Re: LM339 comparator input levels

David Novak wrote:

I find two input voltage specifications in the LM339 data ***, 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
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.


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