ADM3485E

–8–

REV. A

Table I. Comparison of RS-422 and RS-485 Interface Standards

Specification

RS-422

RS-485

Transmission Type

Differential

Differential

Maximum Data Rate

10 MB/s

10 MB/s

Maximum Cable Length

4000 ft.

4000 ft.

Minimum Driver Output Voltage

±2 V

±1.5 V

Driver Load Impedance

100

Ω

54

Ω

Receiver Input Resistance

4 k

Ω min

12 k

Ω min

Receiver Input Sensitivity

±200 mV

±200 mV

Receiver Input Voltage Range

–7 V to +7 V –7 V to +12 V

No. of Drivers/Receivers Per Line

1/10

32/32

Table II. Transmitting Truth Table

Transmitting

Inputs

Outputs

RE

DE

DI

B

A

X1

1

0

1

X1

0

1

0

0

0

X

Hi-Z

Hi-Z

1

0

X

Hi-Z

Hi-Z

Table III. Receiving Truth Table

Receiving

Inputs

Outputs

RE

DE

A–B

RO

0

X

> +0.2 V

1

0

X

< –0.2 V

0

0

X

Inputs O/C

1

1X

X

Hi-Z

ESD/EFT TRANSIENT PROTECTION SCHEME

The ADM3485E uses protective clamping structures on its

inputs and outputs that clamp the voltage to a safe level and

dissipate the energy present in ESD (Electrostatic) and EFT

(Electrical Fast Transients) discharges.

The protection structure achieves ESD protection up to

±8 kV

according to IEC1000-4-2, and EFT protection up to

±2 kV on

all I-O lines.

ESD TESTING

Two coupling methods are used for ESD testing, contact dis-

charge and air-gap discharge. Contact discharge calls for a di-

rect connection to the unit being tested. Air-gap discharge uses

a higher test voltage but does not make direct contact with the

unit under test. With air discharge, the discharge gun is moved

toward the unit under test, developing an arc across the air gap,

hence the term air-discharge. This method is influenced by hu-

midity, temperature, barometric pressure, distance and rate of

closure of the discharge gun. The contact-discharge method,

while less realistic, is more repeatable and is gaining acceptance

and preference over the air-gap method.

Although very little energy is contained within an ESD pulse,

the extremely fast rise time, coupled with high voltages, can

cause failures in unprotected semiconductors. Catastrophic

destruction can occur immediately as a result of arcing or heat-

ing. Even if catastrophic failure does not occur immediately, the

device may suffer from parametric degradation, which may

result in degraded performance. The cumulative effects of con-

tinuous exposure can eventually lead to complete failure.

I-O lines are particularly vulnerable to ESD damage. Simply

touching or plugging in an I-O cable can result in a static dis-

charge that can damage or completely destroy the interface

product connected to the I-O port.

It is extremely important, therefore, to have high levels of ESD

protection on the I-O lines.

It is possible that the ESD discharge could induce latchup in the

device under test, so it is important that ESD testing on the I-O

pins be carried out while device power is applied. This type of

testing is more representative of a real-world I-O discharge

where the equipment is operating normally when the discharge

occurs.

TEMPERATURE – C

1.20

–50

70

–30

–10

10

30

50

1.15

1.05

1.00

0.95

0.80

1.10

0.90

0.85

90

110

0.75

0.70

Figure 19. Supply Current vs. Temperature

TEMPERATURE – C

100

–40

80

–20

0

204060

90

70

60

50

20

80

40

30

10

0

Figure 20. Shutdown Current vs. Temperature