Conformity Magazine - September 2008 - (Page 40)

LVDS (Low Voltage Differential Signaling) The differential signal transmission method allows the serialization of data and its transmission over a pair of PCB traces or wires. This is also called a double-ended system, and is represented as in the diagram in Figure 3. When the driver switches, it changes the direction of the current flow across the resistor, creating a valid “one” or “zero” logic state. The receiver has high DC input impedance; therefore, the majority of the driver current (about 3.5mA in the example in Figure 3) flows across the 100 Ohm termination resistor, generating about 350 mV across the receiver input. This creates a differential current flow across the pair. Because of the low voltage swing from one logic level to the other, the transition times between logic states are short, allowing high switching speeds without affecting the signal integrity. Some of the attributed benefits of the differential mode signaling are: • The timing of the signals can be more precisely defined, and the crossover point of the differential signals is easier to control, than the absolute voltage relative to some other reference (as in the case of single ended lines); • Higher speeds of data transmission are possible, due to the low swings in voltages; • Greater signal to noise ratios, due to the fact that the resulting signal is twice the magnitude at the receiver; • Differential mode noise is cancelled out, due to the opposite magnitudes (layout traces will be key to this). The actual implementation of differential pair in the various types of interfaces varies, but the general topology is as indicated in the diagram shown in Figure 4 for practical proposes. In general, it is composed of a source, a physical medium, a receiver (or sink), and a control protocol. Differential Mode and Common Mode Noise in Differentials Signals Figure 2: Evolution of various digital interfaces in the PC industry Figure 3: Simplified diagram of LVDS driver and receiver via 100Ohm differential impedance media1 (Reprinted courtesy of National Semiconductor) There is an important basic principle about the generation and emission of noise in differential signals. Since differential mode signals are transmitted in opposite phases, any differential noise (traveling opposite direction through the D+ and D- lines) is cancelled out. This represents an advantage because of the low risk of EMI emissions (see Figure 5). Common mode noise on the other hand (which travels in the same direction through D+ and D- lines), normally returns to Figure 4: Simplified differential mode signal method 0 Conformity September 2008

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2008

Conformity Magazine - September 2008 Contents Editor’s Note Commission Adopts Regulations for Internet-Based TRS Services Another Company Fined for Non-Compliance with HAC Requirements FCC Extends “Do-Not-Call” Registry Compliance Requirements Marketing Company Gets $1.4 Million Fine for Junk Faxes EU Commission Codifies Directive on TTE Market Competition EU Commission Releases Updated Standards List for the R&TTE Directive Updated Standards List for the “New” Low Voltage Directive Released New Standards List Released for the EU’s Machinery Directive The ESD Association: Voluntary Standards in Action ESD Open Forum Medical EMC Standards: Towards the 4th Edition of IEC 60601-1-2 iNARTE Informer Pre-Pulse Voltage in the HBM ESD Model Digital Interfaces and EMC Performance ESD Design Buyer’s Guide Product News Commission Publishes Updated Standards List for the EU’s Personal Protective Equipment Directive FDA Links “Human Factors” to Adverse Events with Medical Devices CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.