AN-960 Просмотр технического описания (PDF) - Analog Devices

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AN-960

RS-485/RS-422 Circuit Implementation Guide

Analog Devices 

AN-960

APPLICATION NOTE

ISOLATION

In RS-485 applications, there are often long links, which can

cause the ground potential at different nodes on the bus to be

slightly different. This causes ground currents to flow through

the path of least resistance through either the common earth

ground or the ground wire. If the same electrical system is

used to connect the power supplies of all nodes to the same

earth ground, the ground connection may have reduced noise.

Note, however, that motors, switches, and other electrically

noisy equipment can still induce ground noise into the system.

When different nodes are situated in different buildings,

different power systems are required. This is likely to increase

the impedance of the earth ground and the ground currents

from other sources are more likely to find their way into the

link’s ground wire. Isolating the link reduces or even eliminates

these problems. 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 (see Figure 13).

POINT A

ISOLATOR

POINT B

INFORMATION FLOW

NO GROUND CURRENT FLOW

ISOLATION

BARRIER

PROTECT HUMANS/

EQUIPMENT

ELIMINATE GROUNDING

PROBLEMS

IMPROVE SYSTEM

PERFORMANCE

Figure 13. Galvanic Isolation Allows Information Flow While Preventing

Ground Current Flow

The signal lines, as well as the power supply, must be isolated.

Power isolation is achieved by an isolated dc-dc supply, such as

Analog Devices, Inc. isoPower™, and signal isolation is achieved

via Analog Devices iCoupler® technology. See Figure 14 for an

example of how to achieve signal and power isolation using the

ADM2485.

LOCAL

POWER

SUPPLY

5V

100nF

VDD1

TxD

RTS

UART

RxD

5V CENTER TAPPED

TRANSFORMER

DA2303-AL RECTIFIER

1:11.5

VDD1

22µF

LDO

+5V ISO

IN

OUT

ADP3330

10µF

SD GND ERR

D1

D2

OSC

ENCODE

ENCODE

ADM2485

GND 2

DECODE

D

DECODE

100nF

VDD2

A

RT

B

DE OUT

RS-485 CABLE

DECODE

ENCODE

R

RE

GND1

DIGITAL ISOLATION iCoupler

RS-485 TRANSCEIVER

GND1

ISOLATION

BARRIER

GND2

Figure 14. Signal and Power Isolation using the ADM2485

Rev. 0 | Page 8 of 12

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