thumb|upright=1.5|A signal transmitted differentially. Notice the increased amplitude at the receiving end.
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires in a twisted-pair or ribbon cable or traces on a printed circuit board.
Electrically, the two conductors carry voltage signals which are equal in magnitude, but of opposite polarity. The receiving circuit responds to the difference between the two signals, which results in a signal with a magnitude twice as large.
The symmetrical signals of differential signalling may be referred to as balanced, but this term is more appropriately applied to balanced circuits and balanced lines which reject common-mode interference when fed into a differential receiver. Differential signalling does not make a line balanced, nor does noise rejection in balanced circuits require differential signalling.
Differential signalling is to be contrasted to single-ended signalling which drives only one conductor with signal, while the other is connected to a fixed reference voltage.
Advantages
Contrary to popular belief, differential signalling does not affect noise cancellation. Balanced lines with differential receivers will reject noise regardless of whether the signal is differential or single-ended,
Data rate examples
Data rates of some interfaces implemented with differential pairs include the following:
- Serial ATA – 1.5 Gbit/s
- Hypertransport – 1.6 Gbit/s
- Infiniband – 2.5 Gbit/s
- PCI Express – 2.5 Gbit/s
- Serial ATA Revision 2.0 – 2.4 Gbit/s
- XAUI – 3.125 Gbit/s
- Serial ATA Revision 3.0 – 6 Gbit/s
- PCI Express 2.0 – 5.0 Gbit/s per lane
- 10 Gigabit Ethernet – 10 Gbit/s (four differential pairs running at 2.5 Gbit/s each)
- DDR SDRAM – 3.2 Gbit/s (differential strobes latch single-ended data)
Transmission lines
The type of transmission line that connects two devices (chips, modules) often dictates the type of signalling. Single-ended signalling is typically 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 signalling, however, is typically 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 signalling speeds become faster, wires begin to behave as transmission lines.
Use in printed circuit boards (especially computers)
Differential signalling 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.
Stripline
The traditional solution is to use a stripline, which forms a transmission line.
Microstrip pair
Microstrips also form transmission lines without needing an additional PCB layer, as a stripline does. A balanced pair of microstrip lines can very often suffice. 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.
Twisted traces
Using PCB vias it is also possible to create physically twisted traces with improved crosstalk characteristics compared to coupled microstrips.
High-voltage differential signalling
High-voltage differential (HVD) signalling uses high-voltage signals. In computer electronics, high voltage normally means 5 volts or more.
SCSI-1 variations included a high voltage differential 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 generic term high-voltage differential signalling describes a variety of systems. Low-voltage differential signalling (LVDS), on the other hand, is a specific system defined by a TIA/EIA standard.
Polarity switching
Some integrated circuits dealing with differential signals provide a hardware option (via strapping options, under firmware control, or even automatic) to swap the polarity of the two differential signals, called differential pair swapping, polarity reversion, differential pair inversion, polarity inversion, or lane inversion. This can be utilized to simplify or improve the routing of high-speed differential pairs of traces on printed circuit boards in hardware development, to help to cope with common cabling errors through swapped wires, or easily fix common design errors under firmware control.
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