DS89C387_06NNN Datasheet, PDF (6/12 Pages) National Semiconductor (TI) – Twelve Channel CMOS Differential Line Driver

Typical Application
* R_T is optional although highly recommended to reduce reflection.
FIGURE 5. Two-Wire Balanced System, RS-422
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Application Information
SKEW
Skew may be thought of in a lot of different ways, the next few
paragraphs should clarify what is represented by “Skew” in
the datasheet and how it is determined. Skew, as used in this
databook, is the absolute value of a mathematical difference
between two propagation delays. This is commonly accepted
throughout the semiconductor industry. However, there is no
standardized method of measuring propagation delay, from
which skew is calculated, of differential line drivers. Elucidat-
ing, the voltage level, at which propagation delays are mea-
sured, on both input and output waveforms are not always
(Circuit 1)
consistant. Therefore, skew calculated in this datasheet, may
not be calculated the same as skew defined in another. This
is important to remember whenever making a skew compar-
ison.
Skew may be calculated for the DS89C387, from many dif-
ferent propagation delay measurements. They may be clas-
sified into three categories, single-ended, differential, and
complementry. Single-ended skew is calculated from t_PHL and
t_PLH measurements (see Figures 6, 7). Differential skew is
calculated from t_PHLD and t_PLHD measurements (see Figures
8, 9). Complementry skew is calculated from t_PHL and t_PLH
measurements (see Figures 10, 11).
(Circuit 2)
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FIGURE 6. Circuits for Measuring Single-Ended Propagation Delays (See Figure 7)
Waveforms for Circuit 1
Waveforms for Circuit 2
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FIGURE 7. Propagation Delay Waveforms for Circuit 1 and Circuit 2 (See Figure 6)
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In Figure 2, VX, where X is a number, is the waveform voltage
level at which the propagation delay measurement either
starts or stops. Furthermore, V1 and V2 are normally identi-
cal. The same is true for V3 and V4. However, as mentioned
before, these levels are not standardized and may vary, even
with similar devices from other companies. Also note, NC (no
connection) in Figure 1 means the pin is not used in propa-
gation delay measurement for the corresponding circuit.
The single-ended skew provides information about the pulse
width distortion of the output waveform. The lower the skew,
the less the output waveform will be distorted. For best case,
skew would be zero, and the output duty cycle would be 50%,
assuming the input has a 50% duty cycle.
(Circuit 3)
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FIGURE 8. Circuit for Measuring Differential Propagation
Delays (See Figure 9)
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