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Common-mode rejection ratio

The common-mode rejection ratio (CMRR) is a fundamental performance metric for differential amplifiers and operational amplifiers (op amps), quantifying the device's ability to suppress signals applied simultaneously and equally to both input terminals while amplifying the difference between them.^[1] It is mathematically defined as the absolute value of the ratio of the differential-mode gain (A_d) to the common-mode gain (A_{cm}), expressed as \text{CMRR} = |A_d / A_{cm}|, where an ideal value is infinite, indicating perfect rejection of common-mode signals.^[2] Typically specified in decibels as \text{CMRR}_{dB} = 20 \log_{10}(|A_d / A_{cm}|), practical op amps achieve 70 dB to 120 dB at low frequencies, with performance degrading at higher frequencies due to reduced open-loop gain.^[2]^[3] CMRR is essential in applications requiring high precision, such as instrumentation amplifiers, data acquisition systems, and current-sensing circuits, where it minimizes errors from common-mode noise sources like electromagnetic interference, ground potential differences, or power-line hum.^[3] In differential amplifier configurations, the overall CMRR is influenced not only by the op amp's inherent CMRR but also by external factors, including resistor mismatches (e.g., a 0.5% tolerance can limit system CMRR to about 40 dB) and layout-induced imbalances.^[3] High CMRR ensures signal integrity by converting unwanted common-mode voltages into negligible output contributions, thereby improving the signal-to-noise ratio in noisy environments.^[4] Measurement of CMRR involves techniques that apply controlled common-mode and differential inputs while monitoring output changes, often using precision resistor networks or power-supply perturbation methods to achieve accuracies beyond 100 dB; however, real-world limitations like component tolerances and frequency response must be accounted for in specifications.^[2]^[4] Advances in op amp design, such as those incorporating precision CMOS processes, have pushed typical CMRR values toward 150 dB or higher in specialized devices.^[3]

Fundamentals

Common-mode and differential-mode signals

In balanced transmission systems, such as two-wire lines, signals can be decomposed into differential-mode and common-mode components to distinguish between the intended information-carrying signal and unwanted interference. The differential-mode signal is defined as the voltage difference between the two conductors, expressed as V_{dm} = V_1 - V_2, where V_1 and V_2 are the voltages on the respective lines relative to a common reference. This component carries the primary information in applications like audio transmission over balanced microphone cables or data transmission in RS-485 networks, where the signal is intentionally antisymmetric across the pair to reject external noise.^[5] In contrast, the common-mode signal is the average voltage across the two conductors, given by V_{cm} = \frac{V_1 + V_2}{2}, representing noise or interference that appears equally and in-phase on both lines, often due to electromagnetic coupling from nearby sources or ground potential differences. In two-wire or twisted-pair configurations, such as those used in telephone or video lines, this common-mode voltage typically arises from external disturbances like power-line hum or radio-frequency interference, which affect both conductors symmetrically without contributing to the useful signal.^[5] These components form an orthogonal basis for analyzing signals in two-conductor transmission lines, allowing the total signal to be expressed as the sum of independent common-mode and differential-mode modes that propagate separately under balanced conditions. In phasor representation, the common-mode phasor aligns along the axis of equal voltages on both lines, while the differential-mode phasor is perpendicular, orthogonal to it, enabling decoupling in vector space for independent treatment in circuit analysis.^[6]

Definition and formula

The common-mode rejection ratio (CMRR) is a key performance metric for differential amplifiers, defined as the ratio of the differential-mode gain A_d to the common-mode gain A_{cm}, which quantifies the amplifier's ability to suppress unwanted common-mode signals while amplifying the desired differential signals.^[2] This ratio indicates how effectively the device rejects noise or interference that appears equally on both input terminals, such as ground loops or electromagnetic interference in balanced systems.^[7] The primary mathematical expression for CMRR is given by

\text{CMRR} = \left| \frac{A_d}{A_{cm}} \right|,

where A_d is the gain for differential inputs and A_{cm} is the gain for common-mode inputs.^[2] In practice, CMRR is often expressed in decibels (dB) using the logarithmic scale for easier comparison across frequencies and devices:

\text{CMRR (dB)} = 20 \log_{10} \left| \frac{A_d}{A_{cm}} \right|.

