Increased EMI immunity in CMOS operational amplifiers using an integrated common-mode cancellation circuit

This paper presents an on-chip common-mode cancellation circuit which increases the immunity to electromagnetic interference (EMI) of integrated CMOS operational amplifiers when EMI is injected into their inputs. The circuits have been designed in the UMC 180nm CMOS technology. Two case studies have been considered: first, the common-mode cancellation circuit has been used in a Miller amplifier and secondly, in a folded cascode opamp topology. Circuit simulations and mathematical derivations illustrate how the proposed common-mode cancellation input stage strongly reduces the EMI susceptibility of both opamps, even in the presence of large amplitude interferences. The output offset voltages which are obtained when EMI amplitudes up to 3.3 Vpp are injected in the noninverting inputs of the proposed amplifiers when these are connected in a voltage follower configuration, are reduced by nearly one order of magnitude.