a forum for the COiris ai ONGCCiCoUCre TUEiLCop COM CeCe signal processing ,
`SNCea OeVCReaaata MICROMACHINED ACCELEROMETERS (page 3)
`SgeamoatseTLLL EU, and RS-232: issues and answers (page 6)
`
`APPLE 1023
`
`ANALOG
`DEVICES
`
`Volume 30, Number 4, 1996
`
`
`
`APPLE 1023
`
`1
`
`

`

`Editor’s Notes
`THIRTY YEARS OF ANALOG DIALOGUE
`In Volume 15-1 (1981), in cele-
`bration of 15 years in print, we
`listed the first 15 years of Analog
`Dialogue cover features. At that
`time, we wrote: “. . . will mark the
`15th year of publication of this
`journal, and the 13th year of our
`stewardship. While adding a copy
`of the [most recent] issue to our
`bulging binder, we nostalgically
`turned the pages of issues long forgotten. The amount of
`technological progress reported in them surprised even us. Before
`the list expands beyond the capacity of this column, we thought
`you might be interested in seeing a roll of just the cover stories
`alone, though much significant progress never attained the cover.”
`In this spirit, in celebration of our 30th year in print (and 28th of
`stewardship), here are the second 15 years of cover themes:
`
`1981 15-2 D/A converters for graphic displays
`1982 16-1 High-performance hybrid-circuit isolation amp (AD293)
`16-2 Dual monolithic multiplying DACs (AD7528)
`16-3 Monolithic instrumentation amplifier (AD524)
`and thermocouple preamp (AD594)
`1983 17-1 CMOS ICs for digital signal processing
`17-2 Quad CMOS DAC with buffered voltage outputs
`1984 18-1 Amplifier noise basics revisited
`18-2 Monolithic V-out µP-compatible 12-bit DAC (AD667)
`18-3 An intelligent vision system for industrial image analysis
`1985 19-1 Multifunction analog IC computes y(z/x)m, etc. (AD538)
`1986 20-1 Low-cost, high-performance, compact iso amps (AD202/4)
`20-2 Fast, flexible CMOS DSP µP (ADSP-2100)
`1987 24-1 Monolithic process-control transmitters (AD693)
`21-2 Complete 8-bit 400-ksps analog I/O (AD7569)
`1988 22-1 Chipset for 50-Mbit/s digital data recovery (AD890/891)
`22-2 200-MSPS 8-bit IC ADC with 250-MHz BW (AD770)
`1989 23-1 Isolated sensor-to-serial with a screwdriver (6B Series)
`23-2 Single-chip DSP microcomputer (ADSP-2101)
`23-3 DC-120-MHz IC log amp—accurate compression (AD640)
`23-4 Pin electronics for high-speed ATE (AD1315/1521/22)
`1990 24-1 Monolithic 75-MSPS 10-bit flash converter (AD9060)
`24-2 Mixed-signal processor: DSP/ADC/DAC (ADSP-21msp50)
`24-3 RAM-DAC upgrade enhances VGA graphics (AD7148)
`1991 25-1 Pro-Logic decoder: Dolby“Surround Sound” (SSM2125)
`25-2 ADSP-21020 floating-point high-speed DSP
`1992 26-1 Mixed-signal chips drive digital radio (AD7001/7002)
`26-2 Wideband “linear in dB” VCA (AD600/602)
`1993 27-1 Fast precise 155-Mbps fiberoptic timing recovery (AD802)
`27-2 Single-chip micromachined accelerometer (ADXL50)
`1994 28-1 Dual-setpoint single-chip temperature controller (TMP01)
`28-2 Complete, low-distortion 500-MHz IC mixer (AD831)
`28-3 SHARC Floating-point DSP: tops in memory, performance
`1995 29-1 Integrated stereo codecs for multimedia (AD1843/1845)
`29-2 Meeting the challenges of high speed
`29-3 Considerations in low-power, single-supply system design
`1996 30-1 Read-channel processor uses PRML with MR heads
`30-2 DSP-based chip set for ac motor control (ADMC201)
`30-3 CMOS DACs optimized for transmit path (AD976x family)
`30-4 Dual-axis accelerometer (ADXL250)
` A
`Dan.Sheingold@analog.com
`
`THE AUTHORS
`Howard Samuels (page 3) is
`a Product Development Engineer
`in ADI’s Micromachined Products
`Division in Wilmington, MA. He
`joined Analog after graduating
`from Carnegie Mellon University
`in 1982 with a BS in EE and
`Applied Mathematics. He holds
`several patents and has published
`articles in trade magazines.