This logarithmic form is preferred because it aligns with measurement techniques and highlights the exponential nature of gain suppression, where even small improvements in A_{cm} yield significant dB increases.^[2] When equal-amplitude inputs are applied in the differential and common-mode cases, this is equivalent to \text{CMRR} = |V_{od} / V_{oc}|, where V_{od} and V_{oc} are the corresponding output voltages.^[2] CMRR is typically specified in dB, with higher values indicating better performance; an ideal amplifier achieves infinite CMRR as A_{cm} approaches zero, fully eliminating common-mode contributions at the output.^[2] In real devices, finite CMRR means that a portion of the common-mode input signal appears as an error at the output, superimposed on the differential signal and potentially degrading signal integrity, especially in noisy environments.^[7] This interference can be modeled as an effective input offset voltage proportional to the common-mode voltage divided by the CMRR.^[2]

Applications in Circuits

In operational amplifiers

Operational amplifiers (op-amps) function as high-gain differential amplifiers, featuring open-loop differential gains A_d typically ranging from $10^5 to $10^7 (or 100 dB to 140 dB) and very low common-mode gains A_{cm}, which renders the common-mode rejection ratio (CMRR) essential for suppressing unwanted common-mode noise in configurations such as inverting and non-inverting amplifiers.^[8]^[9] In these circuits, the op-amp's ability to amplify differential signals while rejecting common-mode voltages—such as power-line interference or ground noise—ensures accurate signal processing, particularly in precision analog applications where even small common-mode components can degrade performance.^[2] Typical CMRR values for op-amps at direct current (DC) range from 80 dB to 120 dB, reflecting the device's inherent balance in its differential input stage; however, this performance diminishes at higher frequencies due to mismatches in the input transistors or parasitic capacitances.^[2]^[3] In feedback loops, inadequate CMRR introduces output offset errors proportional to the common-mode voltage, potentially shifting the amplified signal and reducing overall circuit accuracy; for instance, in non-inverting configurations, the error can be expressed as E_{RTO} = (1 + \frac{R_2}{R_1}) \cdot \frac{V_{in}}{CMRR}, highlighting the need for high CMRR to minimize such deviations.^[2] A prominent example is the instrumentation amplifier, often built using op-amps, where CMRR exceeding 100 dB is required to faithfully capture low-amplitude biomedical signals like electrocardiograms (ECGs) amid substantial common-mode noise from the body or environment.^[10] The frequency dependence of CMRR in op-amps is commonly modeled as CMRR(f) = \frac{CMRR_0}{1 + j \frac{f}{f_c}}, where CMRR_0 is the low-frequency value and f_c represents the corner frequency, typically following the op-amp's open-loop gain roll-off at 20 dB per decade due to internal pole frequencies.^[2]^[3] This roll-off underscores the importance of selecting op-amps with appropriate bandwidth for frequency-specific applications to maintain effective noise rejection.

In differential amplifiers

The long-tailed pair configuration forms the core of many differential amplifiers, consisting of two matched bipolar junction transistors (BJTs) with their emitters connected together and tied to a constant tail current source that splits the bias current equally between the transistors under balanced conditions. This structure inherently suppresses common-mode signals by maintaining a virtual ground at the emitters for differential inputs, while the tail current source—often implemented with a high-value resistor or an active device—ensures minimal variation in emitter voltage for common-mode inputs. The common-mode rejection ratio (CMRR) in this topology relies critically on transistor matching, including minimal differences in base-emitter voltage drops (ΔV_{BE}) and tail supply current (ΔI_{SS}), as mismatches introduce imbalances that allow common-mode signals to appear at the output.^[11] In a BJT differential amplifier, the mismatch-limited CMRR can be approximated as

