Inside iCoupler Technology—Isolated RS-485
By Hein Marais, Applications Engineer
RS-485 differential communication networks are often used in harsh industrial and instrumentation applications. These networks are capable of
communicating over distances as far as 4000 feet. When communicating over long distances in these harsh environments the following problems
can occur:
• Ground loop currents due to the ground potential of different nodes being at different potentials
• Induced ground noise due to motors, inductive switching loads, and other noisy electrical equipment
• Harmful electrical surges
Galvanic isolation is a perfect solution if there is no guarantee that the potential at the earth grounds at different nodes in the system are within the
common-mode range of the transceiver. Galvanic isolation allows information flow, but prevents current flow.
Traditionally, RS-485 networks were isolated using optocouplers for isolating the signals and an isolated dc-to-dc converter for power isolation
(see Figure 1).
VIN
5V
0.1uF
ISOLATION
BARRIER
VCC
D1
TRANSFORMER
DRIVER
EN
D1
ON/OFF
+
10uF
MLC
IN
22uF
OUT
10uF
ADP3330
SD
GND
ISO +5V
GND
HIGH SPEED OPTO
TxD
TXD
UART
LOW SPEED OPTO
VCC
RS-485
TRANSCEIVER
RTS
DE
HIGH SPEED OPTO
RxD
ADM485
RE
A
RT
B
RxD
GND
Figure 1. Traditional isolated RS-485 node.
Through the use of Analog Devices iCoupler® digital isolation chip scale transformer technology and isoPower® dc-to-dc converter technology, several
improvements can be achieved over LEDs and photodiodes used in optocouplers:
• Performance: 2× to 4× higher data rate and timing accuracy
• Ease of use: standard CMOS interface without current-transfer-ratio
headaches
• Integration: wafer level fabrication enables lower size and cost
• Allows for bidirectional communications by combining the Tx and Rx
channels in the same package
• Reliability: CMOS level reliability eliminates long-term signal
degradation of LEDs
• Integrates an isolated dc-to-dc converter
• Power: 7× to 10× reduction
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The ADM2682E signal and power isolated RS-485 transceiver integrates Analog Devices i Coupler technology to combine a
3-channel isolator, a three-state differential line driver, a differential input receiver, and Analog Devices iso Power dc-to-dc converter into a single package. The devices are powered by a single 5 V or 3.3 V supply, realizing a fully integrated signal
and power isolated RS-485 solution (see Figure 2).
VIN
5V
0.1𝛍F
ISOLATION
BARRIER
VCC
D1
TRANSFORMER
DRIVER
EN
D1
ON/OFF
+
10𝛍F
MLC
IN
22𝛍F
OUT
10𝛍F
ADP3330
SD
GND
ISO +5V
VIN
3V OR 5V
0.1𝛍F
ISOLATION
BARRIER
10𝛍F
MLC
10uF
MLC
VDD1
VDD2
RTS
HIGH SPEED OPTO
UART
TxD
TxD
UART
LOW SPEED OPTO
TXD
DE
HIGH SPEED OPTO
RxD
RxD
RE
VCC
ADM2687E/
ADM2682E
ISOLATED
RS-485
TRANSCEIVER
A
B
RT
Y
Z
RS-485
TRANSCEIVER
RTS
0.1𝛍F
GND
A
GND1
GND2
RT
ADM485
B
RE
RxD
GND
NEW METHOD
TRADITIONAL METHOD
Figure 2. ADM2682E signal and power isolated RS-485 transceiver.
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