LM393 Input Common Mode Voltage Range question

If you have one input >3.5V and the other at 2.5V, the CM is immaterial because the comparator is overdriven into its output state. We have repeatedly told you that as long as one input remains below 3V, you can take the other one to 36V regardless of Vcc and the comparator operation will be the same. How difficult is this to understand? The CM specifications apply to the analog switching performance of the comparator at threshold, i.e. both inputs ~ equal, and not whole Volts after the switch has taken place.

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I guess I'm stupid Fred, since I just don't know when to listen to you instead of taking the datasheet at face value.

Ok, I just didn't see it explained like that in the Ron Mancini material or anywhere else for that matter. I'm not saying that I don't believe you, I'd just like to know how to determine things like this for myself.

Was Mancini talking about opamps or comparators? Is the LM393 an opamp or a comparator? Keep at it, it should come to you eventually.

Fred's explanation is correct.

For a comparator only one input need be =< VCM

For an OpAmp BOTH inputs are =< VCM, only because an OpAmp is defined as being in its linear region ONLY when both inputs are equal... otherwise it's a comparator too.

...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     |
             
     It\'s what you learn, after you know it all, that counts.

"Fred Bloggs"

So this means that the exact same terminology is meaningless in the comparator world? No wonder I find this confusing. Perhaps you could point me to something like the Mancini document but pertaining to comparators instead so that I no longer bother you with my trivial questions?

However if you can't put together a response without being rude and condescending, then do us both a favor and don't bother.

Thanks for the explanation Jim, I'm just trying to make sense of these parameters.

[snip]

You apparently haven't lurked here long enough to realize that Fred is treating you kindly... you should see his responses when he decides someone is a dummy ;-)

...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 | |

| 1962 | It's what you learn, after you know it all, that counts.

The best write-up of comparator operation, theory, and use is in Walt Jung's OP Amp Cookbook.

--- No.

Here's how common mode works:

Let's say that you have a +5V supply and an LM393 with 2.5V on the + input and 2.3V on the - input. The output will go open-collector, just like it's supposed to.

Now let's say that we have 2.5V on the + input and 2.7V on the - input. In this case the output will go to 0V. Again, just like it's supposed to.

Next, let's say that we increase the voltage on the + input to 3.1V and put 2.9V on the - input. The output will again go open-collector.

Finally, let's keep the + input at 3.1V and raise the voltage on the

- input to 3.3V. Will the output go to 0V? Theres's no telling, because we've exceeded the common mode range of the LM393 by allowing _both_ signals to to go higher than Vcc-2V.

---

--- No. As long as _one_ input is below the common-mode voltage of the part it will function properly.

---

--- There are any number of parts available. Go to TI, National, Linear, etc., and search for rail-to-rail input devices.

-- John Fields Professional Circuit Designer

I have. I'm firmly getting the impression that I'm on that list now. I'm not trying to be argumentative with him, but I would like a more technically based explanation than "We have repeatedly told you......".

OK, thanks. Now I hope that if I read something in there, and then ask about it here, that you won't point out that it's actually an op-amp book. ;-)

--
Perhaps Fred\'s getting a little irked because this is the kind of
stuff that should be posted to seb, where no one cares if it takes a
few iterations before the querant "gets" it.

"John Fields"

you......".

Well, I thought it was a straight-forward question. I had what I thought was an authoritative definition of the parameter and I had a datasheet with that parameter's value. It seemed a simple matter to do the math as I layed it out. Jim's explanation was helpful because he elaborated that the parameter was only really important when switching of the output was about to occur and that after switching it wasn't that important. That was piece that I was getting from the Mancini document or Fred's input. It seems sensible now since an op-amp is expected to remain linear over a broad range of inputs, and the comparator is at the opposite extreme by basically eliminating the linear region. In the comparator what happens after one input passes the other (and the output switches) is mostly irrelevant.

Am I getting close yet? :-)

Keep in mind... there are generally only two differences, circuit-wise, between an OpAmp and a comparator: OpAmps have compensation to make them stable under unity (usually) feedback, comparators do not; and comparator output stages are more amenable to connecting to logic.

...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     |
             
     It\'s what you learn, after you know it all, that counts.

Walt

ask

That's the impression I was under until he made the remark about Mancini's book being about op-amps, not comparators. I think I get it now (more or less, less I suppose).

I see that's available from Amazon, but I also see an apparently newer book (1st edition 2005): "Op Amp Applications Handbook ISBN 0750678445". This is an Analog Devices book as opposed to the Sams cookbook. I can get this book locally, do you think it would be a decent substitute for the "cookbook"?

--
Not really...

Didn\'t you read my explanation?

OK, I\'ll try again.

If you have a comparator with a Vcc of 5V and a common mode range of
Vcc - 2V, then that means that at least one of the inputs must be at
3V or less for the output to switch properly.  It doesn\'t matter
_which_ input, as long as it\'s one or both of them.

However, if _both_ inputs are above 3V, then all bets are off and
the state of the output won\'t depend on whether the + input is more
positive or more negative than the - input.  In other words, if both
inputs are more positive than 3V, the comparator stops being a
comparator.

I think this is the one that is downloadable from Analog

martin

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I think I understand exactly what you are saying. I'm trying to understand why the definition of the parameter (in terms of an op-amp) doesn't say that. IOW, why are the comparators special. If I understand Jim, the paramater is only important when the inputs are close to the switching point. For example, if one input is at 3V and the other input rises to exceed 3V, the output will be predictable. If the rising input continues to rise (meaning the mathematical average voltage of the inputs now exceeds the input common mode voltage range parameter), the output will remain predictable in that it wont suddenly invert on you. However, you will need to have things back within the specs before you can have another predictable transition of the output. Right?