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LM339 comparator input levels

I find two input voltage specifications in the LM339 data sheet, Vidr (input differential range = 36V) and Vicmr (input common mode range,

-0.3V to Vcc). I understand Vidr, but how do I interpret Vicmr? Is this stating the voltage range of the inputs if they move together at the same rate? In other words, the DC offset that effects both inputs equally?

Since the Vidr specification does not refer to Vcc, it appears that I can set IN- = 1V, Vcc = 5V and let IN+ swing from 0V to 36V. Is this correct? I am worried about letting either input exceed Vcc.

Thanks, David

Reply to
David Novak
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yes, i think you have it. CMR = Common mode rejection. Simple test. Take both inputs driven by the the same source with the unit set up to have equal gain on both inputs. the Rejection ratio is that of the difference that changes as you increase the input. the ideal Op-amp would have 0 change on the output from low to high on both inputs.

--
  and yes, you can allow it to swing to 36 volts.

and if the question ever comes up about offset voltages.
  that usually means if both inputs are set to 0 for example.
in a +/- Dual rail supply, the offset is usually the maximum
voltage that should appear at the output.
   but then again, i\'ve seen batches of cheap op-amps actually
perform far lower in the offset than specified in the specs.
   THe LN741 has offset inputs for bias to balance that problem, that
was nice to have to tweak up the problem how ever, you had to maintain
a steady temperature around the chips once set.
Reply to
Jamie

You are correct. It is fine to exceed Vcc with either input. This device is fairly unusual in this respect.

Reply to
John Popelish

Thanks for all the help! I received the following from an ON Semi FAE.

"On page 6 of the data sheet it states (clarifies) that Vdif can indeed be greater than the supply, as long as neither input goes negative by more than .3 V.

The common mode range is the input voltage where the part will function properly. This device has PNP inputs so once the input signal approaches the positive supply rail (within 1.5 - 2.0 V)the transistors turn off. So if you have a 5 V supply and the inputs are 0 V and 36 V, it will work ok since the Vdif is met and one input is within the common mode range. If the input signals are 5 and 36 V, Vdif is met but the common mode isn't so the output will be unknown. Neither of the input transistors are on so the output is not controlled. However, the part won't be damaged."

I had completely missed this page in the data sheet. It is worth noting that the previous paragraph on the same page recommends input resistors of < 10K and a < 10mV positive feedback.

David

Reply to
David Novak

Your last paragraph is correct.

However the specification is confusing. Inputs CAN exceed VCC but, for proper comparator decision output, ONE input must be below VCC by about 1.5V. (The National data sheet shows this.)

...Jim Thompson

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Reply to
Jim Thompson

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