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dual-axis, high- g , i mems ? accelerometers adxl278 rev. a 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 ri ghts of analog devices. trademarks and registered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ?2005 analog devices, inc. all rights reserved. features complete dual-axis acceleration measurement system on a single monolithic ic available in 35 g /35 g , 50 g /50 g , or 70 g /35 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 tight sensitivity tolerance and 0 g offset capability largest available prefilter clipping headroom 400 hz, 2-pole bessel filter single-supply operation compatible with sn/pb and pb-free solder processes applications vibration monitoring and control vehicle collision sensing shock detection general description the adxl278 is a low power, complete, dual-axis accelerometer with signal conditioned voltage outputs that are on a single monolithic ic. this product measures acceleration with a full-scale range of (x-axis/y-axis) 35 g /35 g , 50 g / 50 g , or 70 g /35 g (minimum). the adxl278 can also measure both dynamic acceleration (vibration) and static acceleration (gravity). the adxl278 is the fourth-generation surface micromachined i mems? 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 adxl278 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 adxl278 is available in a 5 mm 5 mm 2 mm, 8-terminal ceramic lcc package. functional block diagram 05365-001 adxl278 v dd v s v dd2 v dd3 differential sensor exc demod amp y out 400hz bessel filter timing generator differential sensor exc demod amp x out 400hz bessel filter self-test figure 1. .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 2 of 12 table of contents specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 4 esd caution .................................................................................. 4 pin configuration and function descriptions ............................. 5 theory of operation ........................................................................ 7 applications ....................................................................................... 8 power supply decoupling ............................................................8 self-test ..........................................................................................8 clock frequency supply response .............................................8 signal distortion ...........................................................................8 outline dimensions ..........................................................................9 ordering guide .............................................................................9 revision history 5/05rev. 0 to rev. a .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 3 of 12 specifications 1 at t a = ?40c to +105c, 5.0 v dc 5%, acceleration = 0 g, unless otherwise noted. table 1. model no. AD22284 model no. ad22285 model no. ad22286 parameter conditions min typ max min typ max axis min typ max unit sensor output full-scale range i out 100 a 37 55 x 70 g y 37 g nonlinearity 0.2 2 0.2 2 0.2 2 % package alignment error 1 1 1 degree sensor-to-sensor alignment error 0.1 0.1 0.1 degree cross-axis sensitivity ?5 +5 ?5 +5 ?5 +5 % resonant frequency 24 24 24 khz sensitivity, ratiometric (over temperature) v dd = 5 v, 100 hz 52.25 55 57.75 36.1 38 39.9 x 25.65 27 28.35 mv/ g y 52.25 55 57.75 mv/ g offset zero-g output voltage (over temperature) 2 v out ? v dd /2, v dd = 5 v ?150 +150 ?150 +150 x ?100 +100 mv y ?150 +150 mv noise noise density 10 hz ? 400 hz, 5 v 1.1 3 1.4 3 x 1.8 3.5 m g /hz y 1.1 3 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 25c to t min or t max 2 2 2 hz self-test output change (cube vs. v dd ) 3 v dd = 5 v 440 550 660 304 380 456 x 216 270 324 mv y 440 550 660 mv logic input high v dd = 5 v 3.5 3.5 3.5 v logic input low v dd = 5 v 1 1 1 v input resistance pull-down resistor to gnd 30 50 30 50 30 50 k output amplifier output voltage swing i out = 400 a 0.25 v dd ? 0.25 0.25 v dd ? 0.25 0.25 v dd ? 0.25 v capacitive load drive 1000 1000 1000 pf prefilter headroom 280 400 560 g cfsr @ 400 khz 6 4.5 3 v/v 6 v/v power supply (v dd ) 4.75 5.25 4.75 5.25 4.75 5.25 v functional range 3.5 6 3.5 6 3.5 6 v quiescent supply current v dd = 5 v 2.2 2.9 2.2 2.9 2.2 2.9 ma temperature range ?40 +105 ?40 +105 ?40 +105 c 1 all minimum and maximum specifications are guarante ed. typical specifications are not guaranteed. 2 zero g output is ratiometric. 3 self-test output at v dd = (self-test output at 5 v) (v dd /5 v) 3 . .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 4 of 12 absolute maximum ratings table 2. parameter rating acceleration (any axis, unpowered) 4,000 g acceleration (any axis, powered) 4,000 g v s ?0.3 v to +7.0 v all other pins (com ? 0.3 v) to (v s + 0.3 v) output short-circuit duration (any pin to common) indefinite operating temperature range ?65c to +150c storage temperature ?65c to +150c 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 esd (electrostatic discharge) sensitive device. electrosta tic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without de tection. although th is product 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. .