Propagation Delay, Pulse-Width Distortion and Propagation
Delay Skew
puts 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
Propagation delay is a figure of merit which describes
how quickly a logic signal propagates through a sys-
tem. 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 Figure8).
longest propagation delay, either tPLH or tPHL
.
As mentioned earlier, tPSK can determine the maximum
parallel data transmission rate. Figure 20 is the timing
diagram of a typical parallel data application with both
the clock and the data lines being sent through opto-
couplers. 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.
Pulse-width distortion (PWD) results when tPLH and tPHL
differ in value. PWD is defined as the difference be-
tween 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. Typi-
cally, PWD on the order of 20-30% of the minimum pulse
width is tolerable; the exact figure depends on the par-
ticular application (RS232, RS422, T-l, etc.).
Propagation delay skew represents the uncertainty of
where an edge might be after being sent through an
optocoupler. Figure 20 shows that there will be uncer-
tainty in both the data and the clock lines. It is important
that these two areas of uncertainty not overlap, other-
wise 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 appli-
cation is twice tPSK. A cautious design should use a slightly
longer pulse width to ensure that any additional uncer-
tainty in the rest of the circuit does not cause a problem.
Propagation delay skew, tPSK, is an important parameter to
consider in parallel data applications where synchroniza-
tion of signals on parallel data lines is a concern. If the
parallel data is being sent through a group of optocou-
plers, differences in propagation delays will cause the
data to arrive at the outputs of the optocouplers at differ-
ent 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.
The tPSK specified optocouplers offer the advantages of
guaranteed specifications for propagation delays, pulse-
width distortion and propagation delay skew over the
recommended temperature, input current, and power
supply ranges.
Propagation delay skew is defined as the difference be-
tween 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 op-
erating temperature). As illustrated in Figure 19, if the in-
DATA
I
F
50%
INPUTS
CLOCK
1.5 V
V
O
I
50%
F
DATA
OUTPUTS
t
PSK
V
1.5 V
O
CLOCK
t
PSK
t
PSK
Figure 19. Illustration of propagation delay skew - tPSK.
Figure 20. Parallel data transmission example.
21
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