`Howard has designed signal conditioners and isolators, including
`the AD210 3-port isolator. When not at work, he enjoys caring for
`and taking pictures of his new baby.
`Matt Smith (page 6) is a Senior
`Applications Engineer at our
`Limerick, Ireland, facility. He is
`responsible for Interface and
`Supervisory products. He holds a
`B.Eng from the University of
`Limerick. His leisure interests
`include playing squash, motor
`maintenance and more recently,
`woodworking.
`Bill Slattery (page 9) has
`marketing responsibility for
`products of the Digital Video
`Group at ADI’s Limerick facility,
`including video DACs, RAM-
`DACs, and video encoders &
`decoders. Earlier, he worked as a
`Senior Engineer in the Applications
`Group in Limerick. Bill has a BSc
`(Eng) degree from the University of
`Dublin (Trinity College) and an MBA from the University of
`Limerick. A licensed private pilot, he enjoys flying his airplane.
`Jürgen Kühnel (page 13) is a
`Senior Marketing Engineer for
`Power Management products,
`located in Munich. Since joining
`ADI in 1984, he has had various
`roles in Sales and Marketing in
`Central Europe. His Dipl. Ing. is
`from the Technische Fachhochschule
`in Berlin, and he worked for several
`years as a system designer in
`medical and chemical instrumentation. In his spare time he likes
`riding on two wheels (powered or not) and designing electronic
`systems for his model railroad.
`
`[More authors on page 22]
`Cover: The cover illustration was designed and executed by
`Shelley Miles, of Design Encounters, Hingham MA.
`
`One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106
`Published by Analog Devices, Inc. and available at no charge to engineers and
`scientists who use or think about I.C. or discrete analog, conversion, data handling
`and DSP circuits and systems. Correspondence is welcome and should be addressed
`to Editor, Analog Dialogue, at the above address. Analog Devices, Inc., has
`representatives, sales offices, and distributors throughout the world. Our web site is
`http://www.analog.com/. For information regarding our products and their
`applications, you are invited to use the enclosed reply card, write to the above address,
`or phone 617-937-1428, 1-800-262-5643 (U.S.A. only) or fax 617-821-4273.
`
`2
`
`ISSN 0161–3626 ©Analog Devices, Inc. 1996
`
`Analog Dialogue 30-4 (1996)
`
`2
`
`

`

`Single- and Dual-Axis
`Micromachined
`Accelerometers
`ADXL150 & ADXL250: New complete
`low-noise 50-g accelerometers
`by Howard Samuels
`The ADXL150 and ADXL250 represent the newest generation
`of surface-micromachined monolithic accelerometers* from
`Analog Devices. Like the landmark ADXL50 (Analog Dialogue
`27-2, 1993), the new devices include both the signal conditioning
`circuitry and the sensor, fabricated together on a single monolithic
`chip—providing acceleration measurement at very low cost with
`high reliability. As with the ADXL50, the sensor structure is a
`differential capacitor, but it is modified to take advantage of the
`experience gained from producing millions of ADXL50s, further
`advancing the state of the art of micromachined sensor design.
`The sensor: The silhouettes in Figure 1 compare the sensors used
`in the ADXL50 and the ADXL150. Both sensors have numerous
`fingers along each side of the movable center member; they
`constitute the center plates of a paralleled set of differential
`capacitors. Pairs of fixed fingers attached to the substrate interleave
`
`TETHER
`
`with the beam fingers to form the outer capacitor plates. The beam
`is supported by tethers, which serve as mechanical springs. The
`voltage on the moving plates is read via the electrically conductive
`tether anchors that support the beam.
