ADM2485 High Speed Isolated RS-485 Transceiver with Integrated Transformer Driver Preliminary Data Sheet (Rev. PrI)
High Speed Isolated RS-485 Transceiver
with Integrated Transformer Driver
Preliminary Technical Data
ADM2485
Rev.PrI
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However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
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registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
2006 Analog Devices, Inc. All rights reserved.
FEATURES
Half-duplex isolated RS-485 transceiver
Integrated oscillator driver for external transformer
PROFIBUS compliant
Complies with ANSI TIA/EIA RS-485-A-1998 and
ISO 8482:1987(E)
16 Mbps data rate
5 V or 3 V operation (V
DD1
)
High common-mode transient immunity: >25 kV/s
Isolated DE OUT status output
Thermal shutdown protection
Safety and regulatory approvals:
UL recognition--2500 V
RMS
for 1 minute per UL 1577
(pending)
CSA component acceptance notice #5A
VDE certificate of conformity
DIN EN 60747-5-2 (VDE 0884 Part 2): 2003-01
DIN EN 60950 (VDE 0805):2001-12; EN 60950: 2000
V
IORM
= 560 V peak
Operating temperature range: -40 to 85C
Wide body 16-lead SOIC package
APPLICATIONS
Isolated RS-485/RS-422 interfaces
PROFIBUS networks
Industrial field networks
Multipoint data transmission systems
GENERAL DESCRIPTION
The ADM2485 differential bus transceiver is an integrated,
galvanically isolated component designed for bidirectional data
communication on multipoint bus transmission lines. It is
designed for balanced transmission lines and complies with
ANSI TIA/EIA RS-485-A-1998 and ISO 8482:1987(E).
The device employs Analog Devices' iCoupler technology to
combine a 3-channel isolator, a 3-state differential line driver,
and a differential input receiver into a single package. An on-
chip oscillator outputs a pair of square waveforms which drive
an external transformer to provide isolated power with an
external transformer. The logic side of the device can be
powered with either a 5 V or a 3 V supply while the bus side is
powered with an isolated 5 V supply.
The ADM2485 driver has an active high enable. The driver
differential outputs and the receiver differential inputs are
connected internally to form a differential input/output port
that imposes minimal loading on the bus when the driver is
disabled or when V
DD1
or V
DD2
= 0 V. Also provided is an active
high receiver disable that causes the receive output to enter a
high impedance state.
The device has current-limiting and thermal shutdown features
to protect against output short circuits and situations where bus
contention might cause excessive power dissipation. The part is
fully specified over the industrial temperature range and is
available in a 16-lead wide-body SOIC package.
FUNCTIONAL BLOCK DIAGRAM
RTS
V
DD1
DE OUT
GND
1
A
B
V
DD2
GND
2
TxD
RxD
GA
L
V
A
N
IC ISOLA
T
ION
4-
OSC
D1 D2
Figure 1
ADM2485
Preliminary Technical Data
Rev. PrI | Page 2 of 13
SPECIFICATIONS
2.7 V
DD1
5.5 V, 4.75 V V
DD2
5.25 V, T
A
= T
MIN
to T
MAX
, unless otherwise noted.
Table 1.
