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Topic: Level shifting op amp? (Read 2009 times) previous topic - next topic

MarkT

Ah, yes I get LM339 and 74HC393 mixed up sometimes!

Modern and especially high speed comparators do typically have a small amout of built in hysteresis,
or a hysteresis programming pin - it prevents bursts of oscillation on slow waveforms and speeds up
response (at the expense of precision).

I suspect you only have to layout the circuit on the chip the right way round for stray capacitance to
add a little (short-term) hysteresiss (the other way round would reduce the slew rate via Miller effect).  

Fast comparators are basically unstable very high gain amplifiers (try to use one as an opamp and it will oscillate),
so there are always going to be internal feedback paths wanted or not, it makes sense to ensure positive
feedback wins over negative.  If the feedback is only capacitive then the dc offset precision need not
be affected - in otherwords the in-built hysteresis might not have any effect on low speed performance.
(other than to remove/reduce oscillatory bursts at cross-overs)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

meme

#16
Jul 15, 2017, 10:42 pm Last Edit: Jul 15, 2017, 10:48 pm by meme
After looking your conversation it's better to go with comparator than op amp which has many pitfalls.

So, I'll just buy both LM 339 and LM 393. I would really prefer one which give output voltage swing as close to ground that is zero to 3.3 V in my case. Once, I have the clear signal, then due can pretty much do the rest.

I have checked both datasheets, they both have low offset voltage.

EDIT:
It was pretty evident that, LM 741 was a bad choice. The voltage doesn't swing close to zero. 

Thanks,
Meme

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