Hi,
I am currently designing a two-stage open loop comparator (differential amp + current sink inverter output stage) using Cadence.
In order to ensure a "safe" and error-free operation of the comparator, it is said that the inputs of the device should lie within the defined input-common mode range (ICMR). To my understanding this means: If both inputs are within the ICMR, then all transistors should operate in saturation mode. Saturation mode, NOT just turned-on (i.e. saturation OR linear)?! Is that true? Correct me if I am wrong.
Maybe somebody could give a definition of the ICMR with respect to my specific application. i.e. Why does it matter?
Moreover, I encountered an issue when shifting one of the inputs to the lower ICMR-range and kept the other exactly in the center of the range. In the DC-simulation, one of my input transistors switched into linear operation mode. Is this behaviour normal or did I possibly calculate something wrongly?
Your help and hints are well appreciated :)
P.S. How can you "simulate" the ICMR in Cadence and verify you calculation results?
Input CM range can be an operating condition of your circuit. It serves to design the circuit that you want to observe with the comparator to operate in the region where the comparator behaves as specified.
Your understanding is almost correct. The ICMR is the DC range at which the input voltages are allowed to be, such that all the other specs of the comparator are still fulfilled. In the design you show, it applies to the DC gate voltages of M1 and M2. If it is lower than Vdssat(M5)+Vgs(M1/M2), the current source M5 will struggle and e.g. your slew rate specification would be compromised.
On the other hand, ICMR can also be a requirement for your circuit, if the inputs you want to observe are already defined. If it is e.g. stated as rail-to-rail ICMR, it means that your circuit must accommodate that range, in which case you'll have to make your design more complex, in case of the example, by providing the input circuit (M1-M5) in its flipped version (upside-down with all NMOSTs replaced by PMOSTs and vv), as well.
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