Parameter Min
Typ
Max
Unit
Test
Conditions/Comments
DRIVER
Differential
Outputs
Differential Output Voltage, V
OD
5
V
R =
, Figure 3
2.1
5
V
R = 50 (RS-422), Figure 3
2.1
5
V
R = 27 (RS-485), Figure 3
2.1
5
V
V
TST
= -7 V to 12 V, V
DD1
4.75,
Figure 4
|V
OD
| for Complementary Output States
0.2
V
R = 27 or 50 , Figure 3
Common-Mode Output Voltage, V
OC
3
V
R = 27 or 50 , Figure 3
|V
OC
| for Complementary Output States
0.2
V
R = 27 or 50 , Figure 3
Output Short-Circuit Current, V
OUT
= High
60
200
mA
-7 V V
OUT
+12 V
Output Short-Circuit Current, V
OUT
= Low
60
200
mA
-7 V
V
OUT
+ 12 V
Bus Enable Output
Output High Voltage
V
DD2
-0.1
V
I
ODE
= 20 A
V
DD2
-0.3 V
DD2
-0.1
V
I
ODE
= 1.6 mA
V
DD2
-0.4 V
DD2
-0.2
V
I
ODE
= 4 mA
Output Low Voltage
0.1
V
I
ODE
=
-20 A
0.1 0.3 V I
ODE
= -1.6 mA
0.2 0.4 V I
ODE
= -4 mA
Logic
Inputs
Input High Voltage
0.7 V
DD1
V
TxD, RTS, RE
Input Low Voltage
0.25 V
DD1
V
TxD, RTS, RE
CMOS Logic Input Current (TxD, RTS, RE)
-10
0.01 10 A TxD, RTS, RE = V
DD1
or 0 V
RECEIVER
Differential
Inputs
Differential Input Threshold Voltage, V
TH
-200
200 mV -7 V V
CM
+12V
Input Hysteresis
70
mV
-7 V
V
CM
+12V
Input Resistance (A, B)
20
30
k
-7
V
V
CM
+12V
Input Current (A, B)
0.6
mA
V
IN
= +12 V
-0.35
mA
V
IN
=
-7 V
RxD Logic Output:
Output High Voltage
V
DD1
-0.1
V
I
OUT
= 20 A, V
A
-V
B
=0.2 V
V
DD1
-0.4 V
DD1
-0.2
V
I
OUT
= 4 mA, V
A
-V
B
=0.2 V
Output Low Voltage
0.1
V
I
OUT
= -20 A, V
A
-V
B
=-0.2 V
0.2 0.4 V I
OUT
= -4 mA, V
A
-V
B
=-0.2 V
Output Short Circuit Current
7
85
mA
V
OUT
= GND or V
CC
Three-State Output Leakage Current
1
A
0.4 V
V
OUT
2.4 V
Transformer
Driver
Oscillator Frequency
500
kHz
Switch On resistance
0.5
1.5
Start-Up Voltage
2.2
2.5
V
Preliminary Technical Data
ADM2485
Rev. PrI | Page 3 of 13
Parameter Min
Typ
Max
Unit
Test
Conditions/Comments
POWER SUPPLY CURRENT
Logic Side
1.3
mA
RTS = 0 V, V
DD1
= 5.5 V
1.0
mA
2 Mbps, V
DD1
= 5.5 V, Figure 5
4.0
mA
16 Mbps, V
DD1
= 5.5 V, Figure 5
0.8
mA
RTS = 0 V, V
DD1
= 3 V
1.1
mA
2 Mbps, V
DD1
= 3 V, Figure 5
2.1
mA
16 Mbps, V
DD1
= 3 V, Figure 5
Bus Side
3.0
mA
RTS = 0 V
43.0
mA
2 Mbps, RTS = V
DD1
, Figure 5
58.0
mA
16 Mbps, RTS = V
DD1
, Figure 5
COMMON-MODE TRANSIENT IMMUNITY
1
25
kV/s
V
CM
= 1 kV, Transient Magnitude =
800 V
HIGH FREQUENCY COMMON-MODE NOISE
IMMUNITY
100
mV
V
HF
= +5V, -2 V < V
TEST2
< 7 V,
1 < f
TEST
< 50 MHz, Figure 6
1
CM is the maximum common-mode voltage slew rate that can be sustained while maintaining specification-compliant operation. V
CM
is the common-mode potential
difference between the logic and bus sides. The transient magnitude is the range over which the common-mode is slewed. The common-mode voltage slew rates
apply to both rising and falling common-mode voltage edges.
TIMING SPECIFICATIONS
2.7 V
DD1
5.5 V, 4.75 V V
DD2
5.25 V, T
A
= T
MIN
to T
MAX
, unless otherwise noted.
Table 2.
Parameter Min
Typ
Max
Unit
Test
Conditions/Comments
DRIVER
Maximum Data Rate
16
Mbps
Propagation Delay t
PLH
, t
PHL
25 45 55 ns R
LDIFF
= 54
, C
L1
= C
L2
= 100 pF; See Figure 7.
RTS-to-DE Propagation Delay
20
35
55
ns
See Figure 8.
Pulse-Width Distortion, t
PWD
5 ns R
LDIFF
= 54
, C
L1
= C
L2
= 100 pF; See Figure 7 and
Figure 12.
Switching Skew, t
SKEW
2
5
ns
R
LDIFF
= 54
, C
L1
= C
L2
= 100 pF; See Figure 7 and
Figure 12.
