AN520 Datasheet, PDF (1/10 Pages) Silicon Laboratories – CMOS ADVANCED GALVANIC ISOLATORS FOR MEDICAL ELECTRONICS

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CMOS ADVANCED GALVANIC ISOLATORS FOR MEDICAL ELECTRONICS
1. Introduction
Safety standards for ac line-powered medical electronic systems require galvanic isolation to protect patients and
operators from electrically-induced trauma. The direct connection between machine and patient together with the
presence of conductive body fluids and gels increase the risk of injury; therefore, isolators used in these systems
must be robust and reliable.
Optocouplers and transformers are commonly used within medical system isolation circuits, and their deficiencies
are well known to the design community. Optocouplers are notoriously slow and exhibit wide performance
variations over temperature and device lifetime. They are single-ended devices, which exhibit poor common-mode
transient immunity (CMTI). In addition, optocouplers are fabricated in Gallium Arsenide (GaAs) processes with
intrinsic wear-out mechanisms that cause permanent reductions in LED emission at elevated temperatures and/or
LED currents. This degradation reduces optocoupler reliability, performance, and service life. While transformers
offer higher speed and better reliability than optocouplers, they cannot pass dc and low-frequency signals, thus
imposing limits on system timing (e.g., ON-time and duty cycle). Transformers also tend to be large and power-
inefficient and often require additional external components for core reset.
2. CMOS Isolator Overview
Unlike optocouplers, complementary metal oxide semiconductor (CMOS) isolators offer substantial advantages in
performance, reliability, operating stability, power savings, and functional integration. Unlike transformers, CMOS
isolators operate from dc to 150 Mbps, require less space (up to six isolation channels per package), and are more
power-efficient. These advantages are made possible by the following fundamental technologies underlying CMOS
isolators.
 Standard CMOS Process Technology Silicon Dioxide Based Capacitive Isolation Barrier instead of
GaAs Process Technology
CMOS is a well understood process technology with 40+ years of learning and offers 5.5 times lower failures-in-
time (FIT) rate than GaAs-based optocouplers. The silicon dioxide isolation barrier offers a time dependent
dielectric breakdown (TDDB) of 60 years compared to less than 15 years in optocouplers. It also offers a mean
time-to-failure (MTTF) of 87 years. The operating temperature range is –40 to +125 °C as compared to –40 to
+85 °C for optocouplers. This wide operating temperature range leads to greater parametric stability over
voltage and temperature, and lower operating power versus optocouplers.
 Improved Performance
 Shorter propagation delay time and PWD, wider operating temperature range and greater parametric operating
permit greater system stability than when using optocouplers.
 High Frequency Carrier instead of Light
RF technology further reduces isolator operating power and adds the benefits of precise frequency
discrimination for superior noise rejection. Device packaging is also simpler compared to optocouplers.
 Fully Differential instead of Single-Ended Isolation Path
The differential signal path and high receiver selectivity enable CMTI above 25 kV/µs, excellent external RF
field immunity to 300 V/m, and magnetic field immunity greater than 1000 A/m for error-free operation. These
attributes make CMOS isolators well-suited for deployment in harsh operating environments where strong
electric and magnetic fields are present, such as in motor-control circuits and medical MRI systems.
 Proprietary EMI Suppression Techniques
CMOS isolators meet the emission standards of FCC Part B and are tested to automotive J1750 (CISPR) test
standards.
For more information on CMOS isolators emissions, susceptibility, and reliability vs. optocouplers, see the Silicon
Laboratories website at www.silabs.com/isolation.
Rev. 0.2 4/13
Copyright © 2013 by Silicon Laboratories
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