Comparator common-mode voltage

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LM139 datasheet has this footnote for the input common mode voltage range parameter, which is specified as min = 0v, max = Vcc-1.5 V in the electrical characteristics table:

Quote:
The upper end of the common-mode voltage range is Vcc-1.5 V, but either or both inputs can go to +30 V without damage.

So what does this mean? What's the point of the +30v part?

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The +30V means there are no clamp diodes to Vcc, and the input pins can drive above the rails (usually one at a time)

 

For correct operation, one IP pin needs to be within the common mode range nominally 0v ~(Vcc-1.5)v

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Yes, and that's how it always been with comparators. But in thins case, "either or both" implies that both can go up to 30v, hence the question.

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alexru wrote:

Yes, and that's how it always been with comparators. But in thins case, "either or both" implies that both can go up to 30v, hence the question.

 

Sure, it says both can go to 30V without damage, but the device does not operate like a Comparator when that happens.

Think Max rating vs operating specs.

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My guess it's a 30v part for level shifting. Since its a comparator with a open drain output I'm thinking you could have your inputs to the comparator in a different voltage range than the input of the driven device can tolerate. With one pullup to logic high of the connected device you could safely drive a input pin on a microprocessor even if the inputs being compared are in a higher voltage range. 

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Alex, I checked my TI datasheet for the LM139, original October 1979, revised in November 2006, and cannot find a specification that

either or both inputs can go to +30 V without damage.

The max allowed input voltage for both input pins include V- and V+, but the CMR figures are only valid from V- to V+ minus 1.5V

 

I attached the DS

 

Nard

 

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I've seen this in a DS for the ST part, specifically this one http://www.digikey.com/product-d...

 

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That note 3 applies only when Vcc = 30V. Page 5/19 of your DS (the ST one, as found at digikey) ; the note itself is on page 6/19

 

So: the max allowed input voltage for both input pins include V- and V+, but the CMR figures are only valid from V- to V+ minus 1.5V

 

The LM319 is an old workhorse, but the PNP inputs make it quite unique : compare works even when the signal goes as low as V-

 

Cheers

 

Nard

 

 

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And just for people coming here from search engines, TI DS actually spells out what happens if both of them are higher than common mode:

Quote:
When IN– and IN+ are both higher than common mode, the output is low and the output transistor is sinking current

 

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