Digital Signal Processing Reference
In-Depth Information
(a)
(b)
Figure 13.5
Measured eye diagram showing the link (a) with no equalization and (b) with equal-
ization. (
From:
[13]. Used with permission.)
13.3.1 Why Is the Differential Impedance Twice the Odd-Mode Impedance?
Figure 13.6 shows a 50
differential driver connected to a pair of traces that have
an odd-mode impedance of 50
Ω
Ω
. From (13.1) this setup has a differential imped-
ance of 100
, and we will prove this by calculating the voltages and currents
launched by the driver.
During the time
t
, the driver is transmitting a logic high value by sending a high
voltage on the
DO
+ line and a low voltage on the
DO
Ω
line. To find the differential
impedance, we must determine the values of these voltages and of the launched cur-
rent. With this we will use Ohm's law to find the impedance.
The termination resistance
R
term
placed across the diff-pair converts the
launched differential current into a differential voltage that is detected by the diff-
amp receiver. The received voltage swings about a common-mode voltage of 0.5V.
The diff-amp input terminals have very high impedance and they draw no current.
By using the voltage divider principle as described in Chapter 6, we might con-
clude that the 1-V, 50
−
transmis-
sion line. In fact, 0.75V is launched. To understand why this is so, we redraw the
circuit as shown in Figure 13.7, diagramming the common-mode voltage and the
current flow. The diff-amp receiver inputs draw no current and so can be ignored.
Ω
driver would launch a 0.5-V signal down the 50
Ω