`The polysilicon support springs (tethers) are highly reliable. Many
`devices have been tested by deflecting the beam with the equivalent
`of > 250× the force of gravity, for > 7 × 1010 cycles, with zero
`failures, as part of the product qualification process.
`The ADXL50’s tethers extend straight out from the beam in an
`‘H’ configuration. On the ADXL150, however, the tethers are
`folded, reducing the size of the sensor and halving the number of
`anchors (Figure 2). Since each anchor adds parasitic capacitance,
`the smaller number of anchors reduces capacitive load, increasing
`the sensor’s acceleration sensitivity. In addition, the tether geometry
`minimizes sensitivity to mechanical die-stress; this allows the
`ADXL150 to be packaged in standard cerdip and surface-mount
`cerpak packages, which require higher sealing temperatures (and
`associated thermal stress) than metal cans. The folded tether was
`first used in the ADXL05 low-g accelerometer; its higher sensitivity
`makes die stress more of a concern.
`In addition to the sense fingers projecting from both sides of the
`beam, the ADXL150 has 12 force fingers (visible near both ends
`
`BEAM
`BEAM
`
`SENSE
`SENSE
`FINGERS
`FINGERS
`
`AXIS OF
`AXIS OF
`ACCELERATION
`ACCELERATION
`
`FORCE
`FORCE
`FINGERS
`FINGERS
`
`ANCHOR
`ANCHOR
`
`TETHER
`TETHER
`
`FOLDED TETHER
`
`Figure 2. Partial aerial SEM view of one end of the
`ADXL150’s sensor.
`
`IN THIS ISSUE
`Volume 30, Number 4, 1996, 24 Pages
`Editor’s Notes, Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
`Single- and dual-axis micromachined accelerometers (ADXL150, ADXL250) . 3
`EMC, CE Mark, IEC801 . . . What’s it all about? . . . . . . . . . . . . . . . . . . 6
`Integrated digital video encoders—studio quality video at consumer video prices 9
`Selecting mixed-signal components for digital communication systems—II . . 11
`Voltage regulators for power management . . . . . . . . . . . . . . . . . . . . . . . . 13
`New-Product Briefs:
`Three new op amp families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
`A/D and D/A converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
`DSPs and Mixed-signal processors . . . . . . . . . . . . . . . . . . . . . . . . 18
`Mixed bag: Circuit protectors, Temperature to current,
`Switched-cap regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
`Ask The Applications Engineer—23: Current-feedback amplifiers—II . . . 20
`Worth Reading, More authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
`Potpourri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
`
`3
`
`Figure 1. Silhouette plots of ADXL50 (upper) and ADXL150
`(lower). Axis of motion is vertical.
`
`*For technical data, visit our Web site, http://www.analog.com. Data is also avail-
`able in North America around the clock by Analogfax™, 1-800-446-6212;
`request 2060; or use the reply card. Circle 1
`
`Analog Dialogue 30-4 (1996)
`
`3
`
`

`

`of the beam), used for self-test actuation. The plates of a parallel-
`plate capacitor attract each other with an electrostatic force of:
`F = εAV 2
`2d 2
`
`
`where ε is the permittivity of the material between the plates, A is
`the area of the plates, V is the voltage across the capacitor, and d is
`the distance between the plates.
`In normal operation, the fixed fingers on either side of the force
`fingers are at the same voltage potential as the beam and its fingers.
`With no voltage between the force fingers on the beam and the
`fixed fingers on the substrate, there is no electrostatic force.
`However, when a digital self-test input pin is activated, the fixed
`fingers on one side of the force section are driven to a nonzero dc
`voltage, applying a force to the sense fingers, deflecting the beam.
`The forcing voltage is laser-trimmed to produce a net electrostatic
`force on the beam equivalent to a 10-g acceleration. This voltage
`will depend on the specific electrical and mechanical characteristics
`of each individual device.
`The self-test circuitry operates independently of the normal
`accelerometer signal path. When self-test is activated, the deflection
`it produces is measured by the device in the same way as a
`deflection produced by accelerating the entire device. Since the
`full-scale deflection of the sensor is only about 1.5% of the gap
`between the capacitor fingers, the self-test response is nearly
`constant, adding to the deflection caused by any existing
`acceleration. Like an externally applied acceleration, the deflection
`produced by the self-test circuitry makes full use of the
`measurement circuitry of the normally functioning accelerometer
`to generate an output, so it is a highly reliable indicator of the
`device’s ability to function correctly.
`Circuit Architecture: As Figure 3 shows, the fixed fingers are
`driven with antiphase square waves. Unlike the ADXL50, which
`uses a dc bias between the excitations and the beam as a means of
`providing a force-balance feedback path, the ADXL150 employs
`an open-loop architecture. With zero average dc voltage on the
`beam, the excitation square waves can swing to the power supply
`rails, with the beam biased at one half the supply voltage. The
`larger amplitude of the 100-kHz excitation in the ADXL150 results
`in reduced sensitivity to electronic device noise and is a contributing
`factor to its improved noise performance.
`If the beam is perfectly centered, both sides of the differential
`capacitor have equal capacitance, and the ac voltage on the beam
`is zero. However, if the beam is off center due to an applied
`acceleration (or self-test deflection), the differential capacitor
`
`VS/2
`
`VST
`
`+ –
`
`VS
`2 – VST
`
`VO
`
`TWO-POLE
`FILTER
`G = 3
`
`– +
`
`R
`
`5R
`
`DEMOD
`
`FORCE
`FINGERS
`
`VS/2
`
`CLOCK/
`TIMING
`
`VS/2
`
`VS/2
`
`ZERO-g
`ADJUST
`
`TEST
`INPUT
`
`ST
`
`SENSE
`FINGERS
`
`VS
`
`RI
`
`VO
`
`ROS
`
`C
`
`RF
`
`VS/2
`
`VO'
`
`ADXL150
`
`VS
`
`SENSOR
`
`AMP
`
`DEMOD
`
`R
`
`5R
`
`VS/2
`
`ADXL150 WITH ADDITIONAL GAIN AV = RF/RI
`VS
`RF
`OFFSET ADJUSTMENT RANGE OF ± ,
`2
`ROS
`1
`AND ADDITIONAL SINGLE POLE LPF, –3dB = .
`2πRFC
`
`Figure 3. ADXL150 electrical block diagram.
`
`Figure 4. ADXL150 with an external op amp for additional
`gain and filtering.
`
`4
`
`Analog Dialogue 30-4 (1996)
`
`becomes unbalanced. The beam waveform is a square wave with
`amplitude proportional to the amount of displacement, and hence,
`acceleration magnitude. The phase of the beam voltage relative to
`the excitation determines the acceleration polarity.
`The beam output is connected directly to a noninverting amplifier,
`which provides buffering for the high impedance beam node, as
`well as gain for the 100-kHz output signal.
`The output is demodulated in a synchronous demodulator that
`samples the amplifier output after it has settled in each half of the
`excitation cycle. By detecting the difference between the amplifier’s
`output levels for the two states, the offset voltage of the amplifier
`is eliminated, much like that of a chopper stabilized amplifier. Since
`the demodulator is phase synchronized with the excitation, the
`output signal polarity correctly indicates the direction of the applied
`acceleration.
`The ADXL150 has a 2-pole gain-of-3 Bessel low-pass filter on
`board [the ADXL250—see below—includes a 2-pole filter for each
`channel]. These filters can be used to prevent aliasing of high-
`frequency components in the demodulator output with A/D
`converter clock frequencies in associated data-acquisition circuitry.
`A second input to the filter is connected to a resistive divider with
`a gain of 1/6, brought out to a package pin. It provides a convenient
`offset adjustment point for the accelerometer, with a net gain of
`+0.5 for the applied voltage.
`Because extensive use is made of CMOS logic, and the open-loop
`architecture allows simpler signal conditioning circuitry, the device
`draws only 1.8 mA of supply current at 5 V (including the 2-pole
`output filter), a >80% reduction from the ADXL50.
`The increased excitation levels used, along with carefully executed
`chopper modulation/demodulation techniques, yield a noise
`density of just 1 mg/√Hz, less than 1/6 that of the ADXL50. The
`improved dynamic range enables the ADXL150 to be used in
`applications such as machine health, vibration monitoring, shock
`sensing, and instrumentation.
`The ADXL150 has a sensitivity of 38 mV/g, measured at the output
`pin. The full scale range is ± 50 g, for a total signal swing of 3.8 V,
`with a single 5-V supply. This significant output voltage range allows
`the designer to take full advantage of the input range of a single-
`supply A/D converter, such as might be found in a microprocessor
`system.
`The output voltage is given by the relationship:
`+ α⋅0.038 V
`5 V
`
`⎞⎠⎟
`
`1 2
`
`⎛⎝⎜
`
`VO =V S
`
`
`
`4
`
`

`

`α is the applied acceleration expressed in gs (1 g ≈ 9.8 m/s2), and
`VS is the power supply and reference voltage, nominally 5 V. If VS
`is also used as the reference for a ratiometric A/D converter, the
`system will reject variations in VS. With zero applied acceleration,
`the output of the ADXL150 is VS/2, which is half scale of the A/D
`converter. Even if Vs is not exactly 5 V, the digital output code of
`the A/D converter still reads half scale. For any applied acceleration,
`the output of the A/D converter will be essentially independent of
`changes in VS.
`Without external manipulation of the filter’s offset, the device
`provides a convenient reference point at one half the power supply
`voltage. An external operational amplifier can be used (Figure 4),
`for additional gain with respect to this voltage to increase the
`sensitivity of the accelerometer. An additional external capacitor
`can be used in this circuit to add a third pole after the internal
`two-pole filter. The offset can be adjusted by current injected into
`the summing node of the external amplifier.
`The ADXL250 adds a new dimension: The ADXL250, a single
`monolithic chip (Figure 5), measures both the x and y coordinates
`of acceleration in a given plane (e.g., forward-back and side-to-
`side). Because the sensitive axis of the ADXL150’s sensor is in the
`plane of the chip, twin sensors can be fabricated on the same die,
`with one rotated 90 degrees from the other. The ADXL250 is the
`world’s first commercially available two-axis monolithic
`accelerometer.
`Both channels share the clock generator, demodulator timing, self
`test logic, and bias voltage. Each sensor receives the clock signals
`via its own CMOS inverter drivers, and the signals generated by
`the sensors are treated completely independently.
`The single self-test pin activates both sensors simultaneously,
`simplifying the interface to a microprocessor. As in the ADXL150,
`the test signal deflects each sensor by an amount equivalent to a
`10-g acceleration. Each channel has its own offset adjustment pin
`and its own output voltage pin. Both channels have the same
`sensitivity.
`The total power-supply current of the two-channel ADXL250, is
`typically 3.5 mA (5 mA maximum, including the output filters—
`just half the typical supply current of the earlier ADXL50). Both
`devices have A and J versions, specified for temperature ranges –
`40 to +85°C and 0 to +70°C. Prices (100s) start at $12.45
`(ADXL150JQC) and $19.95 (ADXL250JQC).
`How do I use them? The ADXL150 is a complete sensor on a
`chip. Just connect a single 5-V power supply (with clean output,
`bypassed to ground by a decent-quality ceramic capacitor) and
`connect the output to its readout destination.
`
`ST
`
`If the self-test pin is left open-circuited, an internal pulldown
`resistor ensures normal operation. With nothing connected to the
`offset adjust pin, the output voltage is unmodified.
`To adjust the output zero-g voltage level, use the offset adjust pin.
`The offset can be adjusted by applying an analog dc voltage,
`including the supply voltage or ground. Computer control can be
`achieved in various ways, e.g., by a serial or parallel D/A converter,
`or by a modulated duty cycle with an R-C averager. A choice of
`three offset adjustment values can be achieved with a three-state
`digital output bit and a series resistor.
`The ADXL150 and ADXL250 were developed by multidisciplinary
`product teams in ADI’s Micromachined Products Division,
`Wilmington, MA.
` A
`
`MOUNTING AND MECHANICAL CONSIDERATIONS
`When an accelerometer is mounted on a PC board, the IC
`becomes part of a larger mechanical system. Accelerations of
`50 g cause the sensor to deflect within the IC package; in
`addition, the PC board and its mounting structure will deflect
`and deform. The motion of the board generates a false
`acceleration signal, which the accelerometer can sense. If the
`resonant frequency of the supporting structure is within the
`signal band or not much higher than the filter rolloff, the
`vibrations of the PC board and its mounting system will show
`up in the sensor output.
`The best way to minimize these effects is to make the mounting
`scheme as stiff as possible, thereby transmitting the system
`acceleration more faithfully to the sensor and increasing the
`resonant frequency. Since a PC board is much stiffer in its plane
`than perpendicular to its surface, the accelerometer’s sensitive
`axis (both axes, if dual) should be in the plane of the board.
`Because the ADXL150 and ADXL250 have their sensitive axes
`in the plane of the chip, and the surface of the chip is parallel
`to the base of the package, the accelerometers receive the benefit
`of the PC board’s stiffness when simply soldered to the board.
`If the sensitive axis were perpendicular to the plane of the chip
`(as is the case for some bulk-micromachined sensors), soldering
`the package to the board would render the measurement most
`susceptible to PC-board flexibility. A right-angle mounting
`system could be used to orient the sensitive axis parallel to the
`PC board, but the mounting system itself can deform, producing
`false acceleration readings. The mounting system, and any PC
`board stiffeners, add cost to the acceleration measurement. Also,
`the additional mass of the mounting system lowers its resonant
`frequency, causing larger false acceleration signals.
`
`X DEMOD
`
`Y DEMOD
`
`VOUTX
`
`X ZERO-g ADJUST
`
`VOUTY
`
`Y ZERO-g ADJUST
`
`Figure 5. ADXL250 block diagram (L) and partial chip photo showing sensors at right angles in plane of chip (R).
`
`Analog Dialogue 30-4 (1996)
`
`5
`
`5
`
`

`

`EMC, CEMark, IEC801
`.....What’s it all about?
`And new devices to ease the job
`by Matt Smith
`This article examines electromagnetic compatibility (EMC),
`which has assumed increased formal significance since January,
`1996. We discuss here the special requirements and standards that
`are mandatory for all pieces of equipment entering the market in
`the European Union (EU), and we consider the requirements for
`attaining the “CE” (Communauté Européene) mark from an EMC
`point of view. A new generation of RS-232 products, designed to
`meet these requirements, exemplifies the measures that have been
`taken by Analog Devices to achieve EMC at the IC level. These
`measures include inbuilt protection circuitry to provide levels of
`immunity far beyond anything previously available; immunity to
`electrostatic discharges (ESD ) in excess of 15 kV has been achieved—
`measured by new and more-stringent test methods. We also discuss
`protection against overvoltage and electrical fast transients (EFT).
`From the emissions point of view, we examine electromagnetic
`emissions and the measures we have taken in ICs to eliminate
`costly shielding procedures.
`The European Union EMC Directive: In May, 1989, the
`European Union published a Council Directive, 89/336/EEC,
`relating to electromagnetic compatibility of products placed on
`the market within the member states. A later amendment, 92/31/
`EEC, delayed compulsory compliance until January 1, 1996. The
`Directive applies to apparatus which is liable either to cause
`electromagnetic disturbance or itself be affected by such
`disturbance—and thus to all electrical or electronic products. It
`goes beyond the more familiar FCC Class B requirement for
`emissions control since it also addresses immunity as well as
`emissions. While the directive applies only to products marketed
`within the EU, the standards are likely to be adopted worldwide.
`Conformance with the EU Directive on Electromagnetic
`Compatibility requires that products will
`• Have high intrinsic immunity to emissions from other sources
`• Keep their undesirable emissions to within very strict limits
`Manufacturers are responsible for meeting the regulations; from
`January 1, 1996, all electronic products sold in the European Union
`must show conformance by displaying the CE mark.
`Definitions:
`Electromagnetic compatibility (EMC): Ability to operate in, and not
`overly contribute to, an environment of electromagnetic radiation.
`When this goal is met, all electronic equipments operate correctly
`in one another’s presence.
`Electromagnetic interference (ºEMI): Electromagnetic energy emanating
`from one device causing degraded performance in another.
`Electromagnetic immunity, or susceptibility (EMS): Tolerance of the
`presence of electromagnetic energy.
`ELECTROMAGNETIC COMPATIBILITY
`
`EMISSIONS
`
`SUSCEPTIBILITY
`
`EMI
`
`EMS
`
`CONDUCTED
`
`RADIATED
`
`CONDUCTED
`
`RADIATED
`
`6
`
`EMC Testing: Thorough EMC evaluation requires testing of both
`EMI and EMS. Requiring different measurement approaches and
`test methodologies, they are specified in separate Standards. Emitted
`energy may be conducted via the power supply lines or on I/O cables,
`or it may be radiated through space. It can start out by being
`conducted along cables and then be radiated when shielding is
`inadequate. Similarly, electromagnetic immunity, or susceptibility,
`must be tested for both conducted and radiated interference.
`Conducted interference includes electrostatic discharges (ESD) and
`electrical fast transients (EFT).
`Emissions testing is not new, but only now has immunity testing
`become mandatory on commercial products—a result of the EU
`regulations. The standards for commercial immunity testing, both
`conducted and radiated, have evolved over several years
`IEC1000-4-x Immunity Specifications: The basic EMC
`immunity standards in Europe come from the International
`Electrotechnical Commission (IEC). The content and document
`numbers have continually evolved over many years. In the latest
`round, the IEC have assigned IEC1000-4-x to the family of
`immunity standards previously known as the IEC801-x series. For
`example, the specification dealing with ESD immunity, previously
`referred to as IEC801-2, has become IEC1000-4-2.
`Nomenclature Subject
`IEC1000-4
`Electromagnetic Compatibility EMC
`IEC1000-4-1 Overview of Immunity Tests
`IEC1000-4-2
`Electrostatic Discharge Immunity (ESD)
`IEC1000-4-3 Radiated Radio-Frequency Electromagnetic Field Immunity
`IEC1000-4-4
`Electrical Fast Transients (EFT)
`IEC1000-4-5
`Lightning Surges
`IEC1000-4-6 Conducted Radio Frequency Disturbances above 9 kHz
`
`EMC AND I-O PORTS
`It has been estimated that up to 75% of EMC problems occur in
`relation to I-O ports. The I-O port is an open gateway for
`electrostatic discharges or fast transient discharges to enter a piece
`of equipment, and for interfering signals to escape, either by
`conduction of spurious signals on the I-O lines or by radiation
`from the I-O cable. Because of this, the EMC performance of the
`I-O transceiver device connected to the port is crucial to the EMC
`performance of the entire package.
`Electromagnetic Susceptibility of I-O Ports: I-O ports are
`particularly vulnerable to damage from EMI because they may be
`subjected to various forms of overvoltage during “normal”
`operation. Simply plugging or unplugging cables carrying static
`charges can destroy the transceiver. RS-232 serial ports are
`especially vulnerable A standard serial port uses an exposed 9-
`way male D connector. The pins on the connector are all too easily
`accessible, making them a prime target for accidental discharges.
`ESD damage can result from simply picking up a laptop PC after
`walking across a carpeted room.
`
`Analog Dialogue 30-4 (1996)
`
`6
`
`

`

`The traditional method for ensuring immunity against ESD on
`I-O ports, including RS-232, has been to use some form of voltage
`clamping structure such as Tranzorbs, or current-limiting resistors.
`Damage to an integrated circuit is caused by excessive current
`flow, usually induced by high voltages. Protection can be achieved
`using current diversion or current limiting.
`Current Diversion: An integrated circuit may be protected by
`diverting some of the current to ground externally, usually with a
`structure that provides voltage clamping. The voltage clamp must
`switch on quickly and be able to safely handle the current that it is
`diverting away from the IC. Tranzorbs are a popular choice, but
`they are expensive and space consuming. For example, an RS-232
`port has eight I-O lines, each requiring individual protection; the
`protection components can often take up more area than the
`transceiver itself. In today’s laptop computers, where both costs
`and board space must be minimized, this is far from ideal. Another
`disadvantage of external clamping structures is their heavy
`capacitive loading on the I-O lines. This limits the maximum data
`rate, and the charging/discharging on data edges contributes to
`battery drain—another serious drawback in portable equipment.
`Current Limiting: Current-limiting protection using simple series
`resistors is a popular choice where the overvoltages likely to be
`encountered are relatively low. But for ESD protection, where the
`voltages can be as high as 15 kV, it is not a feasible option. The
`resistance value required to keep the current within safe limits
`(200 mA or so) would be so high as to defeat normal operation of
`the transceiver. Other current limiting components such as
`thermistors are occasionally used; but again, protection is achieved
`at the expense of output impedance. Current limiting is often used
`in combination with voltage clamping to achieve a good
`compromise, giving high levels of protection but without degrading
`normal operating specifications. Still, the external structures are
`undesirable in portable, low-cost equipment.
`EMC Emissions on I-O ports: One might not think of RS-232
`ports as likely offenders since the data rates are quite modest. But
`emissions are indeed a concern, and for a number of reasons.
`In recent years, transmission speeds have been pushed up by a
`factor of 10 over the originally intended RS-232 speeds. The now-
`common V.34 modems require data rates in excess of 115 kbps.
`Higher speed modems are now appearing, pushing the rate up to
`133 kbps. ISDN pushes this even higher—up to 230 kbps . Higher
`frequencies, together with high voltages, translate into higher levels
`of emissions. The move towards single-supply, charge-pump-based
`transceivers has resulted in on-chip high-frequency clock oscillators.
`The latest generation of charge-pump-based products uses
`0.1-µF charge-pump capacitors in order to conserve board space,
`but at the price of higher oscillator frequencies, resulting in higher
`levels of emissions. The high voltage switching (20 V), high
`frequencies, and the driving of long, often unshielded cables are a
`recipe for EMI trouble unless great care is taken. The RS-232
`cable serves as a very effective antenna, and even low level noise
`coupled onto the RS-232 cable can radiate significantly.
`
`“FIXES” vs. PREVENTION
`Too often, EMC problems are discovered late in a product design
`cycle and require expensive redesign including shielding, additional
`grounding, voltage-clamping structures, etc. Such “Band-Aid” fixes
`can be time- and space consuming, expensive, and lacking in
`guarantees of success. It helps to understand and eliminate potential
`EMI problems, both emissions and immunity, as early as possible in
`
`Analog Dialogue 30-4 (1996)
`
`the design cycle.* It will be helpful to include, where possible, products
`that have already been tested for compliance and characterized so
`you know just how close to the limits you are running.
`The ADM2xxE family† of RS-232 interface transceiver products
`(Analog Dialogue 30-3, p. 19) is an example of devices that have
`been designed with EMC compliance as an important
`consideration. High levels of inherent immunity to EMI as well as
`low levels of radiated emissions make for fewer headaches for the
`system designer. Benefits include low cost, space saving, inbuilt
`ruggedness and low emissions.
`On-chip immunity: On-chip ESD, EFT and EMI protection
`structures ensure compliance with the requirements of IEC1000-
`4-2, IEC1000-4-3 and IEC1000-4-4. All inputs and outputs are
`protected against electrostatic discharges up to ± 15 kV, and
`electrical fast transients up to ± 2 kV. This ideally suits the devices
`for operation in electrically harsh environments or where RS-232
`cables are frequently being plugged or unplugged. They are also
`immune to high R-F field strengths (1000-4-3), allowing operation
`in unshielded enclosures.
`All this inherent protection means that costly external circuitry
`can be eliminated, saving cost and board space; fewer components
`means increased system reliability; and data-transmission speed,
`often compromised by external protection, is maintained.
`Protection Structure: A simplified version of the protection
`structure used is illustrated below. It basically employs two back-
`to-back diodes. Under normal operating conditions, one or the
`other of these diodes is reverse-biased. If the voltage on the I-O
`pins exceeds ±50 V, reverse breakdown occurs and the voltage is
`clamped, diverting the current through the diodes. Two diodes
`are required because the RS-232 signal lines are