TX IN

OOK

Modulation

Transmitter

Oscillator

TX Signal

Conditioning

Envelope

Detection

RX Signal

Conditioning

Receiver

RX OUT

Capacitive

Isolation

Barrier

Edge

Encoding

Refresh

Logic

Watch Dog

Timer

16

ISO7021

SLLSFA0 – JULY 2019

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9 Detailed Description

9.1 Overview

The ISO7021 device uses edge encoding of data with an ON-OFF keying (OOK) modulation scheme to transmit

the digital data across a silicon dioxide isolation barrier. The transmitter uses a high frequency carrier signal to

pass data across the barrier representing a signal edge transition. Using this method achieves very low power

consumption and high immunity. The receiver demodulates the carrier signal after advanced signal conditioning

and produces the output through a buffer stage. For low data rates, a refresh logic option is available to make

sure the output state matches the input state. Advanced circuit techniques are used to maximize the CMTI

performance and minimize the radiated emissions due the high frequency carrier and IO buffer switching. The

conceptual block diagram of a digital capacitive isolator, Figure 4, shows a functional block diagram of a typical

channel.

9.2 Functional Block Diagram

Figure 4. Conceptual Block Diagram of a Digital Capacitive Isolator

9.3 Feature Description

9.3.1 Refresh

The ISO7021 uses an edge based encoding scheme to transfer an input signal change across the isolation

barrier versus sending across the DC state. The built in refresh function consistently validates that the DC output

state of each isolator channel matches the DC input state. An internal watchdog timer monitors for activity on the

individual inputs and transmits the logic state when there is no input signal transition for more than 100 µs. This

ensures that the input and output state of the isolator always match.

9.3.2 Electromagnetic Compatibility (EMC) Considerations

Many applications in harsh industrial environment are sensitive to disturbances such as electrostatic discharge

(ESD), electrical fast transient (EFT), surge and electromagnetic emissions. These electromagnetic disturbances

are regulated by international standards such as IEC 61000-4-x and CISPR 22. Although system-level

performance and reliability depends, to a large extent, on the application board design and layout, the ISO70xx

family of devices incorporates many chip-level design improvements for overall system robustness. Some of

these improvements include:

•

Robust ESD protection cells for input and output signal pins and inter-chip bond pads.