AD22279

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Single-Axis, High-g, iMEMS® Accelerometers ADXL78 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 that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2010 Analog Devices, Inc. All rights reserved.

FEATURES

Complete acceleration measurement system on a single monolithic IC Available in ±35 g, ±50 g, or ±70 g output full-scale ranges Full differential sensor and circuitry for high resistance to EMI/RFI Environmentally robust packaging Complete mechanical and electrical self-test on digital command Output ratiometric to supply Sensitive axes in the plane of the chip High linearity (0.2% of full scale) Frequency response down to dc Low noise Low power consumption (1.3 mA) Tight sensitivity tolerance and 0 g offset capability Largest available prefilter clipping headroom

400 Hz, 2-pole Bessel filter

Compatible with Sn/Pb and Pb-free solder processes Qualified for automotive applications

APPLICATIONS

Vibration monitoring and control Vehicle collision sensing Shock detection GENERAL DESCRIPTION The ADXL78 is a low power, complete single-axis accelerometer with signal conditioned voltage outputs that are on a single monolithic IC. This product measures acceleration with a full- scale range of ±35 g, ±50 g, or ±70 g (minimum). It can also measure both dynamic acceleration (vibration) and static acceleration (gravity). The ADXL78 is the fourth-generation surface micromachined iMEMS® accelerometer from ADI with enhanced performance and lower cost. Designed for use in front and side impact airbag applications, this product also provides a complete cost- effective solution useful for a wide variety of other applications. The ADXL78 is temperature stable and accurate over the automotive temperature range, with a self-test feature that fully exercises all the mechanical and electrical elements of the sensor with a digital signal applied to a single pin. The ADXL78 is available in a 5 mm × 5 mm × 2 mm, 8-terminal ceramic LCC package. FUNCTIONAL BLOCK DIAGRAM 05368-001 ADXL78 VDD VS VDD2 DIFFERENTIAL SENSOREXC DEMOD AMP XOUT 400Hz BESSEL FILTER TIMING GENERATOR SELF-TEST Figure 1.

Rev. B | Page 2 of 12 TABLE OF CONTENTS

REVISION HISTORY

7/10—Rev. A to Rev. B 5/05—Rev. 0 to Rev. A

Rev. B | Page 3 of 12 SPECIFICATIONS1 At TA = −40°C to +105°C, 5.0 V dc ± 5%, acceleration = 0 g, unless otherwise noted. Table 1. Model No. AD22279 Model No. AD22280 Model No. AD22281 Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit S E N S O R Output Full-Scale Range IOUT ≤ ±100 μA 37 55 70 g Nonlinearity 0.2 2 0.2 2 0.2 2 % Package Alignment Error 1 1 1 Degree Cross-Axis Sensitivity −5 +5 −5 +5 −5 +5 % Resonant Frequency 24 24 24 kHz Sensitivity, Ratiometric (Over Temperature) OFFSET Zero-g Output Voltage (Over Temperature)2 VOUT − VDD/2, VDD = 5 V −200 +200 −150 +150 −150 +150 mV N O I S E Noise Density 10 Hz − 400 Hz, 5 V 1.1 3 1.4 3 1.8 3.5 mg/√Hz Clock Noise 5 5 5 mV p-p FREQUENCY RESPONSE 2-pole Bessel −3 dB Frequency 360 400 440 360 400 440 360 400 440 Hz −3 dB Frequency Drift 25°C to T MIN or TMAX 2 2 2 Hz SELF-TEST Output Change (Cube vs. VDD)3 VDD = 5 V 440 550 660 304 380 456 216 270 324 mV Logic Input High VDD = 5 V 3.5 3.5 3.5 V Logic Input Low VDD = 5 V 1 1 1 V Input Resistance Pull-down resistor to GND 30 50 30 50 30 50 kΩ OUTPUT AMPLIFIER Capacitive Load Drive 1000 1000 1000 pF PREFILTER HEADROOM 280 400 560 g CFSR @ 400 kHz 5 4 3 V/V Functional Range 3.5 6 3.5 6 3.5 6 V Quiescent Supply Current VDD = 5 V 1.3 2 1.3 2 1.3 2 mA TEMPERATURE RANGE −40 +105 −40 +105 −40 +105 °C 1 All minimum and maximum specifications are guaranteed. Typical specifications are not guaranteed. 2 Zero g output is ratiometric. 3 Self-test output at VDD = (Self-Test Output at 5 V) × (VDD/5 V)3.

Rev. B | Page 4 of 12 ABSOLUTE MAXIMUM RATINGS Table 2. Parameter Rating Acceleration (Any Axis, Unpowered) 4000 g Acceleration (Any Axis, Powered) 4000 g VS −0.3 V to +7.0 V All Other Pins (COM − 0.3 V) to (VS + 0.3 V) Output Short-Circuit Duration (Any Pin to Common) Indefinite Operating Temperature Range −65°C to +150°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; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ESD CAUTION

