DS89C387TMEA Datasheet, PDF (7/12 Pages) National Semiconductor (TI) – DS89C387 Twelve Channel CMOS Differential Line Driver

Waveforms for Circuit 3
Waveforms for Circuit 4
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FIGURE 9. Propagation Delay Waveforms
for Circuit 3 (See Figure 8)
For differential propagation delays, V1 should equal V2. Fur-
thermore, the crossing point of DO and DO* corresponds to
zero volts on the differential waveform (see bottom waveform
in Figure 9). This is true whether V3 equals V4 or not. How-
ever, if V3 and V4 are specified voltages, then V3 and V4 are
less likely to be equal to the crossing point voltage. Thus, the
differential propagation delays will not be measured to zero
volts on the differential waveform.
The differential skew also provides information about the
pulse width distortion of the differential output waveform rel-
ative to the input waveform. The higher the skew, the greater
the distortion of the differential output waveform. Assuming
the input has a 50% duty cycle, the differential output will have
a 50% duty cycle if skew equals zero and less than a 50%
duty cycle if skew is greater than zero.
(Circuit 4)
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FIGURE 10. Circuit for Measuring Complementary Skew
(See Figure 11)
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FIGURE 11. Waveforms for Circuit 4 (See Figure 10)
Complementary skew is calculated from single-ended prop-
agation delay measurements on complementary output sig-
nals, DO and DO*. Note, when V3 and V4 are absolute
values, they are identical on DO and DO*; but vary whenever
they are relative values.
The complementary skew reveals information about the con-
tour of the rising and falling edge of the differential output
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 calcu-
lations used whenever Skew (channel) is not independently
characterized and specified in the datasheet. The device to
device skew may be calculated in the same way and the re-
sults are the same. Therefore, the device to device skew is 9
ns and 0 ns maximum and minimum respectively.
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