circuit note CN-0274 circuits from the lab? reference circuits are e ngineered and tested for quick and easy system integration to help solve todays analog, mixed - signal, and rf design challenges. for more i nformation and/or support , visit www.analog.com/cn0274 . devices connected /referenced adxl362 nanopower, 3- axis, 2 g , 4 g , and 8 g digital output mems accelerometer adp195 logic controlled, high - side power switch with reverse curren t blocking ultralow power, 3 - axis, motion activated switch rev. a circuits from the lab? circuits from analog devices have been designed and built by analog devices engineers. standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. however, you are solely responsible for testing the circuit and determining its su itability and applicability for your use and application. accordingly, in no event shall analog devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any circuits from the lab circuits. (continued on last page) 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 ? 2012 analog devices, inc. all rights reserved. evaluation and desig n support circuit evaluation boards cn - 0274 circuit evaluation board (eval - cn0274 - sdpz) system demonstration platform (eval - sdp -c s1z) design and integration files schematics, layout files, bill of materials circuit function and benefits the combination of parts shown in figure 1 prov ides an ultralow power, 3- axis, motion activated power switch solution capable of controlling up to 1.1 a of load current. the circuit is ideal for applications where extended battery life is critical. when the switch is off, the battery current is less th an 300 na, and when the switch is on, it draws less than 3 a. the circuit provides an industry leading , low power motion sensing solution suitable for wireless sensors, metering devices, home healthcare , and other portable applications. the 3- axis acceler ometer controls the high - side switch by monitoring the acceleration in three axes and closes or opens the switch depending on the presence or absence of motion. the adxl362 is a n ultralow powe r, 3- axi s accelero meter that consumes less than 100 na in wake - up mode . unlike accelerometers that use power duty cycling to achieve low power consumption, the adxl362 does not alias input signals by under sampling; it samples co ntinuously at all data rates. there is also an on - chip , 12- bit temperature sensor accurate to 0.5. the adxl362 provides 12 - bit output resolution and has three operating ranges, 2 g , 4 g, and 8 g. it is spec ified over a minimum temperature range of ?40c to +85c. for applications where a noise level less than 480 g /hz is desired, either of its two lower noise modes (down to 120 g /hz) can be selected at a minimal increase in supply current. the adp195 is a high - side load switch designed for operation between 1.1 v and 3.6 v and is protected against reverse current flow from output to input. the device contains a low on - resistance, p- channel mosfet that supports over 1.1 a of continuous load current and minimizes power losses. adxl362 gnd int1 int2 cs sclk miso mosi spi interface v s 3.3v c s v dd i/o 3.3v c io interrupt control awake gnd en vin vs load vout adp195 level shift and slew rate control reverse polarity protection 10773-001 figure 1. ultralow power standalone motion switch (simplified schematic: decoupling and all connections not shown)
cn- 0274 circuit note rev. a | page 2 of 6 basic operation of the adxl362 the adxl362 is a three - axis, ultralow power acceleration measurement system capable of measuring dynamic acceleration (resulting from motion or shock) as well as static acceleration ( that is, gravity). the moving componen t of the sensor is a polysilicon , surface micromachined structure, also referred to as a beam, built on top of a silicon wafer. polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. de flection of the structure is measured using differential capacitors. each capacitor consists of independent fixed plates and plates attached to the moving mass. any acceleration deflect s the beam and unbalance s the differential capacitor, resulting in a se nsor output whose amplitude is proportional to acceleration . phase - sensitive demodulation is used to determine the magnitude and polarity of the acceleration. modes of operation the three basic modes of operation for the adxl362 are standby, measurement, and wake - up. ? placing the adxl362 in standby mode suspends measurement and reduces current consumption to 10 na. any pending data or interrupts are preserved ; however, no new information is processed . the adxl362 powers up in standby mode with all sensor functions turned off. ? measurement mode is the normal operating mode of the adxl3 62 . in this mode, acceleration data is continuously read , and the accelerometer consumes less than 3 a across its entire range of output data rates of up to 400 hz using a 2.0 v supply . all described features are available while operating in this mode. t he ability to continuously output data from the minimum 12.5 hz to the maximum 400 hz data rate while still delivering less than 3 a of current consumption is what defines the adxl362 as an ultralow power accel erometer . under sampling and aliasing do not occur with the adxl362 because it continuously samples the full bandwidth of its sensor at all data rates. ? wake - up mode is ideal for simple detection of the presence or absence of motion at extremely low power consumption (270 na at a 2.0 v supply vo ltage) . wa ke - up mode is useful particularly for implementation of a motion - activated on/off switch, allowing the rest of the system to be powered down until activity is det ected. wa ke - up mode reduces current consumption to a very low level by measuring acceleration only 6 times a second to determine whether motion is present . in wake - up mode, all accelerometer features are available with the exception of the activity timer . all registers are accessible, and real - time data is available from the part. the cn0274 evaluation software u s es the wake - up mode of the adxl362 . that is, the adxl362 is asleep until it detects motion at which point it enters m easurement m ode. power/noise tradeoff the adxl362 offers a few options for decreasing noise at the expense of only a small increase in current consumption. the noise performance of the adxl362 in normal operation, typically 