DIFFERENTIAL SIGNALING - Advanced Signal Integrity for High-Speed Digital Design - page 294

DIFFERENTIAL SIGNALING

7.1 Removal of common-mode noise 299

7.2 Differential crosstalk 300

7.3 Virtual reference plane 302

7.4 Propagation of modal voltages 303

7.5 Common terminology 304

7.6 Drawbacks of differential signaling 305

7.6.1 Mode conversion 305

7.6.2 Fiber-weave effect 310

References 313

Problems 313

When the interconnections between drivers and receivers on a bus are imple-
mented with a dedicated transmission line for each bit, the signaling scheme is
said to be single ended . Buses designed with single-ended signaling generally
work well up to approximately 1 to 2 Gb/s. As data rates increase, it becomes
increasingly difficult to maintain adequate signal integrity because digital systems
are notoriously noisy. For example, large arrays of I/O circuits used to drive dig-
ital information onto the bus induce noise on the power and ground planes called
simultaneous switching noise (for a complete description, see a topic by Hall
et al. [2000]). There are many other sources of noise that can severely distort the
integrity of the digital waveform such as crosstalk (as discussed in Chapter 4)
and nonideal current return paths (as discussed in Chapter 10). With single-ended
signaling, each data bit is transmitted on a single transmission line and latched
into the receiver with the bus clock. The decision of whether the bit is a 0 or a
1 is determined by comparing the received waveform to a reference voltage v ref .
If the received waveform has a voltage greater than v ref , the signal is latched
in as a 1, and if it is below v ref , it is latched in as a logic 0. Noise coupled