\text{CMRR} \approx \frac{2 g_m R_{SS}}{\frac{\Delta g_m}{g_m}},

where g_m is the transconductance of the transistors, R_{SS} is the small-signal tail resistance, and \frac{\Delta g_m}{g_m} quantifies the relative transconductance mismatch between the pair. This highlights the need for high tail impedance and precise matching to achieve high CMRR. This expression emphasizes the role of device symmetry, as even small mismatches in g_m—arising from variations in bias current or transistor parameters—can substantially degrade CMRR by increasing the common-mode gain. In integrated designs, monolithic matching achieves higher CMRR than discrete implementations, often exceeding 80 dB, while discrete circuits may require trimming for comparable performance.^[12] Imbalances in the circuit, such as emitter resistance mismatch, further limit CMRR as it disrupts the equal current splitting and introduces an offset in the common-mode response. Similarly, finite tail resistance or imperfect current sources amplify common-mode gain, underscoring the need for high-impedance tail elements in precision applications. Differential amplifiers find essential use in audio preamplifiers and RF mixers, where CMRR values greater than 60 dB are typically required to suppress ground noise, power supply hum, and electromagnetic interference that could otherwise corrupt the signal. In audio preamps, the configuration rejects common-mode noise from unbalanced sources, enhancing signal integrity in low-level microphone inputs; fully differential outputs in these circuits double the effective CMRR compared to single-ended outputs by balancing noise across both sides. In RF mixers, high CMRR isolates the desired differential RF signal from common-mode LO leakage or environmental noise, improving linearity and spurious rejection in receivers.^[13]^[12] Operational amplifiers can be viewed as an advanced implementation of the differential amplifier stage, incorporating additional gain and compensation for broader applications.

Applications in Transmission Systems

In baluns

Baluns are passive electrical devices designed to interface balanced and unbalanced transmission lines, converting between differential and single-ended signals while providing impedance matching and suppressing common-mode currents, particularly in antenna feeds and cable systems. By ensuring that common-mode signals—those appearing equally on both lines of a balanced pair—are rejected, baluns prevent unwanted noise and interference from propagating, thereby maintaining signal integrity. The common-mode rejection ratio (CMRR) in baluns quantifies this rejection capability as the attenuation of common-mode signals from the balanced to the unbalanced ports, typically expressed in decibels, where higher values indicate better isolation of modes.^[14]^[15] Various types of baluns exhibit differing CMRR performance based on their construction. Transformer baluns, which rely on magnetic coupling via a core, typically achieve CMRR values up to 40 dB, depending on the quality of coupling and materials used. Real-world performance is limited by factors such as leakage inductance and interwinding capacitance, which introduce imbalances. Transmission line baluns, exemplified by the Guanella design, employ quarter-wave transmission line sections to create mode-specific paths, yielding CMRR in the range of 30–50 dB across broad frequency bands by effectively presenting high impedance to common-mode signals while maintaining low loss for differential modes.^[16]^[15] In RF systems, such as cable television networks and Ethernet interfaces, baluns play a critical role in mode conversion and noise suppression. For instance, in cable TV setups, a balun matches 75 Ω coaxial lines to 300 Ω balanced antennas, where high CMRR prevents common-mode interference from degrading signal quality. Similarly, in Ethernet applications, baluns balance twisted-pair signals to reject electromagnetic interference (EMI). Poor CMRR in these contexts can lead to EMI radiation from cables acting as unintended antennas, increasing susceptibility to external noise and violating emissions standards.^[14]^[16]