7 lsb rms at 100 hz bandwidth, is adequate for most applications, depending upon band width and the desired resolution . for cases where lower noise is needed, the adxl362 provides two lower noise , operating modes that trade reduced noise for somewhat higher supply current. table 1 . adxl362 noise vs. supply current mode noise ( g /hz typical) current consumption (a typical) normal operation 380 2.7 low noise 280 4.5 ultralow noise 175 15 table 1 shows the supply current values and noise densities obtained for nor mal operation and the two lower noise modes, at a typical 3.3 v supply . the cn0274 evaluation software uses the normal operation noise mode of the adxl362 . motion detection the adxl362 h as built - in logic that detects a ctivity (acceleration above a certain threshold) and i nactivity ( lack of acceleration above a certain threshold) . detection of an activity or inactivity event is indicated in the s tatus re gister and can also be configured to generate an interrupt . in addition, the activity status of the device , that is, whether it is moving or stationary , is indicated by the awa k e bit. activity and i nactivity detection can be used when the accelerometer is in either m easurement m ode or w ake - u p m ode.
circuit note cn- 0274 re v. a | page 3 of 6 activity detection an activity event is detected when acceleration stays above a specified threshold for a user - specified time period. the two activity detection events are absolute and referenced. ? when using absolute activity detection , acceleration samples are compared to a user set threshold to determine whether motion is present . for example, if a threshold of 0.5 g is set , and the acceleration on any axis is 1 g for longer than the user defined activity time, the activity status is asserted. in many applications, it is advantageous for activity detection to be based not on an absolute threshold but on a deviation from a reference point or orientation . this is particularly useful because it remov es the effect on activity detection of the static 1 g imposed by gravity . when an accelerometer is stationary, its output can reach 1 g , even when it is not moving . in absolute activity, if the threshold is set to less than 1 g , activity is immediately det ected in this case. ? in the referenced activity detection , activity is detected when acceleration samples are at least a user set amount above an internally defined reference, for the user defined amount of time . the reference is calculated when activity de tection is engaged , and the first sample obtained is used as a reference point . a ctivity is only detected when the acceleration has deviated sufficiently from th is initial orientation. the referenced configuration results in a very sensitive activity detec tion that detects even the most subtle motion events. the cn0274 evaluation software uses the referenced mode of operation when searching for activity . inactivity detection an inactivity event is detecte d when acceleration remains below a specified threshold for a specified time . the two inactivity detection events are absolute and referenced. ? in absolut e inactivity detection , a cceleration samples are compared to a user set threshold for the user set time to determine the absence of motion. ? in referenced inactivity detection , a ccelera tion samples are compared to a user specified reference for a user defined amount of time . when the part first enters the awake state, the first sample is used as a reference point , and the threshold is applied around it . if the acceleration stays inside the threshold, the part enter s the asleep state . if an acceleration value moves outside the threshold, this point is then used as a new reference , and the thresholds are reappl ied to this new point. the cn0274 evaluation software uses the referenced mode of operation when searching for inactivity . linking activity and inactivity detection the activity and inactivity detecti on functions can be used concurrently , and processed manually by a host processor, or they ca n be configured to interact in several ways : ? in d efault mo de , a ctivity and inactivity detection are both enabled , and all interrupts must be serviced by a host pro cessor; that is, a processor must read each interrupt before it is cleared and can be used again. ? in li nked m ode , a ctivity and inactivity detection are linked to each other such that only one of the f unctions is enabled at any given time . once activity is detected, the device is assumed moving or awake and stops looking for activity: inactivity is expected as the next event so onl y inactivity detection operates . when inactivity is detected, the device is assumed stationary or asleep . a ctivity is now expecte d as the next event so that only a ctivity detection operates. in this mode, a host processor must service each interrupt before the next is enabled. ? in l oop m ode , m otion detection operates as previously described in linked m ode ; however, interrupts do not need to be serviced by a host processor . this configuration simplifies the implementation of commonly used motion detection and enhances power savings by reducing the amount of power used in bus communication. ? when enabling autosleep mode i n l inked mode or l oop m ode, it cause s the device to autonomously enter wake - up mode when inactivity is detected, and reenter measurement mode when activity is detected. the cn0274 evaluation software uses the autosleep a nd loop modes to demonstrate the functionality of the adxl362 . the awake bit the awa k e bit is a status bit that indicates whether the adxl362 is awake or asleep . the device is awake when it has seen an activity condition, and the device is asleep when it has seen an inactivity condition. the awake signal can be mapped to the int1 or int2 pin and can thus be used as a status output to connect or disconnect power to down stream circuitry based on the a wake status of the accelerometer . used in conjunction with l oop m ode, this configuration implements a trivial, autonomous motion - activated switch. if the turn - on time of the downstream circuitry can be tolerated , this motion switch configuration can save significant system - level power by eliminating the standby current consumption of the rest of the application . this standby current can often exceed the full operating current of the adxl362 .
