Hi
I am using this one:
Riding the input just between the lower and upper threshold.
Problem is that the cross current (IDD to GND) is high (se figure 12)
Anyone know of one that is equally fast, but with less current?
Cheers
Klaus
Hi
I am using this one:
Riding the input just between the lower and upper threshold.
Problem is that the cross current (IDD to GND) is high (se figure 12)
Anyone know of one that is equally fast, but with less current?
Cheers
Klaus
I often use LVDS line receivers as, effectively, screaming fast, fairly low power RRIO comparators.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
No. Trying to use a hysteretic logic device as a linear amplifier is a strict no-no. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
sounds like a challenge :) I can think of a couple of ways, neither of which I'd recommend.
NT
Yes, following your suggestion long ago, I've been using the 65LVDS2 for years, very nice. Idd = 2mA. Add some positive feedback for hysteresis.
>-- Thanks, - Win
Short answer: no. But the 'HC14 is a lot better Idd-wise, (see Fig. 10)
I've made micro-power pre-amplifier front-ends on slow stuff to get around this problem.
Lowering Vdd helps a lot, too.
Cheers, James Arthur
Interesting thought... use one section of a hex-inverter with input and output tied together (as a shunt regulator), the rest of package will pretty much track that bias. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Den onsdag den 6. december 2017 kl. 19.50.56 UTC+1 skrev Jim Thompson:
I've seen one section as an oscillator, the output low passed and used as bias for the other sections
I've used that inverter-as-a-bias scheme where I had control of the device sizes. For amusement, once I get other chores out of the way, I'll try it with something like a 74HCU04. (Don't try buffered, you're asking for trouble.) ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Or, maybe Klaus could put current-limiting resistors or diodes in the gate's power supplies... (don't forget to bypass!), lowering Vdd until the desired operating point (linear, or class C) is reached.
Vdd -+- | [R1] | |\| | \
--| O-- | / |/| | [R2] | ===
That automagically limits Idd.
As another idea, an ordinary gate with that treatment might be pressed into service as a fast, low-power preamp.
It all depends on Klaus' cost, speed, and power budgets, none of which are known.
Cheers, James Arthur
You could try resistors in both power supply leads. It will work for some applications. Its usually not a bad option for normal gates ran as linear amps, but not really so good for Schmitts.
-- Kevin Aylward
I am actually not using it as an amplifier, but as a driver for a FET to turn a load on and off
But my signal on the input is not fast changing, and for the topology I have right now, it is not possible to clean up the ramp (clean up by raising the rise time I mean)
What I really need is a schmitt trigger supplied from 5V, that will allow for the input signal to stay between the lower and upper hysteresis levels
The 74HC14 has max 140ns/V transition time on the input, and my design is near 500ns/V...
Cheers
Klaus
There are also some comparators around with fixed or programmable hysteresis. More expensive.
Somebody makes, essentially, the LVDS line receiver renamed as a comparator. The offset spec is a little better and price goes up about
8:1.-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Found one that has 5 times lower current, and meant to be used like that :-)
I would like to come down lower, but now the design is tolerable
Cheers
Klaus
Could you inject a little noise, or some clock spikes, into the front end?
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Found an even better one:
See figure 16, almost 500 times better than the one I started with :-)
Cheers
Klaus
That's the same part you just listed before. Fig. 16 is the typical additional Idd consumed when operated as a relaxation oscillator.
Fig. 13 is the applicable one for operation in your intended mode, Idd ~= 2.5mA.
I didn't see any dV/dt limitation in the 'HC14 datasheet, nor would I expect one. Where did I miss it?
Cheers, James Arthur
The manufactors are not agreeing on that part. The Diodes part has in "Recommended Op cond":
Cheers
Klaus
Sounds like silliness to me, but you could always use the Nexperia part if you're worried.
Being worried probably makes good sense. IME there is *considerable* performance variation between mfrs on these Schmidt-trigger parts. Notice that Diodes doesn't specify Iq in the transition zone. I'd choose Nexperia for that reason.
The 'HC14 draws about 300uA @ 4.5V, Vin=Vdd/2. If fast enough for you, that's about 1/10th of the
Cheers, James Arthur
Correct, the HC14 is good also
Seems like the AHC types are better in general than the LVC parts. Of course the LVC parts are something like 3 times faster than the AHC, so maybe it's just how it is biased
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