In balanced transmission lines

Balanced transmission lines, such as twisted-pair cables, achieve inherent common-mode rejection through their symmetrical geometry, which causes external electromagnetic interference to induce nearly equal voltages on both conductors, manifesting as a common-mode signal that differential receivers can largely ignore. This balance is quantified by metrics such as longitudinal balance, often exceeding 60 dB for Category 5 (CAT5) cables at frequencies up to 10 MHz, depending on the cable's construction and measurement conditions, as specified in standards like TIA/EIA-568. The overall system CMRR is influenced by both the cable's balance and the receiver's rejection capability.^[17] The twist rate in balanced lines plays a key role in enhancing balance by promoting field cancellation; higher twists per unit length reduce the loop area exposed to magnetic fields, thereby minimizing differential-mode conversion from common-mode noise and improving rejection, particularly in environments with strong alternating magnetic fields like near power lines. Cable length influences performance as well, with longer runs leading to degradation due to cumulative attenuation and minor imbalances in conductor characteristics, such as varying capacitance or resistance along the line.^[18] In practical applications, such as RS-485 data communication links, balanced twisted-pair lines leverage their balance to suppress ground-loop noise and electromagnetic interference, enabling reliable multidrop networks over distances up to 1200 meters in industrial settings. Telephone lines also employ twisted-pair configurations for similar reasons, where the inherent balance prevents noise pickup from parallel power lines or radio sources, ensuring clear voice transmission in legacy POTS systems.^[5] Shielding further improves balance in balanced lines by enclosing the twisted pairs in a braided or foil conductor, which diverts electric fields and boosts rejection to around 60 dB or higher, though it adds distributed capacitance that can degrade signal integrity at higher frequencies by increasing attenuation and crosstalk. Proper grounding of the shield—typically at one end only—avoids ground loops that could otherwise counteract these benefits.^[19]^[18]

Measurement and Limitations

Measurement techniques

The standard method for measuring the common-mode rejection ratio (CMRR) involves applying an equal common-mode voltage V_{cm} to both inputs of the device under test (DUT), such as an operational amplifier or differential amplifier, while ensuring no differential signal is present by shorting the inputs together. The output voltage V_{out} is then measured, and the CMRR is calculated using the formula

\text{CMRR} = 20 \log_{10} \left( \frac{V_{cm}}{V_{out} / A_d} \right),

where A_d is the known differential-mode gain of the DUT, typically determined separately through prior characterization.^[4] This approach directly quantifies the device's ability to reject common-mode signals by comparing the applied common-mode input to the residual output after amplification by the common-mode gain. For operational amplifiers, test setups commonly employ precision voltage sources to generate the common-mode signal, often combined with a spectrum analyzer to assess both DC and AC performance across frequencies. The op-amp is configured in a unity-gain or specified gain mode, with the common-mode signal injected via a balanced source, and the analyzer captures the output spectrum to isolate common-mode artifacts from noise.^[20] In contrast, for baluns in transmission systems, a vector network analyzer (VNA) is used with common-mode port excitation, where the balanced ports are driven in phase to simulate common-mode currents, and S-parameters (e.g., S_{21} and S_{31}) are measured to compute CMRR as the ratio of differential to common-mode transmission, such as \text{CMRR} = \frac{|S_{ds21}|}{|S_{cs21}|}.^[21] To evaluate CMRR over frequency, a sweep is performed using the respective instrumentation, plotting CMRR in dB versus frequency to reveal bandwidth limitations where rejection degrades. Standard 50 Ω terminations are applied to unused ports in VNA setups to maintain impedance matching and minimize reflections during the sweep.^[21] For example, in op-amp testing, the sweep might cover 100 Hz to 100 kHz to identify roll-off points. A common pitfall in lab setups is the introduction of ground loops, which can inject unintended common-mode noise and artificially reduce measured CMRR by 10-20 dB, necessitating isolated power supplies or single-point grounding to mitigate.^[22]

