DS89C387 Datasheet, PDF (5/10 Pages) National Semiconductor (TI) – Twelve Channel CMOS Differential Line Driver

Parameter Measurement Information (Continued)
Input pulse; f = 1 MHz, 50%, t_r ≤ 6 ns, t_f ≤ 6 ns
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FIGURE 4. Differential Rise and Fall Times
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 differ-
ence between two propagation delays. This is commonly ac-
cepted throughout the semiconductor industry. However,
there is no standardized method of measuring propagation
delay, from which skew is calculated, of differential line driv-
ers. Elucidating, the voltage level, at which propagation de-
lays are measured, on both input and output waveforms are
not always 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 comparison.
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 Fig-
ures 8, 9). Complementry skew is calculated from t_PHL and
t_PLH measurements (see Figures 10, 11).
(Circuit 1)
(Circuit 2)
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FIGURE 6. Circuits for Measuring Single-Ended Propagation Delays (See Figure 7)
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