joerg

http://www.analogconsultants.com/

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