Common mode response effects in differential measurements

Abstract

In differential measurements, before digitized, the signal is conditioned by a differential or fully differential circuit which output depends on both the differential-and commonmode 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 IC differential. In this paper we demonstrate that the common-mode response depends on component tolerance and can be band-pass with gain peaking and positive phase shifts inside differential signal passband. Tolerances as low as ±0.01% cannot prevent phase shifts close to +90º. 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 commonmode inputs signals.

Buck Converter Current Measurement Using Differential Amplifier

“…The Common Mode Rejection Ratio (CMRR) [16,17] is a significant parameter for Op-Amp circuits which shows the ability of an Op-amp on Common mode signal or common mode noise rejection. The Op-Amps used for Buck converter current measurement are designed to measure DC currents with high frequency ripples and high frequency pulse currents.…”

Evaluating Common-Mode Voltage Based Trade-Offs in Differential-Ended and Single-Supplied Signal Conditioning Amplifiers

“…On the other hand, fulfilling the input restrictions relating to CMV (6) is often marginalized in practice, especially in singlesupplied OP circuits. Usually, this occurs because most users merely focus on CMV rejection, i.e., CMRR capabilities [17,18]. The other cause might be that they do not differentiate between the system-related CMV from one present on OP inputs.…”

“…Since the same argument applies to u out in (15), their magnitudes cannot be arbitrarily chosen. Namely, according to (15), the CMV applied (i.e., u gG ) affect neither of the u CM,OP (16) nor (17), but it does affect the u in+ = u meas + u gG , and the output voltage of the first OP consequently…”