delay, either t
determine the maximum parallel data transmission rate.
or t . As mentioned earlier, t can
PSK
This problem is frequently observed in devices with input
high input impedence such as CMOS buffered inputs in
either optocoupler or alternate isolator technologies.
In some cases, this not only causes momentary missing
pulses but in some technologies may even cause input
circuitry to latch-up.
PLH
PHL
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.
ACPL-061L/ACPL-C61L/ACNW261L optocoupler family
does not face input latch up issue even at very high CMR
levels, such as those experienced in end equipment level
tests (for example IEC61004-4-4) due to the simple diode
structure of the LED.
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 uncer-
tainty 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.
In some cases achieving the rated data sheet CMR perfor-
mance levels is not possible in the intended application,
often because of the practical need to actually connect
the isolator input to the output of a dynamically changing
signal rather than tying the input statically to VDD1 or
GND1.
This specsmanship issue is often observable with alterna-
tive isolators utilizing AC encoding techniques.
From these considerations, the absolute minimum pulse
width that can be sent through optocouplers in a parallel
To address this requirement for clear transparency on the
achievable end application performance, the ACPL-061L/
ACPL-C61L/ACNW261L series of optocouplers includes an
additional typical performance indication of the dynamic
CMR in the electrical parameter table. What this informa-
tion indicates is the achievable CMR performance whilst
the input is being toggled on or off during the occurrence
of a CMR transient. The logic output of the optocoupler
is mainly controlled by the level of the LED current due
to the short transition rise/fall time of the LED current
(approximately 10ns), the dynamic noise immunity is
essentially the same as the static noise immunity.
application is twice t . A cautious design should use a
PSK
slightly longer pulse width to ensure that any additional un-
certainty in the rest of the circuit does not cause a problem.
The t
specified optocouplers offer the advantages of
PSK
guaranteed specifications for propagation delays, pulse-
width distortion and propagation delay skew over the
recommended temperature, and power supply ranges.
Optocoupler CMR performance
The principal protection against common mode noise
comes down to the fundamental isolation properties of
the optocoupler, this in turn is directly related to the input-
output leakage capacitance of the optocoupler.
To achieve this goal of meeting the maximum inherent
CMR capabilities of the ACPL-061L/ACPL-C61L/ACNW261L
family, some simple consideration needs to be given to
the operation of the LED at the application level.
To provide maximum protection to circuitry connected to
the input or output of the optocoupler the leakage capac-
itance is minimized by having large separation distances
at all points in the optocoupler construction, including
the LED/photodiode interface.
In particular ensuring that the LED stays either on or off
during a CMR transient.
Some common design techniques which are sometimes
used to meet this goal:
In addition to the constructional design, additional circuit
design steps are taking to further mitigate the effects of
commonmodenoise.Themostimportantoftheseistheuse
of a Faraday shield on the photodetector stage. This faraday
shield is effective in optocouplers because the internal
modulation frequency (light) is many orders of magnitude
higher than the common mode noise frequency.
Keeping LED On:
i) Overdrive the LED with a higher than required forward
current.
Keeping LED Off:
i) Reverse bias the LED during the off state.
ii) Minimize the off state impedance across the anode and
cathode of the LED during the off state.
Application level CMR Performance
In application, it desirable that the optocoupler’s common
mode isolation perform as close as possible to that
indicated in the data sheets specifications.
All these methods are fully capability of enabling the
full CMR capabilities off the ACPL-061L/ACPL-C61L/
ACNW261L family to be achieved. But they do come at the
cost of practical implementation issues or a compromise
on power consumption.
The first step in meeting this goal is to ensure maintain-
ing maximum separation between PCB interconnects on
either side of the optocoupler and avoid routing tracks
beneath the optocoupler. Nonetheless, it is inevitable that
a certain amount of CMR noise will be coupled into the
inputs which can potentially result in false-triggering of
the input.
An effective method to meet the goal of maintaining
the LED status during a CMR event with no other design
compromises other the addition of a single low cost
component (resistor).
15
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