cn- 0274 circuit note rev. a | page 4 of 6 interrupts several of the built - in functions of the adxl362 can trigger interrupts to alert the host processor of certain status conditions. interrupts may be mapped to either (or both) of two desig nated output pins, int1 and int2, by setting the appropriate bits in the intmap1 and intmap2 registers . all functions can be used simultaneously . if multiple interrupts are mapped to one pin, the or combination of the interrupts determines the status of th e pin. if no functions are mapped to an interrupt pin, that pin is aut omatically configured to a high impedance (high - z ) state . the pins are placed in this state upon a reset as well. when a certain status condition is detected, the pin that condition is m apped to is activated . the configuration of the pin is active high by default, so that when it is activated , the pin goes high . however, this configuration can be switched to active low by setting the int_low pin in the appropriate intmap register. the int pins may be connected to the interrupt input of a host processor and interrupts responded to with an interrupt routine . because multiple functions can be mapped to the same pin, the status register can be used to determine which condition caused the inter rupt to trigger. the cn0274 evaluation software configures the adxl362 such that when activity is detected, the int1 pin is high , and when inactivity is detect ed, the int 1 pin is low . test results all testing was performed using the e va l - cn0274 - sdpz and the e va l - sdp - cs1z . functionality of the part is demonstr ated by setting the activity threshold at 0.5 g , the inactivity threshold at 0.75 g , and the number of inactivity samples at 20 . when looking for activity, only one acceleration sample on any axis is required to cross the threshold. starting with the circu it oriented so that the battery pack is flat against the table, the printed circuit board ( pcb ) can be slowly rotated 90 in any direction causing the acceleration to cross the threshold as it approaches perpendicular to the initial orientation . figure 2 shows a screen shot of the cn0274 evaluation software showing the adxl362 fir st asleep, looking for activity . then, wh en s ample 11 crosses the threshold, the adxl362 enters the awake state and begins looking for inactivity . the thresholds adjust to show the device is now looking for inactivity . 10773-002 figure 2. screen shot of evaluation software output for better visibility, the x - axis and z - axis plots are disabled using the radio buttons above the chart. t he output of the adp195 , or the interrupt pin itself, was measured using a digital multimeter . when the adxl362 is awake , the interrupt goes high and drive s the en pin of the adp195 high , which in turn drives the gate of the mos fet low, causi ng the switch to close, connecting any downstream circuitry to the power supply . conversely, when the adxl362 is asleep, the interrupt drive s the en pin of the adp195 low , which in turn drives the gate of the mosfet high , causing the switch to open. pcb layout considerations in any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. the pcb should isolate the digital and analog sections as much as possible. the pcb for this system was constructed in a 4 - layer stack up with large area ground plane layers and power plane polygons. see the mt - 031 t utorial for more discussion on layout and grounding , and the mt - 101 tutorial for information on decoupling techniques. decouple t he power supply to the adxl362 with 1 f and 0.1 f capacitors to properly suppress noise and reduce ripple. place t he capacitors as close to the device as possible. ceramic capacitors are advised for all high frequency decoupling. power supply lines should have as large a trace width as pos sible to provide low impedance paths and reduce glitch effects on the supply line. shield c locks and other fast switching digital signals from other parts of the board by digital ground. a photo of the pcb is shown in figure 3 . a complete design support package for this circuit note can be found at www.analog.com/cn0274 - designsupport .
