I. Basic Concepts and Definitions

1.1 Differential-Mode Signal

A differential signal refers to a pair of signals with equal amplitudes and opposite phases, transmitted between a pair of conductors. These signals are the useful signals we need, carrying the true information that the circuit is intended to convey.

The differential-mode signal, also known as the common-mode or symmetrical signal, is characterized by opposite current directions on the two signal lines, with their sum always equaling zero. In practical applications, the differential-mode signal forms the foundation for the proper functioning of electronic systems.

1.2 Common-Mode Signal

A common-mode signal refers to a signal that appears on a pair of conductors with equal amplitude and the same phase. Such signals are typically unwanted interference signals, generated by external electromagnetic interference or internal circuit asymmetry.

A common-mode signal, also known as a ground-induced signal or an asymmetric signal, is characterized by the same electrical potential existing between each of the two conductors and ground. In practical circuits, common-mode signals often constitute noise that is detrimental to the system.

1.3 Mathematical Expressions and Relationships

There are two signals, v₁ and v₂, which can be decomposed into common-mode components and differential-mode components:

Common-mode signal: Vcom = (v₁ + v₂)/2

Differential-mode signal: Vdiff = v₁ - v₂

Conversely, the original signal can be expressed as:

v₁ = Vcom + Vdiff/2

v₂ = Vcom - Vdiff/2

Table: Comparison of Common-Mode and Differential-Mode Signal Characteristics

II. Why is it necessary to distinguish between common-mode and differential-mode signals?

2.1 Actual Engineering Requirements

The core value of distinguishing between common-mode and differential-mode signals lies in effectively suppressing interference and improving signal quality. In practical engineering applications, the useful signals we need to transmit typically exist in differential-mode form, whereas most external disturbances tend to appear in common-mode form.

By distinguishing between these two signal modes, we can design targeted circuits that suppress common-mode interference while preserving and amplifying the differential-mode signal. This distinction significantly improves the system’s signal-to-noise ratio and its ability to resist interference.

2.2 Differences in the Nature of Interference

Common-mode interference: With relatively small amplitude and low frequency (primarily concentrated below 1 MHz), the resulting interference is relatively minor.

Common-mode interference: It has a large amplitude and high frequency (primarily concentrated above 5 MHz), can generate radiation through wires, and causes significant interference.

2.3 Understanding the Metaphor of the Ship and Water

This concept can be understood through a vivid analogy: Imagine two boats floating motionless on the water, each with a person—A and B—standing on board, holding hands with each other.

The differential-mode signal is akin to the relative height difference between two boats. As the boats bob up and down, A can sense the changing tension exerted by B. This relative change carries “information.”

A common-mode signal is akin to the average of the absolute heights of two ships above the seabed. When the water level rises or falls uniformly, A doesn't perceive any change in the tension. Such a uniform change carries "no information."

A well-designed circuit (such as a differential amplifier) should respond only to the differential-mode signal (the useful signal) and remain insensitive to the common-mode signal (the interference).

III. Problems Caused by Common-Mode Signals and the Necessity of Their Suppression

3.1 Main Hazards of Common-Mode Interference

Electromagnetic radiation interference: Common-mode currents flowing through wires can create a radiation antenna effect, causing equipment to exceed the limits in electromagnetic compatibility testing. Experimental data show that a common-mode current as low as 10 mA in the 30 MHz frequency band can generate a radiated field strength exceeding the CLASS B limit by more than 10 dBμV/m.

Conversion to differential-mode interference: When circuit impedances are unbalanced, common-mode interference is converted into differential-mode interference voltage according to the following formula: Vdm = [(Z₁ - Z₂)/(Z₁ + Z₂)] × Vcm. This voltage is directly superimposed onto the signal line, causing ADC sampling errors or communication bit errors.

Device reliability degradation: High-frequency common-mode currents passing through components such as bearings and connectors may trigger discharge corrosion, thereby shortening the equipment's service life.

3.2 Common-Mode Rejection Ratio (CMRR)

The common-mode rejection ratio (CMRR) is a key parameter for measuring a circuit's ability to suppress common-mode signals. It is defined as the absolute value of the ratio between the differential-mode signal gain and the common-mode signal gain. The larger the CMRR value, the stronger the circuit's ability to reject common-mode interference.

CMRR (dB) = 20 log₁₀ (Differential-mode gain / Common-mode gain)

The CMRR of typical operational amplifiers typically ranges from 70 dB to 120 dB, and high-precision op amps can even achieve a CMRR exceeding 120 dB. Under high-frequency conditions, however, the CMRR performance tends to degrade due to the influence of parasitic capacitance.