Rise/Fall Time t
R
, t
F
5
15
ns
R
LDIFF
= 54
, C
L1
= C
L2
= 100 pF; See Figure 7 and
Figure 12.
Enable Time
43
53
ns
See Figure 9 and Figure 14.
Disable Time
43
55
ns
See Figure 9 and Figure 14.
Enable Skew, |t
AZH
-t
BZL
|, |t
AZL
-t
BZH
|
1
3
ns
See Figure 9 and Figure 14.
Disable Skew, |t
AHZ
-t
BLZ
|, |t
ALZ
-t
BHZ
|
2
5
ns
See Figure 9 and Figure 14.
RECEIVER
Propagation Delay t
PLH
, t
PHL
25 45 55 ns C
L
= 15 pF; See Figure 10 and Figure 13.
Differential Skew t
SKEW
5
ns
C
L
= 15 pF; See Figure 10 and Figure 13.
Enable Time
3
13
ns
R
L
= 1 k
, C
L
= 15 pF; See Figure 11 and
Figure 15.
Disable Time
3
13
ns
R
L
= 1 k
, C
L
= 15 pF; See Figure 11 and
Figure 15.
ADM2485
Preliminary Technical Data
Rev. PrI | Page 4 of 13
ABSOLUTE MAXIMUM RATINGS
T
A
= 25C, unless otherwise noted. All voltages are relative to
their respective ground.
Table 3.
Parameter
Rating
V
DD1
-0.5 V to +6 V
V
DD2
-0.5 V to +6 V
Digital Input Voltage (RTS, RE, TxD)
-0.5 V to V
DD1
+ 0.5 V
Digital Output Voltage
RxD
-0.5 V to V
DD1
+ 0.5 V
DE OUT
-0.5 V to V
DD2
+ 0.5 V
Driver Output/Receiver Input Voltage
-9 V to +14 V
Operating Temperature Range
-40C to +85C
Storage Temperature Range
-55C to +150C
Average Output Current per Pin
-35 mA to +35 mA
JA
Thermal Impedance
73C/W
Lead Temperature
Soldering (10 sec)
300C
Vapour Phase (60 sec)
215C
Infrared (15 sec)
220C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
Preliminary Technical Data
ADM2485
Rev. PrI | Page 5 of 13
ADM2485E CHARACTERISTICS
PACKAGE CHARACTERISTICS
Table 2.
Parameter Symbol
Min
Typ
Max
Unit
Test
Conditions
Resistance (Input-Output)
1
R
I-O
10
12
Capacitance (Input-Output)
1
C
I-O
3
pF
f = 1 MHz
Input Capacitance
2
C
I
4
pF
Input IC Junction-to-Case Thermal Resistance
JCI
33
C/W
Output IC Junction-to-Case Thermal Resistance
JCO
28
C/W
Thermocouple located at
center of package underside
1
Device considered a two-terminal device: Pins 1, 2, 3, 4, 5, 6, 7, and 8 shorted together, and Pins 9, 10, 11, 12, 13, 14, 15, and 16 shorted together.
2
Input capacitance is from any input data pin to ground.
REGULATORY INFORMATION
The ADM2485 is to be approved by the following organizations:
Table 3.
Organization Approval
Type
Notes
UL
To be recognized under 1577 component recognition program.
In accordance with UL1577, each ADM2485
is proof-tested by applying an insulation
test voltage 3000 V rms for 1 second (current
leakage detection limit = 5 A).
CSA
To be approved under CSA Component Acceptance Notice #5A.
VDE
To be certified according to DIN EN 60747-5-2 (VDE 0884 Part 2): 2003-01
In accordance with VDE 0884, each ADM2485
is proof-tested by applying an insulation
test voltage 1050 V
PEAK
for 1 second
(partial discharge detection limit = 5 pC).
INSULATION AND SAFETY-RELATED SPECIFICATIONS
Table 4.
Parameter Symbol
Value
Unit
Conditions
Rated Dielectric Insulation Voltage
2500
V rms
1-minute duration.
Minimum External Air Gap (Clearance)
L(I01)
5.15 min
mm
Measured from input terminals to output
terminals, shortest distance through air.
Minimum External Tracking (Creepage)
L(I02)
5.5 min
mm
Measured from input terminals to output
terminals, shortest distance along body.
