Increasing the EMI immunity of CMOS operational amplifiers using an on-chip common-mode cancellation circuit

This paper presents an on-chip common-mode cancellation circuit which increases the common-mode rejection ratio (CMRR) in operational amplifiers, while at the same time increasing their immunity to electromagnetic interference (EMI). This common-mode cancellation circuit can be designed so as to double the differential gain while significantly reducing the common-mode gain. Simulations illustrate how the proposed amplifier exhibits an increased immunity to EMI injected in the opamp´s inputs. A case study example shows that the maximum output offset voltage which is obtained when an EMI amplitude of 1 V_pp is injected in the noninverting input of a Miller amplifier connected as a voltage follower is equal to 50 mV and 200 mV with and without the common-mode cancellation structure respectively. Finally, simulations show that the common-mode deleting circuit is not overly sensitive to mismatch.