circuit note cn- 0274 re v. a | page 5 of 6 10773-003 figure 3. photo of eval - cn0274 - sdpz pcb c ommon variations the maximum continuous operating current of the adp195 is fixed at 1.1 a. for applications requiring more downstream power, a higher current rated switch can b e used in place of the adp195 . by sacrificing approximately 15 a of quiescent current, the adp197 is capable of providing 3 a of current to downstream circuitry. fo r applications requiring less downstream power, the adp190 can be used. it has a continuous current of 500 ma and is available in a smaller wlcsp package than the adp1 95. a second variant of the provided solution is to create a free fall detection system. this function can be implemented using the inactivity interrupt. when an object is in true free - fall, acceleration on all axes is 0 g . thus, free - fall detection is ac hieved by looking for acceleration on all axes to fall below a certain threshold (close to 0 g ) for a certain amount of time. the adxl362 functions as a free - fall detector by setting the inactivity threshold (3 00 m g to 600 m g ) and inactivity time (150 ms to 350 ms). the register setting for these values varies based on the g - range setting of the device. circuit evaluation and t est this circuit uses the eval - sdp - c s1z system demonstration platform (sdp) evaluation board and the eval - cn0274 - sdpz circuit board . the two boards have 120 - pin mating connectors, allowing for the quick setup and evaluation of the perfor mance of the circuit . the e va l - cn0274 - sdpz contains the circuit to be evaluated, as described in this note, and the e va l - sdp - cs1z is used with the cn0274 evaluation software to capt ure the data from the e va l - cn0274 - sdpz . because the adxl362 requires a re latively small amount of power in both the asleep and awake states, it is possible to power the eval - cn0274 - sdpz from the digital data lines coming out of the eval - sdp - cs1z . equipment needed the following equipment is needed: ? a pc with a usb p ort and windows? xp or windows vista? ( 32- bit), or windows? 7 (32 - bit) ? the e va l - cn0274 - sdpz evaluation b oard ? the e va l - sdp - cs1z evaluation board ? the cn0274 evaluation software ? a p ower supply: 3.0 v or 2 aaa batteries. getting started load the e valuation s oftwar e by placing the cn0274 evaluation software cd into the pc. using my computer , locate the drive that co ntains the evaluation software cd and open the readme file. follow the instructions contained in the r eadme file for installing and using the evaluation software.
cn- 0274 circuit note rev. a | page 6 of 6 functional block diagram see figure 4 for the test setup block di agram, and the e va l - cn0274 - sdpz - sch - reva.pdf file for the circuit schematics. this file is conta ined in the cn0274 design support package . eval-sdp-cs1z j1 j3 3v power supply j2 pc optional aaa batteries 120-pin connector usb eval-cn0274-sdpz 10773-004 figure 4. test setup block diagram setup connect the 120 - pin connector on the e va l - cn0274 - sdpz to the connector on the e va l - sdp - cs1z . use n ylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120 - pin connectors. with power to the supply of f, connect a 3.0 v pow er supply to the j3 connector . alternatively, c onnector j2 can be used on the bottom of the pcb to power the entire circuit off two aaa batteries . connect the usb cable supplied with the e va l - sdp - cs1z to the usb port on the pc. note: do no t connect the usb cable to the m ini - usb connector on the sdp board at this time. test apply power to the j3 screw terminal or place batteries in the j2 connector on the bottom of the pcb batter ies (move jumper j6 to the left - hand position for battery operation). launch the cn0274 evaluation software and connect the usb cable from the pc to the mini - usb connector on the e va l - sdp - cs1z . once usb communications are established, the e va l - sdp - cs1z can now be used to send, receive, and capture serial data from the e va l - cn0274 - sdpz . information regarding the e va l - sdp - cs1z can be found in the sdp user guide . information and details regarding t est setup and calibration, and how to use the evaluation software for data capture can be found in the software readme file found a t : www.analog.com/cn0274 - userguide . learn more cn - 0274 design support package: http://www.analog.com/cn0274 - designsupport mt - 031 tutorial, grounding data converters and solving the mystery of agnd and dgnd , analog devices. mt - 101 tutor ia l , decoupling techniques, analog devices . an - 688 application note , phase and frequency response of imems accelerometers and gyros , analog devices data sheets and evaluation boards cn0274 circuit evaluation board (eval - cn0274 - sdpz) system demonstration platform (eval - sdp - cs1z) adxl362 data sheet adp195 data sheet adp195 evaluation board revision history 11/12 rev. 0 to rev. a changes to circuit note title and circuit function and benefits section ................................................................................................. 1 changes to common variations ..................................................... 5 9 / 1 2 rev. 0 : initial version (continued from first page) circuits from the lab circuits are intended only for use with analog devices products and are the intellectual property of analog devi ces or its licensors. while you may use the circuits from the lab circuits in the design of your product, no other license is grant ed by implication or otherwise under any patents or other intellectual property by application or use of the circuits from the lab circuits. information furnished by analog devices is believed to be accurate and reliable. however, circuits from the lab cir cuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warrant y of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by an alog devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. analog devices reserves the right to change any circuits from the lab circuits at any time without notice but is under no obli gation to do so. ? 2012 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. cn1077 3- 0 - 11/12(a)
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