Minimum Internal Gap (Internal Clearance)
0.017 min
mm
Insulation distance through insulation.
Tracking Resistance (Comparative Tracking Index)
CTI
>175
V
DIN IEC 112/VDE 0303 Part 1.
Isolation Group
IIIa
Material Group (DIN VDE 0110, 1/89,).
ADM2485
Preliminary Technical Data
Rev. PrI | Page 6 of 13
VDE 0884 INSULATION CHARACTERISTICS
This isolator is suitable for basic electrical isolation only within the safety limit data. Maintenance of the safety data must be ensured by
means of protective circuits.
An asterisk (*) on packages denotes VDE 0884 approval for 560 V peak working voltage.
Table 5.
Description Symbol
Characteristic
Unit
Installation classification per DIN VDE 0110 for rated mains voltage
150 V rms
I to IV
300 V rms
I to III
400 V rms
I to II
Climatic classification
40/85/21
Pollution degree (DIN VDE 0110, Table 1)
2
Maximum working insulation voltage
V
IORM
560 V
PEAK
Input to output test voltage, Method b1
V
PR
1050
V
PEAK
V
IORM
1.875 = V
PR
, 100% production tested, t
m
= 1 sec, partial discharge < 5 pC
Input to output test voltage, Method a
(After environmental tests, Subgroup 1)
V
IORM
1.6 = V
PR
, t
m
= 60 sec, partial discharge < 5 pC
896
V
PEAK
(After input and/or safety test, Subgroup 2/3)
V
IORM
1.2 = V
PR
, t
m
= 60 sec, partial discharge < 5 pC
V
PR
672 V
PEAK
Highest allowable overvoltage
(Transient overvoltage, t
TR
= 10 sec)
V
TR
4000
V
PEAK
Safety-limiting values (maximum value allowed in the event of a failure. See
thermal derating curve)
Case temperature
T
S
150
C
Input current
I
S
,
INPUT
265
mA
Output current
I
S
,
OUTPUT
335
mA
Insulation resistance at Ts, V
IO
= 500 V
Rs
>10
9
Preliminary Technical Data
ADM2485
Rev. PrI | Page 7 of 13
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
ADM2485
D1
D2
GND1
VDD1
RxD
VDD2
GND2
B
A
2
3
4
5
16
15
14
13
12
TOP VIEW
( Not t o Scale)
RE
RTS
TxD
6
7
8
DE OUT
GND2
11
10
9
GND2
GND2
Figure 2. Pin Configuration
Table 4.
Pin Mnemonic
Function
1
D1
Transformer driver terminal 1.
2
D2
Transformer driver terminal 2.
3 GND
1
Ground, Logic Side.
4 V
DD1
Power Supply Logic Side, 3V or 5V. Decoupling capacitor to GND
1
required, capacitor value should
be between 0.01 F and 0.1 F.
5 RxD Receiver Output data. This output is high when (A B) > 200 mV, and low when (A B) < -200
mV. The output is tri-stated when the receiver is disabled, i.e. when RE is driven high.
6
RE
Receiver Enable input. This is an active-low input. Driving this input low enables the receiver,
while driving it high disables the receiver.
7 RTS
Driver enable input. Driving this input high enables the driver, while driving it low disables the
driver.
8
TxD
Driver input. Data to be transmitted by the driver is applied to this input.
10
DE OUT
Driver Enable status output
12 A
Noninverting Driver Output/Receiver Input. When the driver is disabled or V
DD1
or V
DD2
is powered
down, pin A is put in a high impedance state to avoid overloading the bus.
13 B
Inverting Driver Output/Receiver Input. When the driver is disabled or V
DD1
or V
DD2
is powered
down, pin B is put in a high impedance state to avoid overloading the bus.
9,11,14,15 GND
2
Ground, Bus Side.
16 V
DD2
Power Supply Bus Side, Isolated 5V supply. Decoupling capacitor to GND
2
required, capacitor value
should be between 0.01 F and 0.1 F.
