ACPL-W61L-560E 데이터 시트보기 (PDF) - Avago Technologies
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ACPL-W61L-560E Datasheet PDF : 16 Pages
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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 9).
Pulse-width distortion (PWD) results when tPLH and tPHL
differ in value. PWD is defined as the difference between
tPLH and tPHL. PWD determines the maximum data rate 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, a PWD of 20-30% of
the minimum pulse width is tolerable; the exact figure
depends on the particular application (RS232, RS422,
T-1, etc.).
Propagation delay skew, tPSK, is an important parameter
to consider in parallel data applications where synchroni-
zation of signals on parallel data lines is a concern.
If the parallel data is being sent through a group of opto-
couplers, 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 opto-
couplers which are operating under the same conditions
(i.e., the same supply voltage, output load, and operating
temperature). As shown in Figure 10, 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 10 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 optocou-
plers. 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 10 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, pulse-
width distortion and propagation delay skew over the
recommended temperature, and power supply ranges.
VI
50%
VO
2.5 V,
CMOS
tPSK
DATA
INPUTS
CLOCK
VI
50%
VO
2.5 V,
CMOS
Figure 9. Propagation delay skew waveform
11
DATA
OUTPUTS
tPSK
CLOCK
tPSK
Figure 10. Parallel data transmission example
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