Input voltages beyond the supply rails

[niloufar-navidi]

Hi
I have simulated a simple dynamic comparator and it is working properly with input voltages beyond the supply rails, as I know we don't use input voltages beyond supply rails, Would you please explain why we cannot use inputs beyond the supply rails when the circuit is working properly?

Simulation below is for 0.9 v supply. One of the inputs is: 1.35 V fixed, and the other is a sine wave: 1.35+0.45sin(wt)

 

[erikl]

Yes, it should work, also with a real circuit: even if the standard input protection limits the input voltage to Vsupply + 1 diode forward drop (0.9+0.6 > 1.35V) this 1.35V input potential shouldn't be too large for NMOS gate inputs of a 180nm process.

 

[niloufar-navidi]

Thanks. I have seen that in many papers for SAR ADCs comparator is connected to VDD, although, as you see above it can work properly for much lower supply rails, It's a bit confusing because lowering the supply can definitely lower the dynamic power consumption, so I thought there should be design constraints that designers don't use lower supply voltages.