Question

why differentrial signalling is better then common?

Answer 1

Single-Ended Signaling:

Single-ended signaling is a simple and common way of transmitting an electrical signal from a sender to a receiver. The electrical signal is transmitted by a voltage (often a varying voltage), which is referenced to a fixed potential, usually a 0 V node referred to as "ground."

One conductor carries the signal and one conductor carries the common reference potential. The current associated with the signal travels from sender to receiver and returns to the power supply through the ground connection. If multiple signals are transmitted, the circuit will require one conductor for each signal plus one shared ground connection; thus, for example, 16 signals can be transmitted using 17 conductors.

Single-ended topology

Differential Signaling:

Differential signaling, which is less common than single-ended signaling, employs two complementary voltage signals in order to transmit one information signal. So one information signal requires a pair of conductors; one carries the signal and the other carries the inverted signal.

Single-ended vs. differential: Generic timing diagram

The receiver extracts information by detecting the potential difference between the inverted and non-inverted signals. The two voltage signals are "balanced," meaning that they have equal amplitude and opposite polarity relative to a common-mode voltage. The return currents associated with these voltages are also balanced and thus cancel each other out; for this reason, we can say that differential signals have (ideally) zero current flowing through the ground connection.

With differential signaling, the sender and receiver don't necessarily share a common ground reference. However, the use of differential signaling does not mean that differences in ground potential between sender and receiver have no effect on the operation of the circuit.

If multiple signals are transmitted, two conductors are needed for every signal, and it is often necessary or at least beneficial to include a ground connection, even when all the signals are differential. Thus, for example, transmitting 16 signals would require 33 conductors (compared to 17 for single-ended transmission). This demonstrates an obvious disadvantage of differential signaling.

Differential signaling topology

Benefits of Differential Signaling

However, there are important benefits of differential signaling that can more than compensate for the increased conductor count.

No Return Current

Since we have (ideally) no return current, the ground reference becomes less important. The ground potential can even be different at the sender and receiver or moving around within a certain acceptable range. However, you need to be careful because DC-coupled differential signaling generally requires a shared ground potential to ensure that the signals stay within the interface's maximum and minimum allowable common-mode voltage.

Resistance to Incoming EMI and Crosstalk

If EMI (electromagnetic interference) or crosstalk (i.e., EMI generated by nearby signals) is introduced from outside the differential conductors, it is added equally to the inverted and non-inverted signal. The receiver responds to the difference in voltage between the two signals and not to the single-ended (i.e., ground-referenced) voltage, and thus the receiver circuitry will greatly reduce the amplitude of the interference or crosstalk.

This is why differential signals are less sensitive to EMI, crosstalk, or any other noise that couples into both signals of the differential pair.

Reduction of Outgoing EMI and Crosstalk

Rapid transitions, such as the rising and falling edges of digital signals, can generate significant amounts of EMI. Both single-ended and differential signals generate EMI, but the two signals in a differential pair will create electromagnetic fields that are (ideally) equal in magnitude but opposite in polarity. This, in conjunction with techniques that maintain close proximity between the two conductors (such as the use of twisted-pair cable), ensures that the emissions from the two conductors will largely cancel each other out.

Lower-Voltage Operation

Single-ended signals must maintain a relatively high voltage to ensure adequate signal-to-noise ratio (SNR). Common single-ended interface voltages are 3.3 V and 5 V. Because of their improved resistance to noise, differential signals can use lower voltages and still maintain adequate SNR. Also, the SNR of differential signaling is automatically increased by a factor of two relative to an equivalent single-ended implementation, because the dynamic range at the differential receiver is twice as high as the dynamic range of each signal within the differential pair.

The ability to successfully transfer data using lower signal voltages comes with a few important benefits:

• Lower supply voltages can be used.
• Smaller voltage transitions
  • reduce radiated EMI,
  • reduce power consumption, and
  • allow for higher operating frequencies.

High or Low State and Precise Timing

Have you ever wondered how exactly we decide if a signal is in a logic-high or logic-low state? In single-ended systems, we have to consider the power supply voltage, the threshold characteristics of the receiver circuitry, perhaps the value of a reference voltage. And of course there are variations and tolerances, which bring additional uncertainty into the logic-high-or-logic-low question.

In differential signals, determining the logic state is more straightforward. If the non-inverted signal's voltage is higher than the inverted signal's voltage, you have logic high. If the non-inverted voltage is lower than the inverted voltage, you have logic low. And the transition between the two states is the point at which the non-inverted and inverted signals intersect—i.e., the crossover point.

This is one reason why it is important to match the lengths of wires or traces carrying differential signals: For maximum timing precision, you want the crossover point to correspond exactly to the logic transition, but when the two conductors in the pair are not of equal length, the difference in propagation delay will cause the crossover point to shift.

Applications

There are currently many interface standards that employ differential signals. These include the following:

• LVDS (Low-Voltage Differential Signaling)
• CML (Current Mode Logic)
• RS485
• RS422
• Ethernet
• CAN
• USB
• High-quality balanced audio

Clearly, the theoretical advantages of differential signaling have been confirmed by practical use in countless real-world applications.

Conclusion

Differential signaling allows us to transmit information with lower voltages, good SNR, improved immunity to noise, and higher data rates. On the other hand, the conductor count increases, and the system will need specialized transmitters and receivers instead of standard digital ICs.