Propagation Delay, Pulse- tion delay from low to high (tPLH
)
amount of time required for the
input signal to propagate to the
output, causing the output to
change from high to low. See
Figure 13.
Width Distortion and
is the amount of time required for
Propagation Delay Skew
Propagation Delay is a figure of
merit which describes how
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
quickly a logic signal propagates
through a system. The propaga-
INPUT
5 V CMOS
V
50%
0 V
I
t
t
PHL
PLH
V
OH
2.5 V CMOS
OUTPUT
90%
90%
V
10%
10%
O
V
OL
Figure 13.
Pulse-width distortion (PWD) is
the difference between tPHL and
tPLH and often determines the
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 delay is large
enough it will determine the
maximum rate at which parallel
data can be sent through the
optocouplers.
temperature). As illustrated in
Figure 14, 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
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 trans-
mitted. Typically, PWD on the
order of 20 - 30% of the minimum
pulse width is tolerable. The PWD
specification for the HCPL-0710
is 8 ns (10%) maximum across
recommended operating condi-
tions. 10% maximum is dictated
by the most stringent of the three
fieldbus standards, PROFIBUS.
tPLH or tPHL
.
As mentioned earlier, tPSK can
determine the maximum parallel
data transmission rate. Figure 15
is the timing diagram of a typical
parallel data application with both
the clock and data lines being
sent through the optocouplers.
The figure shows data and clock
signals at the inputs and outputs
of the optocouplers. In this case
the data is assumed to be clocked
off of the rising edge of the clock.
Propagation delay skew is defined
as the difference between the
minimum and maximum propa-
gation delays, either tPLH or tPHL
,
for any given group of optocoup-
lers which are operating under
the same conditions (i.e., the
same drive current, supply volt-
age, output load, and operating
Propagation delay skew, tPSK, is
an important parameter to con-
sider in parallel data applications
where synchronization of signals
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