Differential Signaling: Advantages and Disadvantages

This article explores the pros and cons of differential signaling. It covers the benefits and drawbacks to help you understand when and why it’s used.

What is Differential Signaling?

Introduction:

Differential signaling is a specialized form of two-wire signaling where two complementary signals are transmitted over matched traces or wires.

• It’s a type of balanced signaling.
• The signal is measured by the difference between the positive and negative sides.
• Differential signals generally have lower distortion compared to single-ended circuits.
• It’s used to transmit information electrically using two complementary signals.

• The receiver responds to the difference in electrical potential between the two signals, rather than the difference between a signal wire and ground (as in single-ended signaling). As shown above, input signals are combined, while noise signals are suppressed by the differential receiver.
• Differential signaling is used in both analog and digital systems:
  • Analog Differential Signaling: Balanced Audio
  • Digital Differential Signaling: Twisted pair, PCIe, DisplayPort, USB, HDMI, RS-422, RS-485

Benefits (Advantages) of Differential Signaling

Here are the key advantages of using differential signaling:

• EM Interference Resistance: It’s resistant to electromagnetic (EM) interference because the signals are transmitted on balanced lines.
• Reduced Crosstalk: Helps minimize electronic crosstalk between signals.
• High-Speed Transmission: Well-suited for high-speed transmission lines due to noise elimination and reduced radiation. This leads to better common-mode rejection and improved power supply rejection.
• Higher Signal Swings: Can achieve larger overall signal swings.
• Harmonic Cancellation: Offers inherent cancellation of even-order harmonics. Pre-distortion techniques can further reduce odd harmonics.

Drawbacks (Disadvantages) of Differential Signaling

These are the main disadvantages of differential signaling:

• Increased Trace Count: The most obvious disadvantage is that it requires two traces instead of one for each signal, essentially doubling the board area required.
• More Conductors for Multiple Signals: When transmitting multiple signals, the number of conductors required is higher than with single-ended signaling. For example, transmitting 8 differential signals requires 16 conductors, plus a ground conductor for a common ground between sender and receiver. Single-ended signaling would only need 9 conductor wires in the same scenario.