ADM2485
Preliminary Technical Data
Rev. PrI | Page 8 of 13
TEST CIRCUITS
04604-005
V
OC
R
R
V
OD
Figure 3. Driver Voltage Measurement
04604-006
60
V
OD3
375
375
V
TST
Figure 4. Driver Voltage Measurement
V
DD1
DE
150
GND
1
A
B
V
DD2
GND
2
GALV
ANIC IS
OLATION
04604-004
V
DD2
195
110
195
GND
2
RTS
TxD
RxD
RE
Figure 5. Supply-Current Measurement Test Circuit
V
DD1
DE
GND
1
A
B
V
DD2
GND
2
G
A
L
V
ANI
C I
S
O
L
AT
I
O
N
04604-
010
50
110nF
50
V
TEST2
RTS
TxD
RxD
GND
2
2.2k
100nF
100nF
V
CM(HF)
470nF
22k
F
TEST
,
V
HF
RECEIVE
ENABLE
195
110
195
V
DD2
GND
2
Figure 6. High Frequency Common-Mode Noise Test Circuit
A
B
R
LDIFF
04736-005
C
L1
C
L2
Figure 7. Driver Propagation Delay
V
DD1
DE
GND
1
A
B
V
DD2
GND
2
GALV
ANIC IS
OLATION
04604-008
RTS
TxD
RxD
RE
150
50pF
Figure 8. RTS to DE OUT Propagation Delay
04604-009
V
CC
S2
V
OUT
110
50pF
S1
B
A
TxD
RTS
Figure 9. Driver Enable/Disable
04604-012
C
L
V
OUT
RE
A
B
Figure 10. Receiver Propagation Delay
04604-013
V
CC
S2
V
OUT
R
L
C
L
+1.5V
1.5V
S1
RE
RE IN
Figure 11. Receiver Enable/Disable
DE OUT
DE OUT
DE OUT
Preliminary Technical Data
ADM2485
Rev. PrI | Page 9 of 13
SWITCHING CHARACTERISTICS
04604-011
t
PAHL
t
PBHL
t
PALH
t
PBLH
V
DD1
0V
VO
B
A
VO
0V
VO
0.5V
DD1
0.5V
DD1
t
SKEW
t
F
t
R
10% POINT
10% POINT
90% POINT
90% POINT
1/2VO
t
SKEW
t
PWD
= |t
PALH
t
PAHL
|, |t
PBLH
t
PBHL
|
Figure 12. Driver Propagation Delay, Rise/Fall Timing
04604-019
AB
RxD
0V
0V
1.5V
1.5V
t
PLH
t
SKEW
= |t
PLH
t
PHL
|
t
PHL
V
OH
V
OL
Figure 13. Receiver Propagation Delay
04604-021
t
LZ
t
ZL
t
HZ
t
ZH
V
OH
0.5V
V
OH
+0.5V
AB
AB
RTS
0.7V
DD1
0.3V
DD1
0.5V
DD1
0.5V
DD1
2.3V
2.3V
V
OL
V
OH
0V
Figure 14. Driver Enable/Disable Timing
04604-020
0.7V
DD1
0.3V
DD1
0.5V
DD1
0.5V
DD1
V
OH
V
OL
O/P LOW
O/P HIGH
t
LZ
t
ZL
t
HZ
t
ZH
V
OH
0.5V
V
OH
+0.5V
1.5V
1.5V
RxD
RxD
RE
0V
Figure 15. Receiver Enable/Disable Timing
ADM2485
Preliminary Technical Data
Rev. PrI | Page 10 of 13
CIRCUIT DESCRIPTION
ELECTRICAL ISOLATION
In the ADM2485, electrical isolation is implemented on the
logic side of the interface. Therefore, the part has two main
sections: a digital isolation section and a transceiver section
(see Figure 16). Driver input and data enable, applied to the
TxD and RTS pins, respectively, and referenced to logic ground
(GND
1
), are coupled across an isolation barrier to appear at the
transceiver section referenced to isolated ground (GND
2
).
Similarly, the receiver output, referenced to isolated ground in
the transceiver section, is coupled across the isolation barrier to
appear at the RxD pin referenced to logic ground.
iCoupler Technology
The digital signals are transmitted across the isolation barrier
using iCoupler technology. This technique uses chip scale
transformer windings to couple the digital signals magnetically
from one side of the barrier to the other. Digital inputs are
encoded into waveforms that are capable of exciting the
primary transformer winding. At the secondary winding, the
induced waveforms are then decoded into the binary value that
was originally transmitted.
