Can we talk about common-mode input range for a StrongArm Latch

[doenisz]

Title.

I spent some time with this circuit, trying to tweak parameters for offset, kickback noise etc.



For a VDD of 0.8, I realized that when both inputs are like 751mV and 749mV, it could not charge neither OUTPB nor OUTNB enough to toggle the inverters. However, when it is 1.2V (just to try) and 300m, so that the "common-mode" is the same, it could make the decision correctly. Note that it's not about the differential voltage being huge either, since this comparator can easily detect 0.1mV differences when CM is around 300mV for example.

This led me to thinking, can we talk about common-mode input range for a strongarm latch? Or, is it about both inputs being high so that neither of the nodes are pulled-up high enough to get things rolling with the latching phase. But it's the initial dynamic amplification phase that's failing, so I can treat like the CM input range of an OPAMP?

Confused..

Thanks!

 

[nitishn5]

You have a PMOS input comparator. And a supply, VDD = 0.8V.

Now with input at 0.751V and 0.749V, your PMOS devices M1, M2, M7 and Mb are all dead. How do you expect your comparator to work with such inputs?
With the inputs at 1.2V and 0..3V, still one half your comparator is still dead. The |Vgs| is negative for the transistor with 1.2V input. The reason it could make the decision is that because it is now basically a simple Common source amplifier with the 0.3V input.

There is the Common Mode and Common Mode Range and the Input Differential Range. And I think you are having a confusion here.

Just because you can support a common mode of say 1V does not mean you can give inputs of 100V and -98V to a comparator with a supply of 2V.

 

[doenisz]

You have a PMOS input comparator. And a supply, VDD = 0.8V.

Now with input at 0.751V and 0.749V, your PMOS devices M1, M2, M7 and Mb are all dead. How do you expect your comparator to work with such inputs?
With the inputs at 1.2V and 0..3V, still one half your comparator is still dead. The |Vgs| is negative for the transistor with 1.2V input. The reason it could make the decision is that because it is now basically a simple Common source amplifier with the 0.3V input.

There is the Common Mode and Common Mode Range and the Input Differential Range. And I think you are having a confusion here.

Just because you can support a common mode of say 1V does not mean you can give inputs of 100V and -98V to a comparator with a supply of 2V.

Thank you, that helped me understand this.