LM393 Input Common Mode Voltage Range question

• posted

I've been looking at some op-amp material and I have a question. The datasheet shows a Max value for the "Input Common Mode Voltage Range" spec of Vcc-2 over the full temperature range. Ron Mancini says that this parameter refers to the average voltage at the input pins. If I'm using a 5V Vcc and have one input biased at 2.5V when the other input exceeds about 3.5V aren't I exceeding the spec? IOW, it appears that it's not kosher to drive the other input past 3.5V (3.5V + 2.5V) / 2 =

3V = Vcc-2

Am I understanding this? If so, any recommendations for a single supply comparator with a bigger common mode input range?

• posted

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.

• posted

The

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How did you determine this?

can

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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.

• posted

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

• posted

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     |
• posted

"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.

• posted

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

• posted

[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

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|  James E.Thompson, P.E.                           |    mens     |
• posted

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

• posted

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• posted

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......".

• posted

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. ;-)

• posted

```--
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```
• posted

"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? :-)

• posted

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     |
• posted

Walt

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).

• posted

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"?

• posted

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Not really...

• posted

martin

• posted

now.

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output

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?

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