6N137, HCNW137, HCNW2601, HCNW2611, HCPL-0600, HCPL-0601, HCPL-
0611, HCPL-0630, HCPL-0631, HCPL-0661, HCPL-2601, HCPL-2611, HCPL-
2630, HCPL-2631, HCPL-4661 Data Sheet High CMR, High Speed TTL Compatible Optocouplers
Broadcom AV02-0940EN
28
Propagation Delay, Pulse-Width Distortion, and Propagation Delay Skew
Propagation delay is a figure of merit which describes how
quickly a logic signal propagates through a system. The
propagation delay from low to high (tPLH) is the amount of
time required for an input signal to propagate to the output,
causing the output to change from low to high. Similarly, the
propagation delay from high to low (tPHL) is the amount of
time required for the input signal to propagate to the output
causing the output to change from high to low (see
Figure 8).
Pulse-width distortion (PWD) results when tPLH and tPHL
differ in value. PWD is defined as the difference between
tPLH and tPHL and often determines the maximum data rate
capability of a transmission system. PWD can be expressed
in percent by dividing the PWD (in ns) by the minimum pulse
width (in ns) being transmitted. Typically, PWD on the order
of 20 to 30% of the minimum pulse width is tolerable; the
exact figure depends on the particular application (RS232,
RS422, T-l, etc.).
Propagation delay skew, tPSK, is an important parameter to
consider in parallel data applications where synchronization
of signals on parallel data lines is a concern. If the parallel
data is being sent through a group of optocouplers,
differences in propagation delays will cause the data to
arrive at the outputs of the optocouplers at different times. If
this difference in propagation delays is large enough, it will
determine the maximum rate at which parallel data can be
sent through the optocouplers.
Propagation delay skew is defined as the difference
between the minimum and maximum propagation delays,
either tPLH or tPHL, for any given group of optocouplers
which are operating under the same conditions (i.e., the
same drive current, supply voltage, output load, and
operating temperature). As illustrated in Figure 18, if the
inputs of a group of optocouplers are switched either ON or
OFF at the same time, tPSK is the difference between the
shortest propagation delay, either tPLH or tPHL, and the
longest propagation delay, either tPLH or tPHL.
As mentioned earlier, tPSK can determine the maximum
parallel data transmission rate. Figure 19 is the timing
diagram of a typical parallel data application with both the
clock and the data lines being sent through optocouplers.
The figure shows data and clock signals at the inputs and
outputs of the optocouplers. To obtain the maximum data
transmission rate, both edges of the clock signal are being
used to clock the data; if only one edge were used, the clock
signal would need to be twice as fast.
Propagation delay skew represents the uncertainty of where
an edge might be after being sent through an optocoupler.
Figure 19 shows that there will be uncertainty in both the
data and the clock lines. It is important that these two areas
of uncertainty not overlap, otherwise the clock signal might
arrive before all of the data outputs have settled, or some of
the data outputs may start to change before the clock signal
has arrived. From these considerations, the absolute
minimum pulse width that can be sent through optocouplers
in a parallel application is twice tPSK. A cautious design
should use a slightly longer pulse width to ensure that any
additional uncertainty in the rest of the circuit does not
cause a problem.
The tPSK specified optocouplers offer the advantages of
guaranteed specifications for propagation delays,
pulsewidth distortion and propagation delay skew over the
recommended temperature, input current, and power
supply ranges.
Figure 18: Illustration of Propagation Delay Skew – tPSK
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