Hello,
I would like to use comparator MAX40025A from Analog Devices. In the datasheet, it mentions that the input common range is from 1.5V to VDD + 0.1V which I don't understand the meaning. Does it mean both the input should be in this range, or the average of two inputs should be in this range?
I would like to compare a detector signal with a preset threshold. The signal is a step change voltage signal from 0 to around 400mV in several 10ns, and the threshold is constant at around 50-60mV.
May I ask how can I use the comparator in this case?

Thank you very much.
Siyao

 

Hello,
I would like to use comparator MAX40025A from Analog Devices. In the datasheet, it mentions that the input common range is from 1.5V to VDD + 0.1V which I don't understand the meaning. Does it mean both the input should be in this range, or the average of two inputs should be in this range?
I would like to compare a detector signal with a preset threshold. The signal is a step change voltage signal from 0 to around 400mV in several 10ns, and the threshold is constant at around 50-60mV.
May I ask how can I use the comparator in this case?

Thank you very much.
Siyao

It means that BOTH signal must be within the specified range for it to work properly. If you want to measure signals in the 0-400mV range, then either you need a different comparator (there are hundreds where the common-mode range includes zero) or you need a network to shift the input voltage.

 

I am not familiar with "input common range." The input common mode voltage iis the voltage between the input voltages source common, and the comparator's supply common, usually.
I suggest visiting the Analog Devices website and going through the application notes section for the comparator MAX40025A
Application notes are a great value for understanding how to use components to the greatest advantage.

 

Input Common Mode range, as used by most manufacturers in their data sheets, is a bit misnamed.

Strictly speaking, the common mode of the two input signals is simply the average of the two. But this is virtually never what the data sheets are actually referring to. Instead, they are talking about the allowed input range of each input without regard to the other.

 

Input common mode range is the linear range of an op-amp or a comparator. The linear range is defined by the structure of the input differential pair.

While op-amps will almost always implement negative feedback to force the IN+ and IN- pins to be at the same voltage (ignoring non-idealities) a comparator will have its IN+ and IN- pins at different voltages. Specifically for the MAX40025A:



The inputs should be no lower than 1.5V on a 3.3V supply; e.g., the range is 1.5V to 3.4V. Now, it is possible to have one of those inputs to ground and violate the Vcm spec. Will it damage the part? Hardly. Your mileage may vary though when it comes to the delay and timing specifications.

To be True and Honest™, you should look for a comparator that has a Vcm (also abbreviated as VICR, VICMR, depending on manufacturer to include 0V.

 

So if the common mode input range is 0 to 5V, and one input is 5V the other can be 10? I don’t think so.

I always interpreted it as the absolute min and max voltage that both of the two inputs most be between in order for it to work properly.

I didn't say that at all.

I said EACH input must be in the range without regard to what the other one is. If the other one is 10 V, that is hardly within the range 0 V to 5 V, now is it?

But if you take the spec COMMON MODE voltage input range to mean what it technically says, then one input could be at -5 V and the other at 10 V and that would meet the spec since the common mode voltage is 2.5 V. But this just underscores that when manufactures refer to this spec as the common mode input range, they don't really mean that it is the range that the common mode voltage must fall within. They mean, as I said, that it is the input range that each input must be within, without regard to what the other input happens to be.