CN-0313 Datasheet, PDF (4/5 Pages) Analog Devices – EMC Compliant RS-485 Transceiver Protection Circuits

CN-0313
CIRCUIT EVALUATION AND TEST
Apply 3.3 V to VCC to power the EVAL-CN0313-SDPZ board.
The voltage can be checked on the VCC test points near each
ADM3485E. The transmit and receive paths can be tested by
connecting one of the ADM3485E circuits as shown in Figure 3.
A signal or pattern generator can be connected to DI. The outputs
of the driver can be monitored on the A and B test points, and the
output of the receiver can be monitored on the RO test point.
Jumper configurations are also shown in Figure 3. This test
setup can apply to any of the three circuits.
SIGNAL
GENERATOR
3.3V
POWER
SUPPLY
OSCILLOSCOPE
RO
A
B
RE
C
B
DE
A
B
DI
C
A
Figure 3. Transmit and Receive Test Setup
According to IEC 61000-4-2, ESD testing implies using two
coupling methods, contact discharge and air gap discharge.
Contact discharge implies the discharge gun is placed in direct
connection with the port being tested. With air discharge, the
charged electrode of the discharge gun is moved toward the
port under test until a discharge occurs developing an arc across
the air gap. Apply discharges to the screw terminal connector of
each bus line.
Circuit Note
For IEC 61000-4-4 EFT testing, a capacitive coupling clamp is
used to couple the EFT bursts onto the cable connected to the
bus lines. The coupling capacitance of the clamp depends on the
cable diameter, material of the cables, and cable shielding.
IEC 61000-4-5 surge testing implies the use of a coupling/
decoupling network (CDN) to couple the surge transient into
the bus pins. According to the specification, this must be done
using two 80 Ω resistors for a two port test. Figure 4 shows the
test setup for surge testing. Connect the CDN to the A and B
terminals, and the common of the surge generator to the
ground connection of the four terminal screw connector.
V_CC
ADM3485E
RO
B
PROTECTION
RE
120Ω _COMPONENTS
A
DI
DE
CDN
80Ω
80Ω
Figure 4. IEC 61000-4-5 Surge CDN to ADM3485E Setup
All protection circuits described were characterized in the lab
with the ADM3485E and verified by an external independent
EMC compliance test house.
For further information on the ezLINX iCoupler® isolated interface
development environment, go to
http://wiki.analog.com/resources/eval/ezlinx.
Information on other important EMC issues can be found at
http://www.analog.com/rs485emc.
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