DS89C387 Datasheet, PDF (7/10 Pages) National Semiconductor (TI) – Twelve Channel CMOS Differential Line Driver

Application Information (Continued)
signal of the driver. This is important information because the
receiver will interpret the differential output signal. If the dif-
ferential transitions do not continuously ascend or decend
through the receivers threshold region, errors may occur. Er-
rors may also occur if the transitions are too slow.
In addition, complementary skew provides information about
the common mode modulation of the driver. The common
mode voltage is represented by (DO–DO*)/2. This informa-
tion may be used as a means for determining EMI affects.
Only “Skew” is specified in this datasheet for the DS89C387.
It refers to the complementary skew of the driver. Comple-
mentary skew is measured at both V3 and V4 (see
Figure 11).
More information can be calculated from the propagation de-
lays. The channel to channel and device to device skew may
be calculated in addition to the types of skew mentioned pre-
viously. These parameters provide timing performance infor-
mation beneficial when designing. The channel to channel
skew is calculated from the variation in propagation delay
from receiver to receiver within one package. The device to
device skew is calculated from the variation in propagation
delay from one DS89C387 to another DS89C387.
For the DS89C387, the maximum channel to channel skew
is 9 ns (t_p max–t_p min) where t_p is the low to high or high to
low propagation delay. The minimum channel to channel
skew is 0 ns since it is possible for all 12 drivers to have
identical propagation delays. Note, this is best and worst
case calculations used whenever Skew (channel) is not in-
dependently characterized and specified in the datasheet.
The device to device skew may be calculated in the same
way and the results are the same. Therefore, the device to
device skew is 9 ns and 0 ns maximum and minimum re-
spectively.
TABLE 1. DS89C387 Skew Table
Parameter
Min Typ Max Units
Skew (comp.)
0
0.5
3
ns
Skew (channel)
0
9
ns
Skew (device)
0
9
ns
Note Skew (comp.) in Table 1 is the same as “Skew” in the
datasheet. Also Skew (channel) and Skew (device) are cal-
culations, but are guaranteed by the propagation delay tests.
Both Skew (channel) and Skew (device) would normally be
tighter whenever specified from characterization data.
The information in this section of the datasheet is to help
clarity how skew is defined in this datasheet. This should
help when designing the DS89C387 into most applications.
DS89C387 Equivalent Input/Output Circuits
DS012086-17
FIGURE 12. Driver Output Equivalent Circuit
DS012086-18
FIGURE 13. Driver Input or
Driver Enable Equivalent Circuit
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