ENCODE
ENCODE
DECODE
DECODE
DECODE
ENCODE
TxD
RTS
RxD
V
DD1
V
DD2
GND
1
GND
2
A
B
DIGITAL ISOLATION
TRANSCEIVER
ISOLATION
BARRIER
D
R
RE
DE OUT
Figure 16. ADM2485 Digital Isolation and Transceiver Sections
TRUTH TABLES
The truth tables in this section use these abbreviations:
Letter Description
H High
level
I Indeterminate
L Low
level
X Irrelevant
Z
High impedance (off)
NC
Disconnected
Table 6. Transmitting
Supply Status
Inputs Output
V
DD1
V
DD2
RTS
TxD
A
B
DE
OUT
On On
H H H
L
H
On On
H L
L
H
H
On On
L X
Z
Z
L
On Off
X X
Z
Z
L
Off On
X X
Z
Z
L
Off Off
X X
Z
Z
L
Table 7. Receiving
Supply Status
Inputs Output
V
DD1
V
DD2
A-B
(V)
RE
RxD
On
On
>0.2
L or NC
H
On
On
<-0.2
L or NC
L
On
On
-0.2 < A - B < 0.2
L or NC
I
On
On
Inputs open
L or NC
H
On On X
H
Z
On
Off
X
L or NC
H
Off
On
X
L or NC
H
Off
Off
X
L or NC
L
Preliminary Technical Data
ADM2485
Rev. PrI | Page 11 of 13
THERMAL SHUTDOWN
The ADM2485 contains thermal shutdown circuitry that
protects the part from excessive power dissipation during fault
conditions. Shorting the driver outputs to a low impedance
source can result in high driver currents. The thermal sensing
circuitry detects the increase in die temperature under this
condition and disables the driver outputs. This circuitry is
designed to disable the driver outputs when a die
temperature of 150C is reached. As the device cools, the drivers
are re-enabled at a temperature of 140C.
RECEIVER FAIL-SAFE INPUTS
The receiver input includes a fail-safe feature that guarantees a
logic high RxD output when the A and B inputs are floating or
open-circuited.
MAGNETIC FIELD IMMUNITY
Because iCouplers use a coreless technology, no magnetic
components are present, and the problem of magnetic
saturation of the core material does not exist. Therefore,
iCouplers have essentially infinite dc field immunity. The
analysis below defines the conditions under which this may
occur. The ADM2485's 3 V operating condition is examined
because it represents the most susceptible mode of operation.
The limitation on the iCoupler's ac magnetic field immunity is
set by the condition in which the induced error voltage in the
receiving coil (the bottom coil in this case) is made sufficiently
large, either to falsely set or reset the decoder. The voltage
induced across the bottom coil is given by
-
=
2
n
r
dt
d
V
;
N
n
,
.
.
.
,
2
,
1
=
where, if the pulses at the transformer output are greater than
1.0 V in amplitude:
= magnetic flux density (gauss)
N = number of turns in receiving coil
r
n
= radius of nth turn in receiving coil (cm)
The decoder has a sensing threshold of about 0.5 V; therefore,
there is a 0.5 V margin in which induced voltages can be
tolerated.
Given the geometry of the receiving coil and an imposed
requirement that the induced voltage is, at most, 50% of the
0.5 V margin at the decoder, a maximum allowable magnetic
field is calculated, as shown in Figure 17.
04604-016
MAGNETIC FIELD FREQUENCY (Hz)
MAX
I
MUM ALLO
WABLE
MAG
N
E
T
IC
FLUX
DE
NS
ITY
(k
GAUS
S
)
1k
0.001
100
10
0.1
1
0.01
10k
100k
100M
1M
10M
Figure 17. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.2 kGauss induces a
voltage of 0.25 V at the receiving coil. This is about 50% of the
sensing threshold and does not cause a faulty output transition.
Similarly, if such an event occurs during a transmitted pulse and
is the worst-case polarity, it reduces the received pulse from
>1.0 V to 0.75 V--still well above the 0.5 V sensing threshold of
the decoder.
Figure 18 shows the magnetic flux density values in terms of
more familiar quantities such as maximum allowable current
flow at given distances away from the ADM2485 transformers.
