Common mode response effects in differential measurements

Abstract

Differential measurements yield signals that are conditioned by differential or fully differential circuits which output depends on both the differential- and common-mode input voltages. Differential signals carry the desired information but common-mode voltages are a nuisance and their contribution to the output signal becomes an interference which is usually evaluated from the ratio between the respective gains for the differential- and common-mode input voltages, Common-Mode Rejection Ratio (CMRR). Usually, only the modulus of the CMRR is specified for integrated differential amplifiers. In this paper we demonstrate that the common-mode response strongly depends on component tolerance and can be band-pass with gain peaking and positive phase shifts inside the passband of the differential signal. Tolerances as low as ±0.01% cannot prevent phase shifts close to +90°. The presence of positive phase shifts in the common-mode gain can be suspected whenever the modulus of the CMRR decreases for frequencies below the -3 dB bandwidth of the differential gain but cannot be appraised from that modulus alone. Surprisingly, a high CMRR at low frequencies can worsen that effect. Therefore, common-mode effects in differential circuits can be evaluated only from the separate description of the response to differential and common-mode inputs signals.