LTM2881 查看數據表（PDF） - Linear Technology

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LTM2881

Complete Isolated RS485/RS422 µModule Transceiver + Power

Linear Technology 

LTM2881

Applications Information

The PCB layout in Figures 16a to 16e show the low EMI

demo board for the LTM2881. The demo board uses a

combination of EMI mitigation techniques, including both

embedded PCB bridge capacitance and discrete GND to

GND2 capacitors. Two safety rated type Y2 capacitors

are used in series, manufactured by Murata, part number

GA342QR7GF471KW01L. The embedded capacitor ef-

fectively suppresses emissions above 400MHz, whereas

the discrete capacitors are more effective below 400MHz.

EMI performance is shown in Figure 17, measured using

a Gigahertz Transverse Electromagnetic (GTEM) cell and

method detailed in IEC 61000-4-20, “Testing and Mea-

surement Techniques – Emission and Immunity Testing

in Transverse Electromagnetic Waveguides.”

Cable Length versus Data Rate

For a given data rate, the maximum transmission distance

is bounded by the cable properties. A typical curve of

cable length versus data rate compliant with the RS485

standard is shown in Figure 18. Three regions of this

curve reflect different performance limiting factors in data

transmission. In the flat region of the curve, maximum

distance is determined by resistive loss in the cable. The

downward sloping region represents limits in distance and

rate due to the AC losses in the cable. The solid vertical

line represents the specified maximum data rate in the

RS485 standard. The dashed line at 250kbps shows the

maximum data rate when SLO is low. The dashed line at

20Mbps shows the maximum data rate when SLO is high.

RF, Magnetic Field Immunity

The LTM2881 has been independently evaluated and has

successfully passed the RF and magnetic field immunity

testing requirements per European Standard EN 55024,

in accordance with the following test standards:

EN 61000-4-3 Radiated, Radio-Frequency,

Electromagnetic Field Immunity

EN 61000-4-8 Power Frequency Magnetic Field

Immunity

EN 61000-4-9 Pulsed Magnetic Field Immunity

Tests were performed using an unshielded test card de-

signed per the data sheet PCB layout recommendations.

Specific limits per test are detailed in Table 1.

Table 1

TEST

EN 61000-4-3, Annex D

EN 61000-4-8, Level 4

EN 61000-4-8, Level 5

EN 61000-4-9, Level 5

*Non IEC Method

FREQUENCY

80MHz to 1GHz

1.4MHz to 2GHz

2GHz to 2.7GHz

50Hz and 60Hz

60Hz

Pulse

FIELD STRENGTH

10V/m

3V/m

1V/m

30A/m

100A/m*

1000A/m

10k

LOW-EMI MODE

1k

MAX DATA RATE

NORMAL

MODE MAX

100

DATA RATE

10

10k

RS485 MAX

DATA RATE

100k

1M

10M

DATA RATE (bps)

100M

2881 F18

Figure 18. Cable Length vs Data Rate

For more information www.linear.com/LTM2881

2881fh

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