I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 478

The single-ended receiver is the original design for use with GTL signaling
and was designed to switch within v ref 50 mV across process, voltage, and

operating temperature in order to provide a high noise margin with an 800-mV
swing [Gunning et al., 1992]. The differential receiver is a self-biasing Chappell
amplifier that uses an internal feedback signal to adjust the bias voltage for
proper operation [Chappell et al., 1998].
As mentioned above, differential amplifiers respond to small changes in input
signals, as their symmetry gives low-input offset voltages and makes them rela-
tively insensitive to power supply fluctuations. Differential receivers also reject
common-mode noise, with typical common-mode rejection ratios (CMRRs) of
−

±

20 dB or more.

11.9.2 Modeling

We model differential amplifier-based receivers in the same fashion as we model
single-ended receivers. The simplest model is simply a capacitance to ground in
conjunction with the termination. With differential signals, we have multiple
options for termination, as Figure 11-34 shows. Note that the termination for dif-
ferential signals is typically implemented on the silicon die, resulting in nonlinear
current versus voltage characteristics that we may need to include in a behav-
ioral model. In addition, we need to include the parasite effects and nonlinear
characteristics of the ESD protection devices.

+

Z odd

Z odd

−

(a)

+

2 Z odd

−

(b)

Figure 11-34 Termination options for differential signals: (a) single-ended termination;
(b) differential termination.