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 5 of 12 pin configuration and fu nction descriptions 05365-002 nc = no connect v dd3 1 y out 2 com 3 v dd 7 x out 6 nc 5 st top view (not to scale) adxl278 4 v dd2 8 figure 2. pin configuration table 3. pin function descriptions pin no. mnemonic description 1 v dd3 3.5 v to 6 v 2 y out y channel output 3 com common 4 st self-test 5 nc do not connect 6 x out x channel output 7 v dd 3.5 v to 6 v 8 v dd2 3.5 v to 6 v .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 6 of 12 05365-003 t p t l t 25c to peak t s preheat critical zone t l to t p temperature time ramp-down ramp-up t smin t smax t p t l figure 3. recommended soldering profile table 4. recommended soldering profile profile feature sn63/pb37 pb-free average ramp rate (t l to t p ) 3c/s max 3c/s max preheat minimum temperature (t smin ) 100c 150c maximum temperature (t smax ) 150c 200c time (t smin to t smax ), t s 60 s ? 120 s 60 s ? 150 s t smax to t l ramp-up rate 3c/s 3c/s time maintained above liquidous (t l ) liquidous temperature (t l ) 183c 217c time (t l ) 60 s ? 150 s 60 s ? 150 s peak temperature (t p ) 240c + 0c/?5c 260c + 0c/?5c time within 5c of actual peak temperature (t p ) 10 s ? 30 s 20 s ? 40 s ramp-down rate 6c/s max 6c/s max time 25c to peak temperature 6 min max 8 min max 05365-004 earth's surface xxxxx xxxx 22285 pin 8 y out = 2.500v x out = 2.462v xxxxx xxxx 22285 y out = 2.462v x out = 2.500v xxxxx xxxx 22285 xxxxx xxxx 22285 y out = 2.538v x out = 2.500v y out = 2.500v x out = 2.538v y out = 2.500v x out = 2.500v figure 4. output response vs. orientation .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 7 of 12 theory of operation the adxl278 provides a fully differential sensor structure and circuit path, resulting in the industrys 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. 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. complementary 200 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. 05365-005 unit sensing cell movable frame fixed plates unit forcing cell anchor moving plate plate capacitors acceleration anchor figure 5. simplified view of sensor under acceleration .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 8 of 12 applications power supply decoupling for most applications, a single 0.1 f capacitor, c dc , adequately decouples the accelerometer from noise on the power supply. however, in some cases, particularly where noise is present at the 200 khz internal clock frequency (or any harmonic thereof), noise on the supply can cause interference on the adxl278s output. if additional decoupling is needed, a 50 (or smaller) resistor or ferrite bead cany be inserted in the supply line. additionally, a larger bulk bypass capacitor (in the 1 f to 4.7 f 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 towards those fixed fingers. the entire signal channel is active; therefore, the sensor displacement causes a change in v out . the adxl278s 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 adxl278 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 v f clamping diode between st and v s 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 adxl278 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 adxl278 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 v out is 3 the amplitude of an excitation signal at v dd 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 adxl278s 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 adxl278. also, the electronics may saturate in an overload condition between the sensor output and the filter input. ensuring that internal circuit nodes operate linearly to at least several times the full-scale acceleration value can minimize electrical saturation. the adxl278 circuit is linear to approximately 8 full scale. .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 9 of 12 outline dimensions bottom view 1 3 5 7 0.64 1.90 2.50 2.50 0.38 diameter 0.50 diameter 1.27 1.27 1.27 4 .50 sq 5.00 sq top view r 0.38 0.20 1.78 r 0.20 figure 6. 8-terminal ceramic leadless chip carrier [lcc] (e-8) dimensions shown in millimeters axl278 ordering guide model 1 parts per reel measurement range specified voltage (v) temperature range package description package option AD22284-a-r2 250 35 g /35 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 AD22284-a 3000 35 g /35 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 ad22285-r2 250 50 g /50 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 ad22285 3000 50 g /50 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 ad22286-r2 250 70 g /35 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 ad22286 3000 70 g /35 g 5 ?40c to +105c 8-lead ceramic leadless chip carrier e-8 1 all models are on tape and reel and are pb-free parts. .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 10 of 12 notes .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 11 of 12 notes .com .com .com .com .com 4 .com u datasheet
adxl278 rev. a | page 12 of 12 notes ?2005 analog devices, inc. all ri ghts reserved. trademarks and registered trademarks are the prop erty of their respective owners. d05365C0C5/05(a) .com .com .com .com 4 .com u datasheet

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