Typical Application

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

is calculated from t

ures 8, 9). Complementry skew is calculated from t

t

DS012086-7

FIGURE 4. Differential Rise and Fall Times

DS012086-8

FIGURE 5. Two-Wire Balanced System, RS-422

(Circuit 1)

DS012086-9

(Circuit 2)

DS012086-10

FIGURE 6. Circuits for Measuring Single-Ended Propagation Delays (See

Figure 7)

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