Single-Ended vs Differential Signaling
Single-ended and differential signaling are the two fundamental approaches to transmitting electrical signals. Single-ended is simpler; differential provides common-mode noise rejection that makes it essential for high-speed, long-distance, and noise-sensitive applications.
Single-Ended Signaling
Single-ended signaling uses one signal wire referenced to a shared ground. The signal voltage is measured relative to the common ground rail. Examples: UART, SPI, standard GPIO, TTL logic.
Advantages
- Simple — one wire per signal, shared ground return
- Fewer traces and PCB routing complexity
- Compatible with standard logic families (CMOS, TTL)
- Lower cost for short, low-noise interconnects
Disadvantages
- Sensitive to ground potential differences between boards
- Susceptible to common-mode noise (power supply ripple, EMI)
- Limited speed and distance due to ground impedance effects
- Ground loops cause errors in sensitive measurements
When to use
Use single-ended for short on-PCB connections, low-speed signals within a single board or supply domain, and simple GPIO. Keep traces short and provide good ground planes.
Differential Signaling
Differential signaling uses two complementary wires carrying opposite-polarity signals. The receiver detects the difference (V+ − V−), rejecting any common-mode noise present on both wires equally. Examples: RS-485, CAN, LVDS, USB, Ethernet, HDMI.
Advantages
- Excellent common-mode noise rejection (CMRR typically 60–80 dB)
- Immune to ground potential differences between ends
- Enables high-speed signaling: LVDS at 3+ Gbps, Ethernet at 10 Gbps
- Works over long cables: RS-485 to 1200 m, Ethernet to 100 m
Disadvantages
- Requires two wires per signal — doubles wire count
- Needs matched trace lengths (skew control) for high-speed signals
- Differential drivers/receivers add cost and IC count
- Requires controlled impedance PCB routing for high-speed designs
When to use
Use differential for inter-board and off-PCB signals, high-speed data (USB, HDMI, Ethernet), long cable runs, noisy industrial environments, and any signal above ~50 Mbps.
Key Differences
- ▸Single-ended: 1 wire + shared ground; differential: 2 wires with opposite-polarity signals
- ▸Differential CMRR is 60–80 dB; single-ended has no inherent common-mode rejection
- ▸Differential supports longer cable runs and higher speeds
- ▸Differential requires matched trace/cable pairs; single-ended does not
- ▸All high-speed standards (USB, PCIe, HDMI, Ethernet) use differential signaling
Summary
Single-ended is sufficient for short, on-PCB, low-speed signals in a controlled ground environment. Differential is required for inter-board, high-speed, or long-distance signals where noise, ground differences, or EMC are concerns. When in doubt for anything leaving a PCB, use differential.
Frequently Asked Questions
Why does differential signaling reject noise?
Noise picked up by a differential pair affects both wires equally (common-mode). The receiver subtracts V+ − V−, so common-mode noise cancels out. A 1 V noise burst on both wires results in zero differential noise, while a legitimate 100 mV differential signal is perfectly recovered.
What is LVDS?
LVDS (Low Voltage Differential Signaling) is a high-speed differential standard running at 1.2 V common mode with 350 mV differential swing. It achieves 1–3+ Gbps per lane and is used in FPGAs, camera interfaces (MIPI, FPD-Link), and display links (HDMI internals).
Does differential signaling require impedance matching?
For speeds above ~100 Mbps, yes. Differential traces should be routed as a matched pair with controlled differential impedance (typically 90–120 Ω for most standards). Length matching within the pair should be better than 5–10 ps skew at the receiver. At low speeds, impedance matching is less critical.
What is common-mode vs differential-mode noise?
Common-mode noise appears equally on both conductors (and is rejected by differential receivers). Differential-mode noise appears as a difference between the two conductors (and is indistinguishable from signal). EMI filters target both: common-mode chokes reject common-mode noise; differential capacitors filter differential-mode noise.