Commonly available comparators like the dual LM393 and its quad brother the LM339 have a specification parameter called "Common Mode Input Range". This parameter is specified, in single supply operation, to be 0 VDC up to Vcc - 2V.
The first thing that the datasheet says is that if one or both inputs are outside of this range, the part will not be damaged. That's a good thing. The next item from one of the data sheet footnotes is that the output will be in the correct state if either one of the inputs is within the common mode range. OK, not great, but not horrible.
I understand that when specifications are violated there are no guarantees, but I really would like to know what happens when both inputs are in the Vcc - 2 Volts to Vcc range. Will the comparator change states, according to the relationship between the inputs?
I think the answer is yes. I think the specification which is not guaranteed under these conditions is the propagation delay. In SPICE simulations, this is the only effect I can observe, using at least three different comparator models. I also know that simulations and models do not always give an accurate picture of what is going on, so I'd be interested to hear anybody's thoughts on the matter.
The first thing that the datasheet says is that if one or both inputs are outside of this range, the part will not be damaged. That's a good thing. The next item from one of the data sheet footnotes is that the output will be in the correct state if either one of the inputs is within the common mode range. OK, not great, but not horrible.
I understand that when specifications are violated there are no guarantees, but I really would like to know what happens when both inputs are in the Vcc - 2 Volts to Vcc range. Will the comparator change states, according to the relationship between the inputs?
I think the answer is yes. I think the specification which is not guaranteed under these conditions is the propagation delay. In SPICE simulations, this is the only effect I can observe, using at least three different comparator models. I also know that simulations and models do not always give an accurate picture of what is going on, so I'd be interested to hear anybody's thoughts on the matter.