The Advantages of Differential Signaling

Key Takeaways

• Differential signaling can minimize electromagnetic interference, crosstalk, and reflections if it is balanced. 

• Balanced differential signal pairs carry signals of equal amplitude but with a 180° phase shift. 

• Differential signaling requires lower voltages than the levels in a single-ended signaling system to maintain the same signal-to-noise ratio (SNR).

In high-speed PCB designs, maintaining signal integrity is essential

In high-speed PCB designs, signal integrity can be compromised by single-ended signals. The inability of single-ended signaling to secure critical and sensitive information from electromagnetic interference, crosstalk, reflection, and signal loss has prompted a switch to differential signaling in high-speed circuit boards. The advantages of differential signaling have established it as the ideal way to secure information within traces. 

Differential Signaling  

In high-speed PCBs and devices employing the latest technology, such as Ethernet, IoT, HDMI, USB 3.0, and others, differential signaling is standard. This signaling technique accomplishes data transmission using two wires or traces. The two traces carry separate signals, and the differential trace pairs are mostly balanced. At the receiving end, the voltage difference in the two traces retrieves information. Balanced differential signaling protects data from various issues such as electromagnetic interference, crosstalk, and reflections. If two traces’ signals encounter any of these problems, they get canceled at the receiving end because of the signals in the balanced differential pairs. 

Balanced Differential Signaling

Differential signaling can minimize electromagnetic interference, crosstalk, or reflections only if balanced. Balanced differential signal pairs carry signals of equal amplitude but with 180° phase shift. If not properly balanced, the differential signaling itself becomes a source of electromagnetic interference. 

The crosstalk or electromagnetic interference in differential signals adds up equally to the signals in the traces. As differential pairs carry inverted and non-inverted signals, the effects of reflections, crosstalk, and distortions follow the same pattern. When the difference signal is taken for retrieving data at the receiving end, unwanted effects like crosstalk or distortions are canceled out, and only the desired signal or data is received. 

Grounded Differential Signaling

Ideal differential signaling involves the net current flow through the pairs equaling zero. At zero, the requirement of ground in differential signaling is eliminated. However, most of the differential pairs keep a ground wire, so there can be a total of three wires in the differential signaling system. The inclusion of the ground connection becomes beneficial when multiple signals are transmitted using a differential signaling system. 

The Advantages of Differential Signaling 

Compared to single-ended signaling, differential signaling requires two traces for transmitting data. However, in high-speed PCBs, there are many advantages of differential signaling. Different differential signaling advantages include a high data rate, electromagnetic interference and ground noise immunity, power-saving properties, and precise timing and logic rates.

High Data Rate

The differential signal system in high-speed PCBs offers a high data rate compared to the single-ended signaling system. The number of traces in differential signaling is less than single-ended signaling for transmitting high bit rate data. 

Electromagnetic Interference Immunity 

Electromagnetic interferences, reflections, or crosstalk appear as common-mode noise or voltage at the receiving end in a differential signaling system. The common-mode noise interferes with the difference signal at the receiving end. Therefore, differential signaling systems are immune to the effects of electromagnetic interference or crosstalk. It even cancels harmonics and reduces odd harmonics with pre-distortion.

Ground Noise Immunity 

Though most differential signaling includes a ground system,  it doesn’t share a common ground reference with the receiving end. This characteristic enables immunity from ground noise. The detection of signals is not affected by changes in ground potential between the transmitting and receiving ends. 

Power Saving Due to Lower Voltages

Differential signaling requires lower voltages than those in a single-ended signaling system to maintain the same signal-to-noise ratio (SNR). The SNR is doubled in differential signaling versus single-ended signaling systems. This reduction in voltage levels saves significant power. 

Precise Timing and Logic States

In a balanced differential signaling system, the logic states of the signals can be easily determined. When a non-inverted signal is greater than the inverted signal in differential pairs, it corresponds to logic high and vice versa. The intersection of the non-inverted and inverted signals in the differential traces indicates the transition from one logic state to another. Differential signaling offers a faster switching time with the shorter rise and fall time of logic signals. 

The advantages of differential signaling elevate the design of high-speed circuit boards immune to electromagnetic interferences, crosstalk, reflections, distortions, and other issues. Cadence software offers PCB design and analysis tools for creating high-quality, high-speed circuit boards.

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