Figure 2. Pin Configuration Table 3. Pin Function Descriptions

1 NC Do Not Connect

2 NC Do Not Connect

3 COM Common

4 ST Self-Test

5 NC Do Not Connect

6 X OUT X Channel Output

Rev. B | Page 7 of 12 THEORY OF OPERATION The ADXL78 provides a fully differential sensor structure and circuit path, resulting in the industry’s highest resistance to EMI/RFI effects. This latest generation uses electrical feedback with zero-force feedback for improved accuracy and stability. The sensor resonant frequency is significantly higher than the signal bandwidth set by the on-chip filter, avoiding the signal analysis problems caused by resonant peaks near the signal bandwidth. 05368-005 UNIT SENSING CELL MOVABLE FRAME FIXED PLATES UNIT FORCING CELL ANCHOR MOVING PLATE PLATE CAPACITORS ACCELERATION ANCHOR Figure 5 is a simplified view of one of the differential sensor elements. Each sensor includes several differential capacitor unit cells. Each cell is composed of fixed plates attached to the substrate and movable plates attached to the frame. Displacement of the frame changes the differential capacitance, which is measured by the on-chip circuitry. Figure 5. Simplified View of Sensor Under Acceleration Complementary 400 kHz square waves drive the fixed plates. Electrical feedback adjusts the amplitudes of the square waves such that the ac signal on the moving plates is 0. The feedback signal is linearly proportional to the applied acceleration. This unique feedback technique ensures that there is no net electrostatic force applied to the sensor. The differential feedback control signal is also applied to the input of the filter, where it is filtered and converted to a single-ended signal.

Rev. B | Page 8 of 12 For most applications, a single 0.1 μF capacitor, CDC, adequately decouples the accelerometer from noise on the power supply. However, in some cases, particularly where noise is present at the 400 kHz internal clock frequency (or any harmonic thereof), noise on the supply can cause interference on the ADXL78’s output. If additional decoupling is needed, a 50 Ω (or smaller) resistor or ferrite bead can be inserted in the supply line. Additionally, a larger bulk bypass capacitor (in the 1 μF to 4.7 μF range) can be added in parallel to C DC. SELF-TEST The fixed fingers in the forcing cells are normally kept at the same potential as that of the movable frame. When the self-test digital input is activated, the voltage on the fixed fingers on one side of the moving plate in the forcing cells is changed. This creates an attractive electrostatic force, which causes the frame to move toward those fixed fingers. The entire signal channel is active; therefore, the sensor displacement causes a change in V OUT. The ADXL78 self-test function is a comprehensive method of verifying the operation of the accelerometer. Because electrostatic force is independent of the polarity of the voltage across capacitor plates, a positive voltage is applied in half of the forcing cells, and its complement in the other half of the forcing cells. Activating self-test causes a step function force to be applied to the sensor, while the capacitive coupling term is canceled. The ADXL78 has improved self-test functionality, including excellent transient response and high speed switching capabilities. Arbitrary force waveforms can be applied to the sensor by modulating the self-test input, such as test signals to measure the system frequency response or even crash signals to verify algorithms within the limits of the self-test swing. The ST pin should never be exposed to voltages greater than V S + 0.3 V . If this cannot be guaranteed due to the system design (for instance, if there are multiple supply voltages), then a low VF clamping diode between ST and VS is recommended. CLOCK FREQUENCY SUPPLY RESPONSE In any clocked system, power supply noise near the clock frequency may have consequences at other frequencies. An internal clock typically controls the sensor excitation and the signal demodulator for micromachined accelerometers. If the power supply contains high frequency spikes, they may be demodulated and interpreted as an acceleration signal. A signal appears as the difference between the noise frequency and the demodulator frequency. If the power supply spikes are 100 Hz away from the demodulator clock, there is an output term at 100 Hz. If the power supply clock is at exactly the same frequency as the accelerometer clock, the term appears as an offset. If the difference frequency is outside of the signal bandwidth, the filter attenuates it. However, both the power supply clock and the accelerometer clock may vary with time or temperature, which can cause the interference signal to appear in the output filter bandwidth. The ADXL78 addresses this issue in two ways. First, the high clock frequency eases the task of choosing a power supply clock frequency such that the difference between it and the accelero- meter clock remains well outside of the filter bandwidth. Second, the ADXL78 is the only micromachined accelerometer to have a fully differential signal path, including differential sensors. The differential sensors eliminate most of the power supply noise before it reaches the demodulator. Good high frequency supply bypassing, such as a ceramic capacitor close to the supply pins, also minimizes the amount of interference. The clock frequency supply response (CFSR) is the ratio of the response at V OUT to the noise on the power supply near the accelerometer clock frequency. A CFSR of 3 means that the signal at VOUT is 3× the amplitude of an excitation signal at VDD near the accelerometer internal clock frequency. This is analogous to the power supply response, except that the stimulus and the response are at different frequencies. The ADXL78’s CFSR is 10× better than a typical single-ended accelerometer system. SIGNAL DISTORTION Signals from crashes and other events may contain high amplitude, high frequency components. These components contain very little useful information and are reduced by the 2-pole Bessel filter at the output of the accelerometer. However, if the signal saturates at any point, the accelerometer output does not look like a filtered version of the acceleration signal. The signal may saturate anywhere before the filter. For example, if the resonant frequency of the sensor is low, the displacement per unit acceleration is high. The sensor may reach the mechanical limit of travel if the applied acceleration is high enough. This can be remedied by locating the accelerometer where it does not see high values of acceleration, and by using a higher resonant frequency sensor such as the ADXL78. Also, the electronics may saturate in an overload condition between the sensor output and the filter input. Ensuring that the internal circuit nodes operate linearly to at least several times the full-scale acceleration value can minimize electrical saturation. The ADXL78’s circuits are linear to approximately 8× full scale.

0.075 REF

0.020 DIA

Figure 6. 8-Terminal Ceramic Leadless Chip Carrier [LCC] 1 All models are on tape and reel and are RoHS compliant parts. 3 W = Qualified for Automotive Applications. Reliability reports for these models.

Rev. B | Page 10 of 12 NOTES

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Rev. B | Page 12 of 12 NOTES ©2010 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the prop erty of their respective owners. D05368-0-7/10(B)