ADM2481 Data Sheet
Rev. A | Page 16 of 20
APPLICATIONS INFORMATION
PRINTED CIRCUIT BOARD (PCB) LAYOUT
The ADM2481 signal isolated RS-485 transceiver requires no
external interface circuitry for the logic interfaces. Power supply
bypassing is required at the input and output supply pins (see
Figure 29).
Bypass capacitors are most conveniently connected between
Pin 1 and Pin 2 for V
DD1
and between Pin 15 and Pin 16 for
V
DD2
. The capacitor value must be between 0.01 μF and 0.1 μF.
The total lead length between both ends of the capacitor and
the input power supply pin must not exceed 20 mm.
V
DD1
GND
1
RxD
RE
DE
TxD
GND
1
GND
1
V
DD2
GND
2
NC
B
A
NC
GND
2
GND
2
NC = NO CONNECT
ADM2481
8920-127
Figure 29. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, take
care to ensure that board coupling across the isolation barrier
is minimized. Furthermore, the board layout must be designed
such that any coupling that does occur equally affects all pins
on a given component side.
Failure to ensure this can cause voltage differentials between
pins that exceed the absolute maximum ratings of the device,
thereby leading to latch-up or permanent damage.
ISOLATED POWER SUPPLY CIRCUIT
The ADM2481 requires isolated power capable of 5 V at
100 mA to be supplied between the V
DD2
and GND
2
pins. If
no suitable integrated power supply is available, a discrete
circuit, such as the one in Figure 30, can be used. A center-
tapped transformer provides electrical isolation. The primary
winding is excited with a pair of square waveforms that are 180°
out of phase with each other. A pair of Schottky diodes and a
smoothing capacitor are used to create a rectified signal from
the secondary winding. The ADP3330 linear voltage regulator
provides a regulated power supply to the bus-side circuitry of
the ADM2481.
08920-029
V
CC
V
CC
ISO
TION
BARRIER
SD103C
22µF
++
10µF
SD103C
78253
IN
SD
ERR
NR
OUT
5V
GND
ADP3330
V
CC
V
DD1
V
DD2
GND
1
GND
2
ADM2481
TRANSFORMER
DRIVER
Figure 30. Isolated Power Supply Circuit
