I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 448

The linear model provides an initial output voltage and current of 1.250 V and
25mA when driving the 50- transmission line. Thus, the linear model is a clear
source of inaccuracy for modeling an interconnect system, although it may be
good enough for low-speed designs or for use in the early design stages of a
high-speed link.

We have two options at our disposal if a better match between design and
model is required. The first is to modify the transmitter design to give it a more
linear current-voltage relationship. This is most easily accomplished by placing a
resistive element in series with the output transistor. From Example 11-1 we note
that the transmitter actually conducts more than 95% of the maximum current by
the time the output swing has reached one-half of the maximum value (1.25 V).
As a result, the transmitter has an effective output impedance of approximately
25 across the output voltage range 0 to 1.25 V (the linear region of operation)
and a very high impedance from 1.25 to 2.5 V (the saturation region). By adding
a 25- resistor in series with the effective 25 of the transistor, we can better
approximate the desired 50- impedance, as shown in Figure 11-8.
Since it requires modifying the design, this approach is typically taken only
when a constant impedance over the entire voltage swing is critical to the design
[Esch and Chen, 2004]. An example is the AGP4X interface, which used a
series-terminated interconnect to transmit graphics data at 266Mb/s. The interface
relied on the transmitter to provide the termination, which required linearization
of the voltage-current relationship [Intel, 2002].

I D

50

V DS

W/L = 222

I D

v GS = 2.5 V 25 W

V DS

40

W/L = 222

v GS = 2.5 V

30

I D

50 W

V DS

20

10

0

0 0.5 1.0 1.5 2.0 2.5

V DS (V)

Figure 11-8 Comparison of push-pull transmitter current versus voltage characteristics
to a linear model.