04604-017
MAGNETIC FIELD FREQUENCY (Hz)
MAX
I
MUM ALLOWABLE
CURRE
NT (k
A)
1k
0.01
1000
100
1
10
0.1
10k
100k
100M
1M
10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
Figure 18. Maximum Allowable Current for
Various Current-to-ADM2485 Spacings
At combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board traces could induce
sufficiently large error voltages to trigger the thresholds of
succeeding circuitry. Care should be taken in the layout of such
traces to avoid this possibility.
ADM2485
Preliminary Technical Data
Rev. PrI | Page 12 of 13
APPLICATIONS INFORMATION
PC BOARD LAYOUT
The ADM2485 isolated RS-485 transceiver requires no external
interface circuitry for the logic interfaces. Power supply
bypassing is strongly recommended at the input and output
supply pins (see Figure ).
Bypass capacitors are most conveniently connected between Pin
3 and Pin 4 for V
DD1
and between Pin 15 and Pin 16 for V
DD2
.
The capacitor value should be between 0.01 F and 0.1 F. The
total lead length between both ends of the capacitor and the
input power supply pin should not exceed 20 mm.
Bypassing between Pin 1 and Pin 8 and between Pin 9 and Pin
16 should also be considered unless the ground pair on each
package side is connected close to the package.
D1
D2
GND
1
V
DD1
RxD
RE
RTS
TxD
V
DD2
GND
2
GND
2
B
A
GND
2
DE OUT
GND
2
ADM2485
Figure 19. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, care
should be taken to ensure that board coupling across the isola-
tion barrier is minimized. Furthermore, the board layout should
be designed such that any coupling that does occur equally
affects all pins on a given component side.
Failure to ensure this could cause voltage differentials between
pins exceeding the device's absolute maximum ratings, thereby
leading to latch-up or permanent damage.
APPLICATIONS DIAGRAM
The ADM2485 integrates a transformer driver which when
used with an external transformer and LDO generates an
isolated 5V power supply, to be supplied between the V
DD2
and
the GND
2
pins.
Pins D1 and D2 of the ADM2485 drive a center-tapped
transformer T1, A pair of Schottky diodes and a smoothing
capacitor are used to create a rectified signal from the
secondary winding. The
ADP667
linear voltage regulator
provides a regulated 5V power supply to the ADM2485's bus-
side circuitry (V
DD2
), as shown in Figure 20.
When the ADM2485 is powered by 3V on the logic side a
1CT:2.2CT transformer T1 is required to step up the 3V to 6V,
so that therefore is enough headroom for the
ADP667
LDO to
output a regulated 5V output.
If ADM2485 is powered by 5V on the logic side a 1CT:1.5CT
transformer T1 is required so that therefore is enough
headroom for the
ADP667
LDO to output a regulated 5V
output.
V
CC
ADP667
IN
OUT
GND
SET
SHDN
V
DD2
GND
2
V
DD1
GND
1
V
CC
ISOLATION BARRIER
1N5817
1N5817
22
F
+5V
T1
10
F
D1 D2
ADM2485
100nF
100nF
ISO 5V
Figure 20. Applications Diagram
Preliminary Technical Data
ADM2485
Rev. PrI | Page 13 of 13
OUTLINE DIMENSIONS
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MS-013AA
SEATING
PLANE
0.30 (0.0118)
0.10 (0.0039)
0.51 (0.0201)
0.31 (0.0122)
2.65 (0.1043)
2.35 (0.0925)
1.27 (0.0500)
BSC
16
9
8
1
10.65 (0.4193)
10.00 (0.3937)
7.60 (0.2992)
7.40 (0.2913)
10.50 (0.4134)
10.10 (0.3976)
8
0
0.75 (0.0295)
0.25 (0.0098)
45
1.27 (0.0500)
0.40 (0.0157)
0.33 (0.0130)
0.20 (0.0079)
COPLANARITY
0.10
Figure 21. 16-Lead Wide-Body Small Outline Package [SOIC]
(RW-16)
Dimensions shown in millimeters
ORDERING GUIDE
Model
Data Rate (Mbps)
Temperature Range
Package Description
Package Option
ADM2485BRWZ
1
16
-40C to +85C
16-Lead Wide Body SOIC
RW-16
ADM2485BRWZ-REEL
1
16
-40C to +85C
16-Lead Wide Body SOIC
RW-16
The addition of an "-RL" suffix designates a 13" (1000 units) tape and reel option.
1
Z = Pb-free part.
2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
PR06021-0-3/06(PrI)
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