Factors affecting CMRR

Component mismatch is a primary factor degrading CMRR in both amplifiers and transmission systems. In operational and differential amplifiers, resistor tolerances introduce imbalances in the input network, limiting the overall CMRR to the resistor matching quality. For instance, using 1% tolerance resistors in a unity-gain configuration can restrict the resistor-limited CMRR to approximately 34 dB, far below the op amp's intrinsic capability of over 100 dB.^[23] In balanced transmission lines, physical asymmetries such as bends cause differential-to-common-mode conversion, reducing CMRR.^[24] Temperature variations further impair CMRR through thermal effects on semiconductor parameters and passive components. In op amps, mismatches in transistor characteristics and resistor temperature coefficients lead to CMRR drift, typically on the order of -0.2 dB/°C in precision devices; for example, the OPA376 exhibits a drop from 90 dB at 25°C to 60 dB at 150°C due to these effects.^[25] At higher frequencies, parasitic capacitances between input stages or along transmission lines introduce common-mode paths, causing CMRR to roll off. In op amps, CMRR often declines from 120 dB at DC to below 80 dB above 1 MHz due to these imbalances.^[2] Mitigation strategies include common-mode chokes, which can boost effective CMRR by at least 20 dB by presenting high impedance to common-mode currents while allowing differential signals to pass.^[26] Environmental noise, particularly electromagnetic interference (EMI), couples asymmetrically into systems, reducing effective CMRR below device specifications. The overall system CMRR is determined by the minimum value among components, as imbalances in any stage propagate errors; for example, EMI-induced mismatches can degrade a 90 dB system CMRR to 60 dB or less in noisy environments.^[2]

