DESCRIPTION
DEMO MANUAL DC1746A
LTM2881:
Isolated RS485/RS422
µModule Transceiver + Power
Demonstration circuit 1746A is an isolated RS485/RS422
®
Module
The demo circuit is a 2500V
transceiver + power featuring the LTM®2881.
galvanically isolated
RMS
RS485/RS422 transceiver interface. The demo circuit
features an EMI optimized circuit confi guration and printed
circuit board layout. All components are integrated into
the µModule transceiver. The demo circuit operates from
a single external supply on V
PERFORMANCE SUMMARY
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
V
f
MAX
V
CC
CC2
IORM
Input Supply Range LTM2881-5
Output Voltage I
Maximum Data rate SLO = V
Maximum Working Insulation Voltage GND to GND2 560
Common Mode Transient Immunity 30 kV/µs
. The part generates the
CC
(TA = 25°C)
LTM2881-3
= 0mA to 100mA, DE = 0V 4.7 5 V
LOAD
output voltage V
and communicates all necessary
CC2
signaling across the isolation barrier using LTC’s isolator
µModule technology.
Design fi les for this circuit board are available at
http://www.linear.com/demo
L, LT, LTC, LTM, µModule, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
CC2
4.5
3.0
20 Mbps
400
5
3
5.5
3.6
V
V
DC
RMS
V
V
OPERATING PRINCIPLES
The LTM2881 contains an isolated DC/DC converter, delivering power to V
Isolation is maintained by the separation of GND and GND2
where signifi cant operating voltages and transients can
exist without affecting the operation of the LTM2881. The
logic side ON pin enables or shuts down the LTM2881.
RS485/RS422 signaling is controlled by the logic inputs
DE, DI, TE and RE. Connection to the transceiver pins (A,
B, Y and Z) allows full- or half-duplex operation on the
isolated side of the demo circuit. A full-/half-duplex switch
is included on the demo circuit to ease setting the system
confi guration. The SLO pin confi gures the slew rate of the
driver output pins Y and Z.
at 5V from the input supply, VCC.
CC2
Data is transmitted out the driver pins Y and Z from the
input DI with DE set on. Data is received through the difference in A and B to the output RO with RE set on.
The demo circuit has been designed and optimized for low
RF emissions. To this end some features of the LTM2881
are not available for evaluation on the demo circuit. The
logic supply voltage, V
, is tied to VCC on the demo circuit.
L
All control signals are selectable by jumper programming
only, including ON, RE, DE, TE and SLO. The spare logic
channel D
IN
to D
is not available.
OUT
dc1746af
1
DEMO MANUAL DC1746A
OPERATING PRINCIPLES
EMI mitigation techniques used include the following:
1. Four layer PCB, allowing for isolated side to logic side
‘bridge’ capacitor. The bridge capacitor is formed between an inner layer of fl oating copper which overlaps
the logic side and isolated side ground planes. This
structure creates two series capacitors, each with approximately 0.008" of insulation, supporting the full
dielectric withstand rating of 2500V
. The bridge
RMS
capacitor provides a low impedance return path for
injected currents due to parasitic capacitances of the
LTM2881’s signal and power isolating elements.
2. Discrete bridge capacitors (C3, C4) mounted between
GND2 and GND. The discrete capacitors provide additional attenuation at frequencies below 400MHz.
Capacitors are safety rated type Y2, manufactured by
Murata, part # GA342QR7GF471KW01L.
3. Board/ground plane size has been minimized. This
reduces the dipole antenna formed between the logic
side and isolated side ground planes.
4. Top signal routing and ground fl oods have been optimized to reduce signal loops, minimizing differential
mode radiation.
5. Common mode fi ltering is integrated into the input
pin header and output DB9 connector. Filtering helps
to reduce emissions caused by conducted noise and
minimizes the effects of cabling to common mode
emissions.
6. A combination of low ESL and high ESR decoupling is
used. A low ESL ceramic capacitor is located close to
the module minimizing high frequency noise conduction.
High ESR tantalum capacitors are included to minimize
board resonances and prevent voltage spikes due to hot
plugging of the input supply voltage.
EMI performance is shown in Figure 1, measured using
a gigahertz transverse electromagnetic (GTEM) cell and
method detailed in IEC 61000-4-20, “Testing and Measurement Techniques—Emission and Immunity Testing
in Transverse Electromagnetic Waveguides”.
60
50
40
30
20
10
dBµV/m
0
–10
–20
–30
100 200 300 400 500 1000
0
Figure 1. DC1746A Radiated Emissions
CISPR 22 CLASS B LIMIT
DC1746A-B
DETECTOR = QuasiPeak
RBW = 120kHz
VBW = 300kHz
SWEEP TIME = 17s
# OF POINTS = 501
600 700 800 900
FREQUENCY (MHz)
DC1746A F01
2
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QUICK START PROCEDURE
DEMO MANUAL DC1746A
Demonstration circuit 1746A is easy to set up and evaluate the performance of the LTM2881. Refer to Figure 2
for proper measurement equipment setup and follow the
procedure below.
NOTE: When measuring the input or output voltage ripple
or high speed signals, care must be taken to avoid a long
ground lead on the oscilloscope probe.
1. Install jumpers in following positions: (all are default
except JP5 and SW1)
JP1 ON
JP2 ON
JP3 ON
JP4 ON
JP5 OFF
SW1 HALF DUPLEX
2. With power off, connect the input power supply to V
and GND on pin header J1.
3. Turn on the power at the input.
NOTE: Make sure that the input voltage does not exceed
6V.
4. Check for the proper output voltage. V
be measured between probe points V2 and C.
5. Once the proper output voltage is established, connect
a function generator to pin DI and set to square wave
with a low of 0V, high = V
(20Mbps). Enable output of function generator.
6. Connect oscilloscope to pin RO and observe 10MHz
waveform. This demonstration shows data that is
transmitted from DI, loops back through half-duplex
connection, and out of RO.
. Set frequency to 10MHz
CC
= 5V, this can
CC2
CC
Figure 2. Demo Board Setup
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