4. Mechanism of Common-Mode Interference Generation

4.1 Induction by External Electromagnetic Fields

External electromagnetic waves—such as those from lightning, equipment arcs, and radiation from nearby radio stations—induce the same voltage in the conductor-to-ground loop, thereby creating a common-mode current path. This induction is governed by the law of electromagnetic induction as described in Maxwell’s equations.

4.2 Ground Potential Difference Drive

When the equipment grounding system has impedance, the potential difference between different grounding points can drive current to flow through the signal lines, forming a loop. For example, due to differences in grounding resistance, equipment in the machine room and remote sensors may develop a potential difference of 0.5V, thereby driving common-mode currents on the order of several amperes.

4.3 Parasitic Effects of Transmission Lines

In high-speed circuits, voltage transients cause the parasitic capacitance of the circuit traces to charge and discharge, generating displacement currents that use the earth as their return path. A typical manifestation of this phenomenon is that when the drain voltage of the MOSFET in a switching power supply changes, a common-mode current is formed through the parasitic capacitance of the heat sink.

V. Common-Mode Interference Suppression Techniques and Design Practices

5.1 Common Inhibition Methods

Common-mode choke:

By utilizing a bifilar winding structure, the resulting co-directional magnetic field presents high impedance to common-mode signals while offering very low impedance to differential-mode signals.

When the frequency exceeds 1 MHz, a typical common-mode inductor can provide an impedance of over 600 Ω and attenuate interference currents by more than 20 dB.

Capacitor bypass: Connect a Y-type capacitor between the wire and ground to provide a low-impedance path for high-frequency interference. During design, ensure that f_cutoff = 1/(2πRC) > 10f_noise.

Shielding and Grounding:

Employing a metallic shielding layer with single-point grounding to eliminate ground loop potential differences; using twisted-pair wiring to reduce the loop area and minimize magnetic field induction intensity.

5.2 Key Guidelines for PCB Layout

Differential Pair Routing Rules:

Differential pairs should have equal length, equal width, and be closely spaced and located on the same plane. The most important rule is to match the line lengths; other aspects can be handled flexibly according to design requirements, avoiding unnecessary bends and branches while maintaining differential pair symmetry.

Ground plane design:

Maintain a continuous ground plane to provide a low-impedance return path for common-mode currents, avoid cracks in the ground plane, and prevent common-mode interference from being converted into differential-mode interference.

Isolation and Segregation:

Isolate high-frequency circuits from I/O ports to reduce parasitic capacitive coupling; adopt a partitioned layout for analog and digital circuits to minimize mutual interference.

VI. Real-World Application Cases and Troubleshooting

6.1 Common Application Scenarios

Differential transmission interfaces—such as RS-422/485, USB, and LVDS—leverage the strong anti-interference capability of differential signals to achieve reliable data transmission.

Instrumentation amplifier: With its high common-mode rejection ratio, it precisely extracts weak signals and is widely used in sensor signal conditioning.

Switching power supply: Employs common-mode filtering technology to reduce electromagnetic interference from the power supply to the external environment.

6.2 Troubleshooting Methods

Interference Source Identification:

Lightning, nearby arc discharges, and high-power radiation devices primarily generate common-mode interference on cables.

Motors, switching power supplies, thyristors, and other devices on the same power line primarily generate common-mode interference.

Instrument measurement:

Measure using a spectrum analyzer and current clamp;

By comparing the measurement results from single-line and dual-line configurations, identify interfering components.

VII. Checklist and Summary of Design Key Points

7.1 Common-Mode Rejection Design Checklist

[ ] Are the differential signal lines equal in length, equal in width, and closely spaced?

[ ] Were appropriate common-mode filtering components used?

[ ] Is the grounding system low-impedance and single-point grounded?

[ ] Are the shielding measures in place and properly grounded?

[ ] Are the high-frequency circuits and I/O ports adequately isolated?

7.2 Key Design Points

Symmetry is fundamental: Ensuring perfect symmetry in the differential paths is key to improving the common-mode rejection ratio.

Complete return path: Provides a low-impedance common-mode current return path, preventing common-mode interference from being converted into differential-mode interference.

Comprehensive Mitigation: Employing a combination of methods—including filtering, shielding, and grounding—to adopt a multi-pronged approach for suppressing common-mode interference.

Conclusion: Understanding common-mode and differential-mode signals is fundamental to electronic engineering design. Through this training tutorial, Leakin Technology’s design engineers should master the characteristics and distinctions of these two signal types, enabling them to effectively identify and suppress common-mode interference in practical designs, thereby enhancing product performance and reliability. Continuously staying abreast of signal integrity design techniques will help our company gain a competitive edge in the fiercely contested market.

This document is copyrighted by Shenzhen Leakin Technology Co., Ltd. and may not be distributed externally without permission.

Revision date: June 19, 2019

Applicable Scope: Internal Design Engineer Training