ADM1491E

Rev. 0 | Page 10 of 16

THEORY OF OPERATION

The ADM1491E is an RS-485 transceiver that operates from a

single 5 V ± 5% power supply. The ADM1491E is intended for

balanced data transmission and complies with both TIA/EIA-485-A

and TIA/EIA-422-B. It contains a differential line driver and a

differential line receiver and is suitable for full-duplex data

transmission.

The input impedance of the ADM1491E is 12 kΩ, allowing up

to 32 transceivers on the differential bus. A thermal shutdown

circuit prevents excessive power dissipation caused by bus con-

tention or by output shorting. This feature forces the driver

output into a high impedance state if, during fault conditions,

a significant temperature increase is detected in the internal

driver circuitry.

The receiver contains a fail-safe feature that results in a logic

high output state if the inputs are unconnected (floating).

The ADM1491E features very low propagation delay, ensuring

maximum baud rate operation. The balanced driver ensures

distortion free transmission.

Another important specification is a measure of the skew

between the complementary outputs. Excessive skew impairs

the noise immunity of the system and increases the amount

of electromagnetic interference (EMI).

TRUTH TABLES

Table 6. Truth Table Abbreviations

Letter

Description

H

High level

I

Indeterminate

L

Low level

X

Irrelevant

Z

High impedance (off )

Table 7. Transmitting

Inputs

Outputs

DE

DI

Z

Y

H

H

L

H

H

L

H

L

L

X

Z

Z

Table 8. Receiving

Inputs

Output

RE

A − B

RO

L

≥ +0.2 V

H

L

≤ −0.2 V

L

L

−0.2 V ≤ A − B ≤ +0.2 V

I

L

Inputs open

H

H

X

Z

ESD TRANSIENT PROTECTION SCHEME

The ADM1491E uses protective clamping structures on its

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

dissipate the energy present in ESD (electrostatic). The

protection structure achieves ESD protection up to ±8 kV

human body model (HBM).

ESD Testing

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

charge and air gap discharge. Contact discharge calls for a direct

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 humidity, tem-

perature, barometric pressure, distance, and rate of closure of

the discharge gun. The contact discharge method, though 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 because of arcing or heating. Even if cata-

strophic failure does not occur immediately, the device can suffer

from parametric degradation, resulting in degraded performance.

The cumulative effects of continuous exposure can eventually

lead to complete failure.

C1

R2

HIGH

VOLTAGE

GENERATOR

DEVICE

UNDER TEST

NOTES

1. THE ESD TEST METHOD USED IS THE

HUMAN BODY MODEL (±8kV) WITH

R2 = 1500

Ω AND C1 = 100pF.

Figure 25. ESD Generator

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 destroy the interface product connected

to the I/O port. It is, therefore, extremely important to have high

levels of ESD protection on the I/O lines.

The ESD discharge can induce latch-up in the device under test.

Therefore, it is important to conduct ESD testing on the I/O pins

while device power is applied. This type of testing is more repre-

sentative of a real-world I/O discharge where the equipment is

operating normally when the discharge occurs.