Design of a CMOS opamp input stage immune to EMI

Abstract

In this paper the behaviour of a complementary nMOS-pMOS differential pair in the presence of RF interference superimposed on the input terminals is analysed. An intrinsic nonlinearity cancellation mechanism in this structure is demonstrated, and proper design criteria are presented to exploit this mechanism in order to achieve a very high immunity to RF interference. A high-immunity complementary differential pair has been employed as an input stage of a folded cascode operational amplifier and its improved behaviour in the presence of RF interference has been verified by computer simulations.

Analysis of the Effect of Radio Frequency Interference on the DC Performance of CMOS Operational Amplifiers

“…This effect is equivalent to a change in the offset voltage of the operational amplifier. Reported experimental results [21] as well as qualitative analysis [13][14][15] are clearly indicative that the RFI demodulation in the operational amplifier is essentially originating from the differential input stage rather than in the later single-ended stages.…”

“…This phenomena is attributed either to an inherent square-law nonlinearity [5,11], or to a mixing-effect resulting from the fluctuations of the bias-current of the operational amplifier input differential stage [12][13][14][15][16][17][18][19][20]. While the square-law nonlinearity may arise from the asymmetrical characteristic of the CMOS differential amplifier resulting from mismatched MOS transistors [22], the mixing effect results from the fluctuations induced on the bias current; because of its finite shunt admittance, by the common mode voltage applied to the differential amplifier input terminals [13].…”