References

[1]
CMRR - Analog Devices
Common Mode Rejection Ratio (CMRR) is the ability of a differential amplifier to not pass (reject) the portion of the signal common to both the + and - inputs.Missing: explanation | Show results with:explanation
[2]
[PDF] MT-042: Op Amp Common-Mode Rejection Ratio (CMRR)
The op amp common-mode rejection ratio (CMRR) is the ratio of the common-mode gain to differential-mode gain. For example, if a differential input change of Y.
[3]
None
### Summary of CMRR in Op-Amps and Differential Amplifiers
[4]
[PDF] On the Measurement of Common-Mode Rejection Ratio
Common-mode rejection ratio (CMRR) is the differential voltage gain divided by the common-mode voltage gain, or the change in input offset voltage from a unit ...
[5]
Understanding Common-Mode Signals | Analog Devices
Jun 29, 2003 · A differential-mode signal appears differentially on a pair of wires in an ungrounded cable configuration. A common-mode signal appears equally ...
[6]
[PDF] Modeling the Conversion between Differential Mode and Common ...
Mar 1, 2015 · Changes in electrical balance in a transmission line cause power exchange between differential and common modes, reducing signal power and ...
[7]
Wherefore Art Thou CMRR? - Analog Devices
CMRR is simply the quotient of the differential gain divided by the common-mode gain. It can be expressed in V/V or in dB according to the following ...Missing: formula | Show results with:formula
[8]
[PDF] ECEN 474/704 Lab 6: Differential Pairs
The CMRR is defined as the ratio of the differential gain and common mode gain: 20log. The input common mode voltage is limited in magnitude.<|control11|><|separator|>
[9]
[PDF] MT-033: Voltage Feedback Op Amp Gain and Bandwidth
If the op amp has a single pole, the open-loop gain will continue to fall at this rate as shown in Figure 1A. A practical op amp will have more than one ...
[10]
[PDF] Operational amplifier gain stability, Part 2: DC gain-error analysis
With the same open-loop-gain specifications of 130 dB. (typical) and 114 dB (minimum) at room temperature, and 110 dB (minimum) across the specified temperature.
[11]
Designing a Low-noise, High-resolution, and Portable Four Channel ...
... signal recording is the power-line interference. However, previous studies stated that the required CMRR for attenuating this unwanted signal is from 100 dB ...
[12]
7.5: The Differential Amplifier - Engineering LibreTexts
Jun 5, 2021 · This circuit is commonly referred to as a diff amp or as a long-tailed pair. ... redrawn for common mode rejection ratio (CMRR) analysis.
[13]
Chapter 12: Differential amplifiers - Analog Devices Wiki
Jun 6, 2017 · A differential amplifier multiplies the voltage difference between two inputs (Vin+ - Vin-) by some constant factor Ad, the differential gain.
[14]
[PDF] The BJT Differential Amplifier Basic Circuit - Marshall Leach
Fig. 3 shows the circuit diagram of a differential amplifier. The tail supply is modeled as a current source I/Q having a parallel resistance RQ.Missing: mismatch | Show results with:mismatch
[15]
A Deeper Look into Difference Amplifiers - Analog Devices
Some low-cost op amps have a minimum CMRR in the 60 dB to 70 dB range, making the calculation more complicated. Low Tolerance Resistors. The first suboptimal ...Missing: preamps RF mixers
[16]
Demystifying Transformers: Baluns and Ununs - Mini-Circuits Blog
Jul 9, 2020 · The Common-Mode Rejection Ratio (CMRR) is a measure of how well common-mode signals are attenuated from the balanced port to the unbalanced port ...
[17]
[PDF] Measuring HF Balun Performance - ARRL
The Common Mode Rejection Ratio (CMRR) of a balun is defined in professional literature1 as the ratio of wanted to un-wanted transmitted power. As rejection of ...
[18]
[PDF] BALUN BASICS PRIMER - TI E2E
The amount of attenuation this signal will experience from the balanced to unbalanced port is called common mode rejection ratio (CMRR) and is expressed in dB.
[19]
Copper Cabling for Digital Audio and Video Applications
The graph above shows typical longitudinal conversion loss or balance for Quabbin's 2-pair Cat 5e cable. It has better than 60 dB balance through 10 MHz.
[20]
[PDF] 1 Design of High-Performance Balanced Audio Interfaces
rejection ratio or CMRR (called "longitudinal balance" by telephone engineers) and is usually expressed in dB. There is an excellent treatment of this ...
[21]
[PDF] Common-Mode Noise - Sources and Solutions
Shielding a twisted-pair wire will provide better common mode rejection because the shield provides additional protection from electric and magnetic ...
[22]
How to Measure Op-Amp CMRR - Accel Instruments
Set the network analyzer to calibration mode and sweep over the frequency range (i.e. 100Hz to 100kHz) to be measured. Save the calibration data for later use.
[23]
[PDF] Balun Measurements with a 2-Port Vector Network Analyzer
Jun 8, 2022 · Both Common mode rejection ratios should be reasonably large numbers for a good balun. A ratio of 10 or more (linear magnitude) is reasonable.
[24]
https://www.academia.edu/89491955/Suppression_of_Mode_Conversion_by_Using_Tightly_Coupled_Asymmetrically_Tapered_Bend_in_Differential_Lines
[25]
Increasing the Common-Mode Rejection Ratio of Differential ...
Nov 13, 2018 · The ability of a differential amplifier to reject this common-mode error is given in terms of the common-mode rejection ratio (CMRR).
[26]
Suppression of Mode Conversion by Using Tightly Coupled ...
In this paper, we propose a tightly coupled asymmetrically tapered bend to suppress differential-to-common mode conversion caused by bend discontinuity in a ...
[27]
OPA376 issue in high temperature - Amplifiers forum - TI E2E
The datasheet shows the CMRR changes from 90dB @25C to 60dB @150C, at hot temperature, where CMRR is specified above V(-) supply. The datasheet offers no ...
[28]
How to Compensate for CMRR Variations in Op-Amp - LinkedIn
Mar 20, 2023 · The main reason why CMRR changes with temperature is because the internal resistors and transistors of the op-amp have temperature coefficients ...Missing: semiconductors | Show results with:semiconductors
[29]
The broadband common mode choke, 1:1 balun and 1:1 unun - PA9X
Ideally, a common mode choke should present a minimum of 20 dB suppression of common mode currents. There are several ways to achieve that. Air core choke.