measuring input common mode range

[Arielski]

hi all,

can any one help me how to measure the input common mode range of a comparator.
im looking at a maxim data sheet MAX9015 and it states there that the ICMR of the comparator is from VEE-0.2 to VCC+0.2. this mean that the ICMR is beyond the two rails. im not sure if a buffer configuration can measure the spec'd value. the comparator is a single supply (gnd and VCC).
can somebody show me a test set-up to measure the ICMR.. thanks..

 

[pebe]

Why do you want to measure it? Maxim's data sheet clearly states, "Input Voltage Range Extends 200mV
Beyond-the-Rails".

Why not take it as read?

 

[Arielski]

Knowing how to measure something helps you understand and explain why a test set-up is like that or like this.. most test says that you have to configure ur comparator in the unity gain configuration and sweep the +input from rail to rail.. that is good enough but what if it goes beyond the rail. does the test-set up still give you a correct measurement?

 

[pebe]

The maximum input offset voltage is given as 10mv. So I think if I wanted to test if the device was in spec, I would take one input to 200mV above the positive rail and the other input to 188mV and measure the output. Then reverse the inputs and ensure that the output changed.

Then repeat with the same voltages below the negative rail.

 

[Arielski]

would it be possible to set it up in unity gain config and sweep the +in below the negative rail to above the positive rail and observe the behavior of the output. at +in level where the output goes out of linear response is the ICMR of the device. is this set up not appropriate? what i want is simply not to check if the device meet the specification but also to record what is the value of the ICMR.

 

[pebe]

If you set it up as a unity gain amp, then you will not be able to track the inputs beyond the supply rails, because the output is constrained within the rails.

 

[Arielski]

yes, i thought of that as well and i think the only way i can test the comparator is by open-loop configuration as you suggested previously..