CN-0313 Datasheet, PDF (2/5 Pages) Analog Devices – EMC Compliant RS-485 Transceiver Protection Circuits

CN-0313
CIRCUIT DESCRIPTION
The RS-485 bus standard is one of the most widely used physical
layer bus designs in industrial and instrumentation applications.
RS-485 offers differential data transmission between multiple
systems often over very long distances. Applications for RS-485
include process control networks; industrial automation; remote
terminals; building automation, such as heating, ventilation, air
conditioning (HVAC) and security systems; motor control; and
motion control.
In these real systems, lightning strikes, power source fluctuations,
inductive switching, and electrostatic discharge can cause
damage to communications ports by generating large transient
voltages. Designers must ensure that equipment does not just
work in ideal conditions, but it must also work in real-world
conditions. To ensure that these designs can work in electrically
harsh environments, EMC regulations must be met.
Many EMC problems are not simple or obvious and must be
considered at the start of the product development cycle. Proper
solutions and protection circuits must be part of the total design
effort and not left to the last minute. The protection circuits
must incorporate the input and output structure of the specific
transceiver manufacturer as part of the design.
The IEC 61000 specifications define the set of EMC immunity
requirements. Within this set of specifications, designers must
be concerned with the following three types of high voltage
transients for data communication lines:
• IEC 61000-4-2 electrostatic discharge (ESD)
• IEC 61000-4-4 electrical fast transients (EFT)
• IEC 61000-4-5 surge immunity
ESD and EFT have similar rise times, pulse widths, and energy
levels. The surge transient has longer rise times and pulse widths; as
a result, the surge transient energy can be three to four orders of
magnitude larger than the energy in an ESD or EFT transient.
Due to the similarities between ESD and EFT transients, the
design of the circuit protection can be similar. However, due to
the high energy associated with surge transients, they must be
dealt with differently.
Each solution protects data ports to ESD voltages of 8 kV
contact and 15 kV air discharge, and EFT voltages of 2 kV. The
different solutions provide an increased level of surge
protection up to 6 kV. Protection levels for the circuits are
summarized in Table 1.
Circuit Note
Table 1. Protection Levels Offered by Each of the Three
Protection Circuits in Figure 1
Protection Scheme ESD
EFT (kV) Surge (kV)
1. TVS
8 kV contact,
2
1
15 kV air discharge
2. TVS/TBU/TISP
8 kV contact,
2
4
15 kV air discharge
3. TVS/TBU/GDT
8 kV contact,
2
6
15 kV air discharge
Figure 2 shows a photo of the EVAL-CN0313-SDPZ board. There
are three ADM3485E devices on the board, one for each
protection scheme. Each protection scheme provides ESD and
EFT protection as described and increasing levels of surge
protection.
A complete design support package for the EVAL-CN0313-
SDPZ board including schematics, layout files, and bill of
materials can be found at www.analog.com/CN0313-
DesignSupport.
Figure 2. EVAL-CN0313-SDPZ Board
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