Differential signaling

This article is about electric signals via wires. For an immunological model attempting to explain how T cells survive selection during maturation, see Differential Signaling Hypothesis.

Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet (twisted-pair only), PCI Express and USB. The opposite technique, which is more common but lacks some of the benefits of differential signaling, is called single-ended signaling.

Advantages

Tolerance of ground offsets

In a system with a differential receiver, desired signals add and noise is subtracted away.

At the end of the connection, the receiving device reads the difference between the two signals. Since the receiver ignores the wires' voltages with respect to ground, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal.

Suitability for use with low-voltage electronics

In the electronics industry, and particularly in portable and mobile devices, there is a continuing tendency to lower the supply voltage in order to save power and reduce unwanted emitted radiation. A low supply voltage, however, causes problems with signaling because it reduces the noise immunity. Differential signaling helps to reduce these problems because, for a given supply voltage, it gives twice the noise immunity of a single-ended system.

To see why, consider a single-ended digital system with supply voltage $V_S\,$. The high logic level is $V_S\,$ and the low logic level is 0 V. The difference between the two levels is therefore $V_S - 0\,\mathrm{V} = V_S$. Now consider a differential system with the same supply voltage. The voltage difference in the high state, where one wire is at $V_S\,$ and the other at 0 V, is $V_S - 0\,\mathrm{V} = V_S$. The voltage difference in the low state, where the voltages on the wires are exchanged, is $0\,\mathrm{V} - V_S = -V_S$. The difference between high and low logic levels is therefore $V_S - (-V_S) = 2V_S\,$. This is twice the difference of the single-ended system. Supposing that the voltage noise on one wire is uncorrelated to the noise on the other one, the result is that it takes twice as much noise to cause an error with the differential system as with the single-ended system. In other words, the noise immunity is doubled.

Resistance to electromagnetic interference

This advantage is not actually due to differential signaling itself, but to the common practice of transmitting differential signals on balanced lines.^[1][2] Single-ended signals are still resistant to interference if the lines are balanced and terminated by a differential amplifier. See Balanced line for more details.

Comparison with single-ended signaling

In single-ended signaling, the transmitter generates a single voltage that the receiver compares with a fixed reference voltage, both relative to a common ground connection shared by both ends.

The widely used RS-232 system is an example of single-ended signaling, which uses ±12 V to represent a signal, and anything less than ±3 V to represent the lack of a signal. The high voltage levels give the signals some immunity from noise, since few naturally occurring signals can create that sort of voltage. They also have the advantage of requiring only one wire per signal. However, they also have a serious disadvantage: they cannot run at high speeds. The effects of capacitance and inductance, which filter out high-frequency signals, limit the speed. Large voltage swings driving long cables also require significant power from the transmitting end. This problem can be reduced by using smaller voltages, but then the chance of mistaking random environmental noise for a signal becomes much more of a problem. In many instances single-ended designs are not feasible. Another difficulty is the electromagnetic interference that can be generated by a single-ended signaling system which attempts to operate at high speed.

Examples

Examples of differential signaling include LVDS, differential ECL, PECL, LVPECL, current loop interfaces such as Musical Instrument Digital Interface (MIDI) hardware, RS-422, RS-485, most Ethernet physical layers, USB, Serial ATA (SATA), TMDS, FireWire, and HDMI. LVDS is currently the only scheme that combines low power dissipation with high speed.

Examples of single-ended signaling include RS-232 and PATA.

The lowest-power-dissipation, highest-speed signals in any commercially available system are the on-chip signals in a microprocessor. Those signals are almost always single-ended.

In an electric guitar, the humbucker pickup type uses exactly the same principle to avoid power supply hum at the AC frequency.^{[citation needed]}

Transmission lines

The type of transmission line used to connect two devices (chips, modules) dictates the type of signaling to be used. Single-ended signaling is used with coaxial cables, in which one conductor totally screens the other from the environment. All screens (or shields) are combined into a single piece of material to form a common ground. Differential signaling is used with a balanced pair of conductors. For short cables and low frequencies, the two methods are equivalent, so cheap single-ended circuits with a common ground can be used with cheap cables. As signaling speeds become faster, wires begin to behave as transmission lines.

Use in computers

Differential signaling is often used in computers to reduce electromagnetic interference, because complete screening is not possible with microstrips and chips in computers, due to geometric constraints and the fact that screening does not work at DC. If a DC power supply line and a low-voltage signal line share the same ground, the power current returning through the ground can induce a significant voltage in it. A low-resistance ground reduces this problem to some extent. A balanced pair of microstrip lines is a convenient solution, because it does not need an additional PCB layer, as a stripline does. Because each line causes a matching image current in the ground plane, which is required anyway for supplying power, the pair looks like four lines and therefore has a shorter crosstalk distance than a simple isolated pair. In fact, it behaves as well as a twisted pair. Low crosstalk is important when many lines are packed into a small space, as on a typical PCB.

High-voltage differential signaling

High-voltage differential (HVD) signaling uses high-voltage signals. In computer electronics, "high voltage" normally means 5 volts or more.

SCSI-1 variations included a high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version. SCSI equipment for example allows a maximum total cable length of 25 meters using HVD, while single-ended SCSI allows a maximum cable length of 1.5 to 6 meters, depending on bus speed. LVD versions of SCSI allow less than 25 m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than the older HVD SCSI.

The term high-voltage differential signaling is a generic one that describes a variety of systems. Low-voltage differential signaling or LVDS, on the other hand, is a specific system defined by a TIA/EIA standard.

See also

• Current mode logic (CML)
• Low-voltage differential signaling (LVDS)
• Low-voltage positive emitter-coupled logic (LVPECL)
• Positive emitter-coupled logic (PECL)
• EIA-422 (RS-422)
• Transition Minimized Differential Signaling (TMDS)
• Longitudinal voltage
• Differential amplifier
• Differential pair
• Twisted pair
• Current loop signaling

References

1. ^ Graham Blyth. "Audio Balancing Issues". Professional Audio Learning Zone. Soundcraft. http://www.soundcraft.com/support/white_papers.aspx#. Retrieved 2009-08-25. "Let’s be clear from the start here: if the source impedance of each of these signals was not identical i.e. balanced, the method would fail completely, the matching of the differential audio signals being irrelevant, though desirable for headroom considerations." 
2. ^ "Part 3: Amplifiers". Sound system equipment (Third edition ed.). Geneva: International Electrotechnical Commission. 2000. p. 111. IEC 602689-3:2001. "Only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal."