a Monolithic Accelerometer
`With Signal Conditioning
`ADXL50*
`
`FEATURES
`Complete Acceleration Measurement System
`on a Single Monolithic IC
`Full-Scale Measurement Range: ￿50 g
`Self-Test on Digital Command
`+5 V Single Supply Operation
`Sensitivity Precalibrated to 19 mV / g
`Internal Buffer Amplifier for User Adjustable Sensitivity
`and Zero-g Level
`Frequency Response: DC to 10 kHz
`Post Filtering with External Passive Components
`High Shock Survival: >2000 g Unpowered
`Other Versions Available: ADXL05 (￿5 g)
`
`GENERAL DESCRIPTION
`The ADXL50 is a complete acceleration measurement system on
`a single monolithic IC. Three external capacitors and a +5 volt
`power supply are all that is required to measure accelerations up
`to ± 50 g. Device sensitivity is factory trimmed to 19 mV/g,
`resulting in a full-scale output swing of ±0.95 volts for a ±50 g
`applied acceleration. Its zero g output level is +1.8 volts.
`A TTL compatible self-test function can electrostatically deflect
`the sensor beam at any time to verify device functionality.
`
`For convenience, the ADXL50 has an internal buffer amplifier
`with a full 0.25 V to 4.75 V output range. This may be used to
`set the zero-g level and change the output sensitivity by using
`external resistors. External capacitors may be added to the resis-
`tor network to provide 1 or 2 poles of filtering. No external
`active components are required to interface directly to most
`analog-to-digital converters (ADCs) or microcontrollers.
`The ADXL50 uses a capacitive measurement method. The ana-
`log output voltage is directly proportional to acceleration, and is
`fully scaled, referenced and temperature compensated, resulting
`in high accuracy and linearity over a wide temperature range.
`Internal circuitry implements a forced-balance control loop that
`improves accuracy by compensating for any mechanical sensor
`variations.
`The ADXL50 is powered from a standard +5 V supply and is
`robust for use in harsh industrial and automotive environments
`and will survive shocks of more than 2000 g unpowered.
`
`The ADXL50 is available in a hermetic 10-pin TO-100 metal
`can, specified over the 0°C to +70°C commercial, and –40°C to
`+85°C industrial temperature ranges. Contact factory for avail-
`ability of devices specified for operation over the –40°C to
`+105°C automotive temperature range.
`
`FUNCTIONAL BLOCK DIAGRAM
`
`ADXL50
`
`OSCILLATOR
`DECOUPLING
`CAPACITOR
`
`C2
`
`4
`
`OSCILLATOR
`
`SENSOR
`
`DEMODULATOR
`
`+3.4V
`
`VREF
`OUTPUT
`
`6
`
`REFERENCE
`
`+1.8V
`
`PREAMP
`
`BUFFER
`AMP
`
`SELF TEST
`(ST)
`
`7
`
`5
`
`COM
`
`1
`
`C3
`
`+5V
`
`2
`
`C1
`
`3
`
`C1
`
`DEMODULATOR
`CAPACITOR
`
`8
`
`VPR
`
`10
`
`VIN–
`
`R1
`
`9
`
`R3
`
`R2
`
`VOUT
`
`*Patents pending.
`
`REV. B
`Information furnished by Analog Devices is believed to be accurate and
`reliable. However, no responsibility is assumed by Analog Devices for its
`use, nor for any infringements of patents or other rights of third parties
`which may result from its use. No license is granted by implication or
`otherwise under any patent or patent rights of Analog Devices.
`
`© Analog Devices, Inc., 1996
`
`One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
`Tel: 617/329-4700
`Fax: 617/326-8703
`
`APPLE 1019
`
`1
`
`

`

`ADXL50–SPECIFICATIONS
`
`(TA = TMIN to TMAX, TA = +25￿C for J Grade Only, VS = +5 V, @ Acceleration = 0 g,
` unless otherwise noted)
`
`Parameter
`SENSOR INPUT
`Measurement Range
`Nonlinearity
`Alignment Error1
`Transverse Sensitivity2
`SENSITIVITY
`Initial Sensitivity at VPR
`Temperature Drift3
`ZERO g BIAS LEVEL
`Initial Offset
`vs. Temperature3
`vs. Supply
`NOISE PERFORMANCE
`Voltage Noise Density
`Noise in 100 Hz Bandwidth
`Noise in 10 Hz Bandwidth
`FREQUENCY RESPONSE
`3 dB Bandwidth4
`3 dB Bandwidth4
`Sensor Resonant Frequency
`
`5
`
`SELF TEST INPUT
`Output Change at VPR
`Logic “1” Voltage
`Logic “0” Voltage
`Input Resistance
`+3.4 V REFERENCE
`Output Voltage
`Output Temperature Drift3
`Power Supply Rejection
`Output Current
`
`PREAMPLIFIER OUTPUT
`Voltage Swing
`Current Output
`Capacitive Load Drive
`
`BUFFER AMPLIFIER
`Input Offset Voltage6
`Input Bias Current
`Open-Loop Gain
`Unity Gain Bandwidth
`Output Voltage Swing
`Capacitive Load Drive
`Power Supply Rejection
`POWER SUPPLY
`Operating Voltage Range
`Quiescent Supply Current
`
`TEMPERATURE RANGE
`Operating Range J
`Specified Performance A
`Automotive Grade*
`
`Conditions
`
`Guaranteed Full Scale
`Best Fit Straight Line, 50 g FS
`
`+25°C
`
`at VPR
`
`VS = 4.75 V to 5.25 V
`at VPR
`BW = 10 Hz to 1 kHz
`
`C1 = 0.022 µF (See Figure 22)
`C1 = 0.0068 µF
`
`ST Pin from Logic “0” to “1”
`
`To Common
`
`DC, VS = +4.75 V to +5.25 V
`Sourcing
`
`Source or Sink
`
`Delta from Nominal 1.800 V
`
`DC
`
`IOUT = ±100 µA
`
`DC, VS = +4.75 V to +5.25 V
`
`Min
`
`–50
`
`16.1
`
`1.55/1.60
`
` ADXL50J/A
`Typ
`
`0.2
`± 1
`± 2
`
`19.0
`0.75/1.0
`
`1.80
`± 15/35
`10
`
`6.6
`66
`20
`
`1300
`10
`24
`
`–1.00
`
`50
`
`3.400
`± 10
`1
`
`80
`100
`
`± 10
`5
`80
`200
`
`1
`
`10
`
`800
`
`–0.85
`2.0
`
`3.350
`
`500
`
`0.25
`30
`
`0.25
`1000
`
`4.75
`
`0
`–40
`–40
`
`Max
`
`+50
`
`21.9
`
`2.05/2.00
`
`32
`
`12
`
`–1.15
`
`0.8
`
`3.450
`
`10
`
`VS – 1.4
`
`± 25
`20
`
`VS – 0.25
`
`10
`
`5.25
`13
`
`+70
`+85
`+125
`
`Units
`
`g
`% of FS
`Degrees
`%
`
`mV/g
`% of Reading
`
`V
`mV
`mV/V
`
`mg/√Hz
`mg rms
`mg rms
`
`Hz
`kHz
`kHz
`
`V
`V
`V
`kΩ
`
`V
`mV
`mV/V
`µA
`
`V
`µA
`pF
`
`mV
`nA
`dB
`kHz
`V
`pF
`mV/V
`
`V
`mA
`
`°C
`°C
`°C
`
`NOTES
`1Alignment error is specified as the angle between the true and indicated axis of sensitivity, (see Figure 2).
`2Transverse sensitivity is measured with an applied acceleration that is 90° from the indicated axis of sensitivity. Transverse sensitivity is specified as the percent of
` transverse acceleration that appears at the VPR output. This is the algebraic sum of the alignment and the inherent sensor sensitivity errors, (see Figure 2).
`3Specification refers to the maximum change in parameter from its initial at +25°C to its worst case value at TMIN to TMAX.
`4Frequency at which response is 3 dB down from dc response assuming an exact C1 value is used. Maximum recommended BW is 10 kHz using a 0.007 µF capacitor, refer to
`Figure 22.
`5Applying logic high to the self-test input has the effect of applying an acceleration of –52.6 g to the ADXL50.
`6Input offset voltage is defined as the output voltage differential from 1.800 V when the amplifier is connected as a follower (i.e., Pins 9 and 10 tied together). The voltage at
`Pin 9 has a temperature drift proportional to that of the 3.4 V reference.
`*Contact factory for availability of automotive grade devices.
`All min and max specifications are guaranteed. Typical specifications are not tested or guaranteed.
`Specifications subject to change without notice.
`
`–2–
`
`REV. B
`
`2
`
`

`

`ABSOLUTE MAXIMUM RATINGS*
`Acceleration (Any Axis, Unpowered for 0.5 ms) . . . . . . 2000 g
`Acceleration (Any Axis, Powered for 0.5 ms) . . . . . . . . . . 500 g
`+VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7.0 V
`Output Short Circuit Duration
`(VPR, VOUT, VREF Terminals to Common) . . . . . . . Indefinite
`Operating Temperature . . . . . . . . . . . . . . . . . –55°C to +125°C
`Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
`*Stresses above those listed under “Absolute Maximum Ratings” may cause
`permanent damage to the device. This is a stress rating only; the functional
`operation of the device at these or any other conditions above those indicated in the
`operational sections of this specification is not implied. Exposure to absolute
`maximum rating conditions for extended periods may affect device reliability.
`
`ADXL50
`
`Package Characteristics
`Package
`￿JA
`130°C/W
`
`10-Pin TO-100
`
`Device Weight
`
`￿JC
`30°C/W 5 Grams
`
`ORDERING GUIDE
`
`Model
`
`ADXL50JH
`ADXL50AH
`
`Temperature
`Range
`
`0°C to +70°C
`–40°C to +85°C
`
`CAUTION
`ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
`accumulate on the human body and test equipment and can discharge without detection.
`Although the ADXL50 features proprietary ESD protection circuitry, permanent damage may
`occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
`precautions are recommended to avoid performance degradation or loss of functionality.
`
`WARNING!
`
`ESD SENSITIVE DEVICE
`
`CONNECTION DIAGRAM
`10-Header (TO-100)
`
`NOTES:
`AXIS OF SENSITIVITY IS ALONG
`A LINE BETWEEN PIN 5 AND
`THE TAB.
`THE CASE OF THE METAL CAN
`PACKAGE IS CONNECTED TO
`PIN 5 (COMMON).
`ARROW INDICATES DIRECTION
`OF POSITIVE ACCELERATION
`ALONG AXIS OF SENSITIVITY.
`
`C1
`
`C1
`
`C2
`
`23
`23
`
`+5V
`
`4
`4
`
`1
`1
`
`TOP VIEW
`COM
`
`
`
`55
`
`SENSITIVITY
`SENSITIVITY
`
`AXIS OF
`AXIS OF
`
`10
`10
`
`VIN–
`
`VREF
`
`
`
`66
`
`9
`9
`
`7 8
`7 8
`
`VOUT
`
`ST
`
`VPR
`
`PIN DESCRIPTION
`
`C1
`
`+5 V The power supply input pin.
`C2
`Connection for an external bypass capacitor (nominally
`0.022 µF) used to prevent oscillator switching noise
`from interfering with other ADXL50 circuitry. Please
`see the section on component selection.
`Connections for the demodulator capacitor, nominally
`0.022 µF. See the section on component selection for
`application information.
`COM The power supply common (or “ground”) connection.
`VREF Output of the internal 3.4 V voltage reference.
`ST
`The digital self-test input. It is both CMOS and TTL
`compatible.
`The ADXL50 preamplifier output providing an output
`voltage of 19 mV per g of acceleration.
`VOUT Output of the uncommitted buffer amplifier.
`VIN– The inverting input of the uncommitted buffer amplifier.
`
`VPR
`
`REV. B
`
`–3–
`
`3
`
`

`

`ADXL50
`
`+1g
`
`TAB
`
`PIN 5
`
`Figure 1. Output Polarity at VPR
`Z
`
` TRANSVERSE Z AXIS
`
`SIDE VIEW
`
`X
`
` PIN 5
`
`X
`SENSITIVE (X) AXIS
`
`TAB
`
`Z
`Figure 2a. Sensitive X and Transverse Z Axis
`Y
`
`TRANSVERSE Y AXIS
`
`TOP VIEW
`
`X
`
`PIN 5
`
`X
`SENSITIVE (X) AXIS
`
`TAB
`
`Y
`Figure 2b. Sensitive X and Transverse Y Axis
`
`–Z Axis
`
`Axyz
`
`Ax
`
`X Axis
`
`θxy
`
`θx
`
`Axy
`
`Y Axis
`
`Figure 2c. A Vector Analysis of an Acceleration Acting
`Upon the ADXL50 in Three Dimensions
`
`Polarity of the Acceleration Output
`The polarity of the ADXL50 output is shown in the Figure 1.
`When oriented to the earth’s gravity (and held in place), the
`ADXL50 will experience an acceleration of +1 g. This corre-
`sponds to a change of approximately +19 mV at the VPR output
`pin. Note that the polarity will be reversed to a negative going
`signal at the buffer amplifier output VOUT, due to its inverting
`configuration.
`Mounting Considerations
`There are three main causes of measurement error when using
`accelerometers. The first two are alignment and transverse sen-
`sitivity errors. The third source of error is due to resonances or
`vibrations of the sensor in its mounting fixture.
`Errors Due to Misalignment
`The ADXL50 is a sensor designed to measure accelerations that
`result from an applied force. Because these forces act on the
`sensor in a vector manner, the alignment of the sensor to the
`force to be measured may be critical.
`The ADXL50 responds to the component of acceleration on its
`sensitive X axis. Figures 2a and 2b show the relationship be-
`tween the sensitive “X” axis and the transverse “Z” and “Y”
`axes as they relate to the TO-100 package.
`Figure 2c describes a three dimensional acceleration vector
`(AXYZ) which might act on the sensor, where AX is the compo-
`nent of interest. To determine AX, first, the component of accel-
`eration in the XY plane (AXY) is found using the cosine law:
`AXY = AXYZ (cosθXY) then
` AX = AXY (cosθX)
`Therefore: Typical VPR = 19 mV/g (AXYZ) (cosθXY) cosθX
`Note that an ideal sensor will react to forces along or at angles
`to its sensitive axis but will reject signals from its various trans-
`verse axes, i.e., those exactly 90° from the sensitive “X” axis.
`But even an ideal sensor will produce output signals if the trans-
`verse signals are not exactly 90° to the sensitive axis. An accel-
`eration that is acting on the sensor from a direction different
`from the sensitive axis will show up at the ADXL50 output at a
`reduced amplitude.
`
`Table I. Ideal Output Signals for Off Axis Applied
`Accelerations Disregarding Device Alignment and
`Transverse Sensitivity Errors
`
`% of Signal Appearing
`at Output
`
`Output in gs for a 50 g
`Applied Acceleration
`
`100%
`99 98%
`99.94%
`99.86%
`99.62%
`98.48%
`86.60%
`70.71%
`50.00%
`17.36%
`8.72%
`5.25%
`3.49%
`1.7%
`0%
`
`50 (On Axis)
`49.99
`49.97
`49.93
`49.81
`49.24
`43.30
`35.36
`25.00
`8.68
`4.36
`2.63
`1.75
`0.85
`0.00 (Transverse Axis)
`
`θX
`
`0
`1°
`2°
`3°
`5°
`10°
`30°
`45°
`60°
`80°
`85°
`87°
`88°
`89°
`90°
`
`–4–
`
`REV. B
`
`4
`
`

`

`Table I shows the percentage signals resulting from various θX
`angles. Note that small errors in alignment have a negligible
`effect on the output signal. A 1° error will only cause a 0.02%
`error in the signal. Note, however, that a signal coming 1° off of
`the transverse axis (i.e., 89° off the sensitive axis) will still con-
`tribute 1.7% of its signal to the output. Thus large transverse
`signals could cause output signals as large as the signals of
`interest.
`Table I may also be used to approximate the effect of the
`ADXL50’s internal errors due to misalignment of the die to the
`package. For example: a 1 degree sensor alignment error will
`allow 1.7% of a transverse signal to appear at the output. In a
`nonideal sensor, transverse sensitivity may also occur due to in-
`herent sensor properties. That is, if the sensor physically moves
`due to a force applied exactly 90° to its sensitive axis, then this
`might be detected as an output signal, whereas an ideal sensor
`would reject such signals. In every day use, alignment errors
`may cause a small output peak with accelerations applied close
`to the sensitive axis but the largest errors are normally due to
`large accelerations applied close to the transverse axis.
`Errors Due to Mounting Fixture Resonances
`A common source of error in acceleration sensing is resonance
`of the mounting fixture. For example, the circuit board that the
`ADXL50 mounts to may have resonant frequencies in the same
`range as the signals of interest. This could cause the signals
`measured to be larger than they really are. A common solution
`to this problem is to dampen these resonances by mounting the
`ADXL50 near a mounting post or by adding extra screws to
`hold the board more securely in place.
`When testing the accelerometer in your end application, it is
`recommended that you test the application at a variety of fre-
`quencies in order to ensure that no major resonance problems
`exist.
`
`GLOSSARY OF TERMS
`Acceleration: Change in velocity per unit time.
`Acceleration Vector: Vector describing the net acceleration
`acting upon the ADXL50 (AXYZ).
`g: A unit of acceleration equal to the average force of gravity
`occurring at the earth’s surface. A g is approximately equal to
`32.17 feet/s2, or 9.807 meters/s2.
`Nonlinearity: The maximum deviation of the ADXL50 output
`voltage from a best fit straight line fitted to a plot of acceleration
`vs. output voltage, calculated as a % of the full-scale output
`voltage (@ 50 g).
`Resonant Frequency: The natural frequency of vibration of
`the ADXL50 sensor’s central plate (or “beam”). At its resonant
`frequency of 24 kHz, the ADXL50’s moving center plate has a
`peak in its frequency response with a Q of 3 or 4.
`
`ADXL50
`Sensitivity: The output voltage change per g unit of accelera-
`tion applied, specified at the VPR pin in mV/g.
`Sensitive Axis (X): The most sensitive axis of the accelerom-
`eter sensor. Defined by a line drawn between the package tab
`and Pin 5 in the plane of the pin circle. See Figures 2a and 2b.
`Sensor Alignment Error: Misalignment between the
`ADXL50’s on-chip sensor and the package axis, defined by
`Pin 5 and the package tab.
`Total Alignment Error: Net misalignment of the ADXL50’s
`on-chip sensor and the measurement axis of the application.
`This error includes errors due to sensor die alignment to the
`package, and any misalignment due to installation of the sensor
`package in a circuit board or module.
`Transverse Acceleration: Any acceleration applied 90° to the
`axis of sensitivity.
`Transverse Sensitivity Error: The percent of a transverse ac-
`celeration that appears at the VPR output. For example, if the
`transverse sensitivity is 1%, then a +10 g transverse acceleration
`will cause a 0.1 g signal to appear at VPR (1% of 10 g). Trans-
`verse sensitivity can result from a sensitivity of the sensor to
`transverse forces or from misalignment of the internal sensor to
`its package.
`Transverse Y Axis: The axis perpendicular (90°) to the pack-
`age axis of sensitivity in the plane of the package pin circle. See
`Figure 2.
`Transverse Z Axis: The axis perpendicular (90°) to both the
`package axis of sensitivity and the plane of the package pin
`circle. See Figure 2.
`
`100
`90
`
`10
`0%
`
`1V
`
`0.5V
`
`0.5ms
`
`Figure 3. 500 g Shock Overload Recovery. Top Trace:
`ADXL50 Output. Bottom Trace: Reference Accelerometer
`Output
`
`REV. B
`
`–5–
`
`5
`
`

`

`7 6
`
`5 4 3
`
`NOISE – mV, RMS
`
`ADXL50–Typical Characteristics
`
` 5 g p-p SIGNAL TA = +25°C
`C1 = C2 = 0.022µF
`
`9
`
`036
`
`–3
`–6
`–9
`
`–12
`–15
`–18
`
`NORMALIZED SENSITIVITY – dB
`
`DEMODULATOR CAPACITANCE – µF
`
`0.1
`
`20
`
`.010
`
` Figure 7. RMS Noise vs. Value of Demodulator
` Capacitor, C1
`
`TA = +25°C, ACL = 2
`
`1000
`100
`FREQUENCY – Hz
`
`10000
`
`100
`
`80
`
`60
`
`40
`
`20
`
`OUTPUT IMPEDANCE – Ω
`
`0
`
`10
`
`–21
`
`1
`
`10
`
`100
`FREQUENCY – Hz
`Figure 4. Normalized Sensitivity vs. Frequency
`
`1k
`
`10k
`
`0
`
`10
`
`20
`
`30
`40
`g LEVEL APPLIED
`
`50
`
`60
`
`0.25%
`0.20%
`0.15%
`0.10%
`0.05%
`0.00%
`–0.05%
`–0.10%
`–0.15%
`–0.20%
`–0.25%
`
`LINEARITY IN % OF FULL SCALE
`
`Figure 5. Linearity in Percent of Full Scale
`
`Figure 8. Buffer Amplifier Output Impedance vs. Frequency
`
`TA = +25°C
`
`G = 10
`
`G = 2
`
`10
`
`100
`
`1k
`10k
`FREQUENCY – Hz
`
`100k
`
`1M
`
`30
`25
`20
`15
`10
`5
`0
`
`–5
`–10
`–15
`
`–20
`
`GAIN – dB
`
`Figure 9. Buffer Amplifier Closed-Loop Gain vs. Frequency
`
`–6–
`
`REV. B
`
`C1 = C2 = 0.022µF
`
`1350
`
`1300
`
`1250
`
`1200
`
`1150
`
`–3dB BW – Hz
`
`1100
`–60
`
`–40
`
`–20
`
`0
`
`20
`40
`60
`TEMPERATURE – °C
`Figure 6. –3 dB Bandwidth vs. Temperature at VPR
`
`80
`
`100
`
`120
`
`140
`
`6
`
`

`

`HANGE IN SELF–TEST OUTPUT SWING – mV
`
`ADXL50
`
`0.5
`
`0.0
`
`–0.5
`
`–1.0
`
`–1.5
`
`CHANGE IN SENSITIVITY – %
`
`TA = +25°C
`C1 = C2 = 0.022µF
`
`+0.50
`
`+0.25
`
`0
`
`–0.25
`
`CHANGE IN SENSITIVITY AT VPR – %
`
`–0.50
`
`4.8
`
`4.9
`
`5.0
`5.1
`SUPPLY VOLTAGE – V
`Figure 10. Change in Sensitivity vs. Supply Voltage
`
`5.2
`
`5.3
`
`–60
`
`–40
`
`–20
`
`0
`
`20
`40
`60
`TEMPERATURE – °C
` Figure 13. Percent Change in Sensitivity at VPR vs.
` Temperature
`
`80
`
`100
`
`120
`
`140
`
`TA = +25°C
`VS = +5V + (0.5Vp-p)
`
`80
`
`60
`
`40
`
`+3.4V REF PSRR – dB
`
`TA = +25°C
`VS = +5V + (0.5Vp-p)
`C1 = C2 = 0.022µF
`
`55
`
`45
`
`35
`
`VPR 0g PSRR – dB
`
`25
`
`1
`
`10
`
`100
`1k
`FREQUENCY – Hz
`
`10k
`
`100k
`
`20
`
`1
`
`10
`
`100
`1k
`FREQUENCY – Hz
`
`10k
`
`100k
`
`Figure 11. VPR 0 g PSRR vs. Frequency
`
`Figure 14. +3.4 V REF PSRR vs. Frequency
`
`–1004
`
`–1000
`
`–0.996
`
`–0.992
`
`VREF
`
`SELF–TEST
`
`3.404
`
`3.400
`
`3.396
`
`3.392
`
`3.388
`
`VREF OUTPUT – Volts
`
`40
`
`30
`
`20
`
`10
`
`0
`
`CHANGE IN 0g OUTPUT LEVEL – mV
`
`–60
`
`–40
`
`–20
`
`0
`
`20
`40
`60
`TEMPERATURE – °C
`Figure 12. 0 g Bias Level vs. Temperature
`
`80
`
`100
`
`120
`
`140
`
`3.384
`–60
`
`–40
`
`–20
`
`0
`
`20
`40
`60
`TEMPERATURE – °C
`
`80
`
`100
`
`120
`
`–0.988 C
`140
`
`Figure 15. VREF Output and Change in Self-Test Output
`Swing vs. Temperature
`
`REV. B
`
`–7–
`
`7
`
`

`

`ADXL50
`
`THEORY OF OPERATION
`The ADXL50 is a complete acceleration measurement system
`on a single monolithic IC. It contains a polysilicon surface-mi-
`cro machined sensor and signal conditioning circuitry. The
`ADXL50 is capable of measuring both positive and negative ac-
`celeration to a maximum level of ± 50 g.
`Figure 16 is a simplified view of the ADXL50’s acceleration
`sensor at rest. The actual structure of the sensor consists of 42
`unit cells and a common beam. The differential capacitor sensor
`consists of independent fixed plates and a movable “floating”
`central plate which deflects in response to changes in relative
`motion. The two capacitors are series connected, forming a ca-
`pacitive divider with a common movable central plate. A force
`balance technique counters any impeding deflection due to ac-
`celeration and servos the sensor back to its 0 g position.
`
`TOP VIEW
`
`CS2
`
`CS1
`CENTER
`PLATE
`
`TETHER
`
`BEAM
`
`CENTER
`PLATE
`
`FIXED
`OUTER
`PLATES
`
`UNIT CELL
`CS1 = CS2
`
`CS1
`
`CS2
`
`DENOTES ANCHOR
`
`Figure 16. A Simplified Diagram of the ADXL50 Sensor at
`Rest
`Figure 17 shows the sensor responding to an applied accelera-
`tion. When this occurs, the common central plate or “beam”
`moves closer to one of the fixed plates while moving further
`from the other. The sensor’s fixed capacitor plates are driven
`deferentially by a 1 MHz square wave: the two square wave am-
`plitudes are equal but are 180° out of phase from one another.
`When at rest, the values of the two capacitors are the same and
`therefore, the voltage output at their electrical center (i.e., at the
`center plate) is zero.
`When the sensor begins to move, a mismatch in the value of
`their capacitance is created producing an output signal at the
`central plate. The output amplitude will increase with the
`amount of acceleration experienced by the sensor. Information
`concerning the direction of beam motion is contained in the
`phase of the signal with synchronous demodulation being used
`to extract this information. Note that the sensor needs to be po-
`sitioned so that the measured acceleration is along its sensitive
`axis.
`Figure 18 shows a block diagram of the ADXL50. The voltage
`output from the central plate of the sensor is buffered and then
`applied to a synchronous demodulator. The demodulator is also
`supplied with a (nominal) 1 MHz clock signal from the same
`oscillator which drives the fixed plates of the sensor. The
`
`demodulator will rectify any voltage which is in sync with its
`clock signal. If the applied voltage is in sync and in phase with
`the clock, a positive output will result. If the applied voltage is in
`sync but 180° out of phase with the clock, then the demodu-
`lator’s output will be negative. All other signals will be rejected.
`An external capacitor, C1, sets the bandwidth of the demodulator.
`The output of the synchronous demodulator drives the preamp
`—an instrumentation amplifier buffer which is referenced to
`+1.8 volts. The output of the preamp is fed back to the sensor
`through a 3 MΩ isolation resistor. The correction voltage re-
`quired to hold the sensor’s center plate in the 0 g position is a
`direct measure of the applied acceleration and appears at the
`VPR pin.
`
`TOP VIEW
`
`APPLIED
`ACCELERATION
`
`BEAM
`
`CS1
`
`CS2
`
`UNIT CELL
`CS1 < CS2
`
`DENOTES ANCHOR
`
`Figure 17. The ADXL50 Sensor Momentarily Responding
`to an Externally Applied Acceleration
`When the ADXL50 is subjected to an acceleration, its capacitive
`sensor begins to move creating a momentary output signal. This
`is signal conditioned and amplified by the demodulator and
`preamp circuits. The dc voltage appearing at the preamp output
`is then fed back to the sensor and electrostatically forces the
`center plate back to its original center position.
`At 0 g the ADXL50 is calibrated to provide +1.8 volts at the
`VPR pin. With an applied acceleration, the VPR voltage changes
`to the voltage required to hold the sensor stationary for the du-
`ration of the acceleration and provides an output which varies
`directly with applied acceleration.
`The loop bandwidth corresponds to the time required to apply
`feedback to the sensor and is set by external capacitor C1. The
`loop response is fast enough to follow changes in g level up to
`and exceeding 1 kHz. The ADXL50’s ability to maintain a flat
`response over this bandwidth keeps the sensor virtually motion-
`less. This essentially eliminates any nonlinearity or aging effects
`due to the sensor beam’s mechanical spring constant, as com-
`pared to an open-loop sensor.
`An uncommitted buffer amplifier provides the capability to ad-
`just the scale factor and 0 g offset level over a wide range. An in-
`ternal reference supplies the necessary regulated voltages for
`powering the chip and +3.4 volts for external use.
`
`–8–
`
`REV. B
`
`8
`
`

`

`DENOTES EXTERNAL
`PIN CONNECTION
`
`+3.4V
`
`+3.4V
`
`75Ω
`
`C2
`
`EXTERNAL
`OSCILLATOR
`DECOUPLING
`CAPACITOR
`
`1MHz
`OSCILLATOR
`
`0°
`
`180°
`
`+5V
`
`+5V
`
`SYNC
`
`+5V
`
`COMMON
`
`INTERNAL
`REFERENCE
`
`+0.2V
`
`VREF
`
`+3.4V
`
`+5V
`33kΩ
`
`33kΩ
`
`C1
`
`EXTERNAL
`DEMODULATION
`CAPACITOR
`
`C1
`
`SYNCHRONOUS
`DEMODULATOR
`
`CS1
`
`CS2
`
`BEAM
`
`+1.8V
`
`3MΩ
`
`ADXL50
`
`PREAMP
`
`VPR
`
`+1.8V
`
`LOOP GAIN = 10
`
`INTERNAL
`FEEDBACK
`LOOP
`
`RST
`
`+3.4V
`
`+3.4V +1.8V +0.2V
`
`COM
`
`VIN–
`
`+1.8V
`
`BUFFER
`AMPLIFIER
`
`VOUT
`
`Figure 18. Functional Block Diagram
`
`50kΩ
`
`SELF–TEST
`(ST)
`
`The sensor’s tight mechanical spacing allows it to be electro-
`statically deflected to full scale while operating on a 5 volt sup-
`ply. A self-test is initiated by applying a TTL “high” level
`voltage (>+2.0 V) to the ADXL50’s self-test pin which causes
`the chip to apply a deflection voltage to the beam which moves
`it an amount equal to –50 g (the negative full-scale output of the
`device). Note that the ±10% tolerance of the self-test circuit is
`not proportional to the sensitivity error, see Self-Test section.
`The output of the ADXL50’s preamplifier is 1.8 V at 0 g accel-
`eration with an output range of ± 0.95 V for a ± 50 g input, i.e.,
`19 mV/g. An uncommitted buffer amplifier has been included
`on-chip to enhance the user’s ability to offset the 0 g signal level
`and to amplify and filter the signal. Access is provided to both
`
`the inverting input and the output of this amplifier via pins
`VOUT and VIN–, while the noninverting input is connected inter-
`nally to a +1.8 V reference. The +1.8 V is derived from a
`resistor divider connected to the 3.4 V reference.
`
`BASIC CONNECTIONS FOR THE ADXL50
`Figure 19 shows the basic connections needed for the ADXL50
`to measure accelerations in the ± 50 g range with an output scale
`factor 40 mV/g corresponding to a 2.5 V 0 g level, a ± 2.0 V full-
`scale swing around 0 g and a 3 dB bandwidth of approximately
`1 kHz.
`In general, the designer will need to take into account the initial
`zero g bias when designing circuits. For the ADXL50J this off-
`set is 1.8 V ± 250 mV. When microprocessors and software
`
`ADXL50
`
`+3.4V
`
`6
`
`VREF
`OUTPUT
`
`REFERENCE
`
`+1.8V
`
`OSCILLATOR
`
`SENSOR
`
`DEMODULATOR
`
`PREAMP
`
`BUFFER
`AMP
`
`OSCILLATOR
`DECOUPLING
`CAPACITOR
`
`C2
`0.022µF
`
`SELF TEST
`(ST)
`
`4
`
`7
`
`5
`
`COM
`
`1
`
`C3
`
`2
`
`C1
`
`3
`
`C1
`
`8
`
`VPR
`
`+5V
`
`0.022µF
`DEMODULATOR
`CAPACITOR
`
`R1
`49.9kΩ
`
`10
`
`VIN–
`
`9
`
`R3
`105kΩ
`
`VOUT
`
`R2
`274kΩ
`
`Figure 19. ADXL50 Application Providing an Output Sensitivity of
`40 mV/g, a +2.5 V 0 g Level and a Bandwidth of 1 kHz
`
`REV. B
`
`–9–
`
`9
`
`

`

`ranges listed by keeping R1 > 49.9 kΩ, with the subsequent
`tradeoff that the required values for R3 will become very large.
`The user always has the option of adding external gain and fil-
`tering stages after the ADXL50 to make lower full-scale ranges.
`Measuring Full-Scale Accelerations Less than ￿5 g
`Applications, such as motion detection, and tilt sensing, have
`signal amplitudes in the 1 g to 2 g range. Although designed for
`higher full-scale ranges, the ADXL50 may be adapted for use in
`
`1
`
`9
`
`+5V
`0.1µF
`
`VOUT
`
`BUFFER
`AMP
`
`R3
`
`ADXL50
`
`PRE-AMP
`
`1.8V
`
`4
`
`2 3
`
`5
`
`C2
`0.022µF
`
`0.022µF
`
`C1
`
`C1
`
`COM
`
`VREF
`
`8
`
`6
`
`VPR
`+3.4V
`
`0g
`LEVEL
`TRIM
`
`50kΩ
`
`VX
`
`10
`VIN–
`+1.8V
`
`R1
`
`R2
`
` Figure 20. ADXL50 Circuit Using the Buffer Amplifier to
` Set the Output Scaling and 0 g Offset Level
`low g applications; the two main design considerations are noise
`and 0 g offset drift (BH, KH grades recommended).
`At its full 1 kHz bandwidth, the ADXL50 will typically exhibit
`1 g p-p of noise. With ± 50 g accelerations this is generally not a
`problem, but at a ± 2 g full-scale level the signal-to-noise ratio
`will be very poor. However, reducing the bandwidth to 100 Hz
`or less considerably improves the S/N ratio. Figure 25 shows the
`relationship between ADXL50 bandwidth and noise.
`The ADXL50 exhibits offset drifts that are typically 0.02 g per
`°C but which may be as large as 0.1 g per °C. With the buffer
`amplifier configured for a 2 g full scale, the ADXL50 will typi-
`cally drift 1/2 of its full-scale range with a 50°C increase in
`temperature.
`There are several cures for offset drift. If a dc response is not
`required, for example in motion sensing or vibration measurement
`applications, consider ac coupling the acceleration signal to re-
`move the effects of offset drift. See the section on ac coupling.
`Periodically recalibrating the accelerometer’s 0 g level is another
`option. Autozero or long term averaging can be used to remove
`long term drift using a microprocessor or the autozero circuit of
`Figure 29. Be sure to keep the buffer amplifier’s full-scale out-
`put range much larger than the measurement range to allow for
`the 0 g level drift.
`
`CALCULATING COMPONENT VALUES FOR SCALE
`FACTOR AND 0 g SIGNAL LEVEL
`The ADXL50 buffer’s scale factor is set by –R3/R1 (since the
`amplifier is in the inverter mode).
`
`ADXL50
`calibration are used and there is a desire to eliminate trim po-
`tentiometers, the design should leave room at either supply rail
`to account for signal swing and or variations in initial zero g bias.
`For example, in the circuit in Figure 19, the initial zero g bias of
`± 250 mV will be reflected to the output by the gain of the R3/R1
`network, resulting in an output offset of ± 526 mV worst case.
`The offset, combined with a full-scale signal of 50 g, (+2.0 V)
`will cause the output buffer amplifier to saturate at the supply
`rail.
`The full ± 2.25 V output swing of the buffer amplifier can be
`utilized if the user is able to trim the zero-g bias to exactly
`2.5 V. In applications where the full-scale range will be ± 25 g or
`less, a bias trim such as that shown in Figure 20 will almost al-
`ways be required.
`
`VARYING THE OUTPUT SENSITIVITY AND 0 g LEVEL
`USING THE INTERNAL BUFFER AMPLIFIER
`The uncommitted buffer amplifier may be used to change the
`output sensitivity to provide useful full-scale ranges of ±50 g
`and below. Table II provides recommended resistor values for
`several standard ranges down to ± 10 g. As the full-scale range is
`decreased, buffer amplifier gain is increased, and the noise con-
`tribution as a percentage of full scale will also increase. For all
`ranges, the signal-to-noise ratio can be improved by reducing
`the circuit bandwidth, either by increasing the demodulator ca-
`pacitor, C1, or by adding a post filter using the buffer amplifier.
`
`Table II. Recommended Resistor Values for Setting the
` Circuit of Figure 20 to Several Common Full-Scale Ranges
`
`FS (g)
`
`Buffer
`Gain
`
`SF in
`mV/g
`
`R1
`
`R3
`
` R2
`
`100 k
`105 k
`49.9 k
`40
`2.11
`± 50.0
`100 k
`103 k
`39.2 k
`50
`2.63
`± 40.0
`100 k
`137 k
`40.2 k
`65
`3.42
`± 30.8
`100 k
`113 k
`28.7 k
`75
`3.95
`± 26.7
`100 k
`137 k
`26.1 k
`100
`5.26
`± 20.0
`100 k
`249 k
`23.7 k
`200
`10.53
`± 10.0
`Note that the value of resistor R1 should be selected to limit the
`output current flowing into VPR to less than 25 µA (to provide a
`safety margin). For a “J” grade device, this current is equal to:
`
`(2.05V– The peak full-scaleoutputvoltageatVPR ) – 1.8V
`R1inohms
`
`IPR =
`
`For a ±50 g full-scale range, R1 needs to be 49.9 kΩ or larger in
`value; but at the lower full-scale g ranges, if the VPR swing is
`much less, then it is possible to use much lower resistance val-
`ues. For this table, the circuit of Figure 20 is used, as a 0 g off-
`set trim will be required for most applications. In all cases, it is
`assumed that the zero-g bias level is 2.5 V with an output span
`of ± 2 V.
`Note that for full scales below ± 20 g the self-test is unlikely to
`operate correctly because the VPR pull-down current is not guar-
`anteed to be large enough to drive R1 to the required –1.0 V
`swing. In these cases, the self-test command will cause VOUT to
`saturate at the rail, and it will be necessary to monitor the self-
`test at VPR. Self-test can remain operational at VPR for all g
`
`–10–
`
`REV. B
`
`10
`
`

`

`ADXL50
`loads. Note that a capacitance connected across the buffer feed-
`back resistor for low-pass filtering does not appear as a capaci-
`tive load to the buffer. The buffer amplifier is limited to
`sourcing or sinking a maximum of 100 µA. Component values
`for the resistor network should be selected to ensure that the
`buffer amplifier can drive the filter under worst case transient
`conditions.
`
`SELF-TEST FUNCTION
` The digital self-test input is compatible with both CMOS and
`TTL signals. A Logic “l” applied to the self-test (ST) input will
`cause an electrostatic force to be applied to the sensor which
`will cause it to deflect to the approximate negative full-scale out-
`put of the device. Accordingly, a correctly functioning acceler-
`ometer will respond by initiating an approximate –1 volt output
`change at VPR. If the ADXL50 is experiencing an acceleration
`when the self-test is initiated, the VPR output will equal the alge-
`braic sum of the two inputs. The output will stay at the self-test
`level as long as the ST input remains high and will return to the
`0 g level when the ST voltage is removed.
`A self-test output that varies more than ± 10% from the nominal
`–1.0 V change indicates a defective beam or a circuit problem
`such as an open or shorted pin or component.
`Operating the ADXL50’s buffer amplifier at Gains > 2, to pro-
`vide full-scale outputs of less than ± 50 g, may cause the self-test
`output to overdrive the buffer into saturation. The self-test may
`still be used in the case, but the change in t