MS-2234 Datasheet, PDF (1/3 Pages) Analog Devices – Anatomy of a Digital Isolator

.
Anatomy of a Digital Isolator
by David Krakauer, Product Line Manager,
Analog Devices, Inc.
IDEA IN BRIEF
Digital isolators offer significant, compelling advantages
over optocouplers in terms of size, speed, power
consumption, ease of use, and reliability.
For years, designers of industrial, medical, and other
isolated systems had limited options when
implementing safety isolation: the only reasonable
choice was the optocoupler. Today, digital isolators offer
advantages in performance, size, cost, power efficiency, and
integration. Understanding the nature and interdependence
of three key elements of a digital isolator is important in
choosing the right digital isolator. These elements are
insulation material, their structure, and data transfer
method.
Designers incorporate isolation because of safety regulations
or to reduce noise from ground loops, etc. Galvanic isolation
ensures data transfer without an electrical connection or
leakage path that might create a safety hazard. Yet, isolation
imposes constraints such as delays, power consumption,
cost, and size. A digital isolator’s goal is to meet safety
requirements while minimizing incurred penalties.
Technical Article
MS-2234
Optocouplers, a traditional isolator, incur the greatest
penalties, consuming high levels of power and limiting data
rates to below 1 Mbps. More power efficient and higher
speed optocouplers are available but impose a higher cost
penalty.
Digital isolators were introduced over 10 years ago to reduce
penalties associated with optocouplers. They use CMOS-
based circuitry and offer significant cost and power savings
while significantly improving data rates. They are defined by
the elements noted above. Insulating material determines
inherent isolation capability and is selected to ensure
compliance to safety standards. Structure and data transfer
method are chosen to overcome the cited penalties. All three
elements must work together to balance design targets, but
the one target that cannot be compromised and “balanced”
is the ability to meet safety regulations.
Insulation Material
Digital isolators use foundry CMOS processes and are
limited to materials commonly used in foundries.
Nonstandard materials complicate production, resulting in
poor manufacturability and higher costs. Common
insulating materials include polymers such as polyimide
(PI), which can be spun on as a thin film, and silicon dioxide
(SiO2). Both have well known insulating properties and have
been used in standard semiconductor processing for years.
Polymers have been the basis for many optocouplers, giving
them an established history as a high voltage insulator.
Figure 1. Transformer with Thick Polyimide Insulation Where Current Pulses Create Magnetic Fields to Induce Current on the Secondary Coil (left);
Capacitor with Thin SiO2 Insulation Using Low Current Electric Fields to Couple Across Isolation Barrier (right)
October 2011 | Page 1 of 3
www.analog.com
©2011 Analog Devices, Inc. All rights reserved.