How to measure hystersis and offset of a latched comparator?

[mmohsen]

offset latched comparator

Can anyone till me how to measure the hystersis and offset of the latched comparator (test bench ?)

Regards

 

[Syukri]

Re: Latched comparator

In order to measure offset and delay :

1. Vref = fix dc voltage = Full swing voltage/2 or usually VDD/2

2. Vin = ramp the input from 0 full voltage swing @ VDD

Run the simulation and then u will see the output response that it change from 0V to VDD ( digitally = from logic 0 to logic 1 ).

The distance of the point where Vin and Vref intercept to the actual rise of the vout from 0 to VDD is the offset or maybe just the delay.

 

[sankudey]

Re: Latched comparator

For the Offset:
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In general in simulation u will not find them unless u put some device mismatch in the circuit. Still if the design is having some systematic offset u could find them in the following way. The same applies when u give some mismatch and looking for the random offset coming from mismatch (this is a potential contributer). As u are talking of latched comparator the following applies to that only.

The offset is DC char. of a comparator. It could be measured as syukri has mentioned. But you have to run a long simualtion for the clock speed is higher. The other way is like following.
Give a DC voltage at V=+ (say ICRM- + (ICMR/2)). Now give V- a DC = V+ and run transient simulation. It will give either "1" or "0". Now go on incraesing the DC voltage for V- in step, i.e., say 1mV above/ below the previous voltage and again run simulaiton. Once you are getting a transition, try to fine tune the place by above/below 100uV and then 10uV and again simulation. The simualtions will not take much time. Now you will be getting a point where the comparator is transiting from 0->1 and vice versa.

If the two transion point are exactly centered abot V+ (= Vref) then no offset. Other wise offset is there. Now from the above three voltage levels (Vref, Vpt=> where it transits to 1 and Vnt; these are magnitude not the actual voltage level) you can calculate Offset and Minimum resolution.

Not to verify the above calculation; you give V+ same as Vref and give a square wave having zero and one state equal to the Vpt and Vnt above. The comparator now should change between 0 and 1. But unless you add the offset to either V+ or V-; it won't. Now you add the offset as calculated and watch for it.

Theoritically the square wave sould have same magnitude below and above as minimum resolution for +ve going and -Ve going is generally same.

Hysteresis:
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Alsmos in the same way. Just check the two transition points and difference from the V+ or centre. Hyst. = Vpt-Vnt.



The method above I followed my self and calculated in that way. I don't know whether you can can get it clearly.
gd luck...
sankudey