Texas Instruments | Digital Isolator E-Field Sensitivity | Application notes | Texas Instruments Digital Isolator E-Field Sensitivity Application notes

Texas Instruments Digital Isolator E-Field Sensitivity Application notes
Application Report
SLLA267 – September 2007
Digital Isolator E-Field Sensitivity
Kevin Gingerich
..................................................... High-Performance Analog/Interface and Clock Products
The small differential signal circuit used in Texas Instruments digital isolators is immune to external
electric fields making common-mode voltage the primary constraint. The intensity vector and physical
circuit determine the field-induced voltage that appears across the isolation barrier, and common-mode
transient immunity (CMTI) characterizes the high-frequency isolation characteristics of an isolator. Figure 1
depicts horizontal and vertical electric field intensity vectors, isolated dice, and inter-die bond wires making
the differential circuit.
Figure 1. Electric Fields and Isolated Dice
Differential Noise
The only method of externally creating a voltage around the 944 × 10–9 m2 or smaller differential circuits
with an electric field is to expose only one inter-die bond wire (or a portion) to the field without exposing
the rest of the loop (1). Intuition says this is extremely unlikely. Maxwell’s third equation says this would
require a non-zero net flux from an internal volume and a charge density inside the device. This does not
exist in real-world applications.
See ISO72X Digital Isolator Magnetic-Field Immunity (Rev. A, slla181a.htm, 0 KB, 22 Feb 2006, Abstract) for the influence from
time-varying magnetic fields.
SLLA267 – September 2007
Submit Documentation Feedback
Digital Isolator E-Field Sensitivity
Common-Mode Noise
Common-Mode Noise
The voltage induced by an electric field is equal to the summation of the dot product of the electric field
intensity and differential length vectors along a path or:
v AB = ò E · dL
If the field and path are parallel and the intensity is uniform, the voltage is simply E × L. If L is the width of
an SOIC package (about 6 mm), external field strength greater than 667 × 103 V/m exceeds a 4-kV-rated
isolation barrier. If L is the vertical thickness of a die (about 305 × 10–6 m), external field strength greater
than 1.64 × 103 V/m exceeds a 0.5-V noise margin typical of CMOS logic circuits. Table 1 gives field
strengths from various sources.
Table 1. Electric Field Strengths (1)
Background radiation in space
(N.C–1 or V.m–1)
3 × 10–6
In-house wires
Radio waves
Outside an electrified building
Center of a typical living room
In a fluorescent tube
30 cm from an electric clock
30 cm from a stereo
Laser beam (low power)
Atmosphere (fair weather)
30 cm from electric blanket
Built up by splashing water in a shower
Sunlight (average)
Atmosphere (thunderstorm)
Van de Graaff generator
2 × 106
Breakdown of air
3 × 106
X-ray tube
5 × 106
At cell membrane
At electron in hydrogen atom
Surface of a pulsar
Surface of uranium atom
6 × 1011
2 × 1021
Physics Resource Database, The University of Sydney (Australia), School of Physics
These examples cover a wide range of frequencies. A digital isolator rated to 150 Mbps has a bandwidth
of at least 300 MHz to operate. At higher frequencies, gains roll off and transistors need more signal to
switch, essentially raising the noise margins. Due to the capacitance of the isolation barrier,
common-mode injection to the differential circuit increases with frequency and CMTI specifies it.
Digital Isolator E-Field Sensitivity
SLLA267 – September 2007
Submit Documentation Feedback
The worst-case CMTI rating for these devices is 25 × 10 V/s with a 5-V supply and gives a
common-mode voltage slew rate that will not affect the logic state of the device. If the induced
common-mode voltage is a sinusoid, vCM = V sin(2 pft) and the slew rate is:
dv CM
= 2pfV cos(2pft)
The constraint is 2 pfV cos(2 pft) < 25 × 109 and, taking the magnitude and dividing both sides by 2 p,
fV < 4 × 109. This gives a constraint for the magnitude and frequency of a sinusoidal common-mode
voltage. This also indicates that the full 4-kV rating is good to 1 MHz and drops to 4 V at 1000 MHz. This
is still 667 V/m based on the analysis above and conservatively assumes the circuit will still respond at 1
GHz (note that few of the listed sources have much, if any, radiation at this frequency).
Clearly, the capacitive isolators from TI have orders of magnitude more immunity to electric fields than the
much larger circuits connected to the isolator. Therefore, in most applications, the external circuits or
shielding determines the overall equipment susceptibility from external electric fields. Unless a user is
planning on operating the isolator unshielded in a thunderstorm or on the surface of a pulsar, the
capacitive-isolation technology family from TI gives huge margins with which to work.
SLLA267 – September 2007
Submit Documentation Feedback
Digital Isolator E-Field Sensitivity
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements,
improvements, and other changes to its products and services at any time and to discontinue any product or service without notice.
Customers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s
standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this
warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should
provide adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask
work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services
are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such
products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under
the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is
accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an
unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Information of third parties
may be subject to additional restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service
voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business
practice. TI is not responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would
reasonably be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement
specifically governing such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications
of their applications, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related
requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any
applications-related information or support that may be provided by TI. Further, Buyers must fully indemnify TI and its
representatives against any damages arising out of the use of TI products in such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is
solely at the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in
connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products
are designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any
non-designated products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Data Converters
Digital Control
Power Mgmt
Optical Networking
Low Power
Video & Imaging
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2007, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF