carry and vice versa. Uses of differential pairs The technique minimizes electronic crosstalk and electromagnetic interference, both noise emission and noise acceptance, and can achieve a constant or known characteristic impedance, allowing impedance matching techniques important in a high-speed signal transmission line or high-quality balanced line and balanced circuit audio signal path. Differential pairs include: Twisted-pair cables, shielded and unshielded Microstrip and stripline differential pair routing techniques on printed circuit boards Differential pairs generally carry differential or semi-differential signals, such as high-speed digital serial interfaces including LVDS differential ECL, PECL, LVPECL, Hypertransport, Ethernet over twisted pair, Serial Digital Interface, RS-422, RS-485, USB, Serial ATA, TMDS, FireWire, and HDMI, etc., or else high quality and/or high frequency analog signals (e.g. video signals, balanced audio signals, etc.). 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 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. A balanced pair of microstrip lines is a convenient solution