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Design >> Analog Design >> Comparators input beyond supply rails
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Message started by niloun on Nov 10^th, 2017, 12:33pm

Title: Comparators input beyond supply rails
Post by niloun on Nov 10^th, 2017, 12:33pm

Hi everyone

I have simulated a StrongArm comparator which its inputs are beyond its supply (for instance: Vdd=0.9, Vi+=1.35+0.45*sinwt, Vi-=1.35) and surprisingly it works properly (even in the simulation corners). If it is so, why don't we decrease the supply to save power, what is wrong with decreasing the supply as much as possible for any input range?

Thanks in advance

Title: Re: Comparators input beyond supply rails
Post by DanielLam on Nov 10^th, 2017, 2:22pm

Oxide stress. The Vgs of the input devices can also be > Vdd and runs the risk of overstressing the oxide.

Btw for this nmos input comparator, the higher the common mode, the higher the input thermal noise.

Also, lower Vdd will give lower speed.

Title: Re: Comparators input beyond supply rails
Post by niloun on Nov 10^th, 2017, 11:48pm

DanielLam wrote on Nov 10^th, 2017, 2:22pm:

THanks DanielLam. I have attached two image from Razavi's survey on StrongArm. So you mean that in phase "b" transistors M1 and M2 receive an unusual amount of voltage which may cause damage to them?

Title: Re: Comparators input beyond supply rails
Post by neoflash on Nov 14^th, 2017, 4:44pm

If you don't need speed from the regeneration latch, you can reduce the supply to save power. Or alternatively, reduce the size of transistor at expense of larger offset.

niloun wrote on Nov 10^th, 2017, 12:33pm:

Title: Re: Comparators input beyond supply rails
Post by niloun on Nov 14^th, 2017, 11:40pm

neoflash wrote on Nov 14^th, 2017, 4:44pm:

Thanks, so there will be no problem if one wants to fabricate this circuit? Since I have heard about issues including : "bulk current" (which I have checked in the simulations and it was OK) or "latch up" or " oxide stress" which Daniel mentioned?

Title: Re: Comparators input beyond supply rails
Post by Frank_Heart on Nov 17^th, 2017, 12:08am

If your input device Vgs can handle (Vi+=1.35+0.45*sinwt), you don't have stress problem.

With lower supply for latch, as DanielLam and neoflash mentioned, your dynamic gain is small and speed for latch would be low.

And, how about the supply for the logics after comparator? You probably need a level-shifter to bring the voltage swing to normal supply range.

-Frank

Title: Re: Comparators input beyond supply rails
Post by niloun on Nov 17^th, 2017, 1:33am

Frank_Heart wrote on Nov 17^th, 2017, 12:08am:

Thanks for your answer Frank.
You are right but we can lower the supply of the subsequent blocks and use a level-shifter in the final stage if it is necessary to have the original logic levels. Am I wrong?

Title: Re: Comparators input beyond supply rails
Post by Frank_Heart on Nov 17^th, 2017, 8:19am

Niloun, I agree with you. As soon as the overall speed/noise performances meet system requirements. It's a good idea to lower down supply for saving power.

-Frank