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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
Introduction
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.
E
E
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.
(1)
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.
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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:
A
v AB = ò E · dL
B
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)
SOURCE
Background radiation in space
FIELD STRENGTH E
(N.C–1 or V.m–1)
3 × 10–6
In-house wires
0.01
Radio waves
~0.1
Outside an electrified building
~0.1
Center of a typical living room
~3
In a fluorescent tube
10
30 cm from an electric clock
15
30 cm from a stereo
90
Laser beam (low power)
100
Atmosphere (fair weather)
~250
30 cm from electric blanket
250
Built up by splashing water in a shower
800
Sunlight (average)
103
Atmosphere (thunderstorm)
104
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
(1)
107
6 × 1011
~1014
2 × 1021
Physics Resource Database, The University of Sydney (Australia), School of Physics
(www.physics.usyd.edu.au/teach_res/db/d0006b.htm)
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.
2
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Conclusion
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:
9
dv CM
= 2pfV cos(2pft)
dt
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).
Conclusion
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.
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