MXD2002B

Low Cost, ±10 g Dual Axis Accelerometer with Digital Outputs MXD2002A/B FEATURES Sck ( opt i ona l ) I n t e r nal O s c il la t o r Tem p e r at ur e S ens o r V o lt age R e fe r e n c e To ut Dual axis accelerometer fabricated on a monolithic CMOS IC On-chip mixed mode signal processing Resolution better than 6 milli-g 50,000 g shock survival rating 30 Hz bandwidth 2.70V to 5.25V single supply operation Small (5mm x 5mm x 2mm) surface mount package Continuous self test Independently programmable axis (factory special) C LK Vr e f H e at er C ont r o l C o n tin o u s S e lf T e s t X a x is LPF A/ D D out X F a ct o r y A d j u st O f fs e t & G a in APPLICATIONS Automotive – Vehicle Security/Active Suspension/ABS HED Angle Control/Tilt Sensing Security – Gas Line/Elevator/Fatigue Sensing Office Equipment – Computer Peripherals/PDA’s/Mouse Smart Pens/Cell Phones Gaming – Joystick/RF Interface/Menu Selection/Tilt Sensing White Goods – Spin/Vibration Control Y ax i s LPF A/ D D out Y 2- A X I S SENSO R Vd d Gn d Vda MXD2002A/B FUNCTIONAL BLOCK DIAGRAM The MXD2002A/B is a very low cost, dual axis accelerometer fabricated on a standard, submicron CMOS process. The MXD2002A/B measures acceleration with a full-scale range of ±10 g. (The MEMSIC accelerometer product line extends from ±1 g to ±10 g with custom versions available above ±10 g.) It can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). The MXD2002A/B design is based on heat convection and requires no solid proof mass. This eliminates stiction and particle problems associated with competitive devices and provides shock survival up to 50,000 g, leading to significantly lower failure rates and lower loss due to handling during assembly. The MXD2002A/B provides a digital output (ref. other MEMSIC data sheets for analog or ratiometric analog outputs). The outputs are digital signals with duty cycles (ratio of pulsewidth to period) that are proportional to acceleration. The duty cycle outputs can be directly interfaced to a micro-processor. Information furnished by MEMSIC is believed to be accurate and reliable. However, no responsibility is assumed by MEMSIC 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 MEMSIC. The typical noise floor is .0015% duty cycle / Hz allowing signals below 6 milli-g to be resolved at 1 Hz bandwidth. The MXD2002A/B is available in a low profile LCC surface mount package (5mm x 5mm x 2mm height). It is hermetically sealed and operational over a -40°C to +105°C temperature range. Due to the standard CMOS structure of the MXD2002A/B, additional circuitry can easily be incorporated into custom versions for high volume applications. Contact the factory for more information. MEMSIC, Inc. 100 Burtt Road, Andover, MA 01810 Tel: 978.623.8188 Fax: 978.623.9945 www.memsic.com MEMSIC MXD2002A/B Rev.E Page 1 of 8 5/15/2004 MXD2002A/B SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0 unless otherwise noted, VDD, VDA = 5.0V unless otherwise specified) MXD2002A/B Parameter SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2 Transverse Sensitivity3 SENSITIVITY DOUTX and DOUTY Change over Temperature (uncompensated)4 Change over Temperature (compensated) 4 ZERO g BIAS LEVEL 0 g Offset5 0 g Duty Cycle5 0 g Offset over Temperature NOISE PERFORMANCE Noise Density, % Duty Cycle FREQUENCY RESPONSE 3dB Bandwidth TEMPERATURE OUTPUT Tout Voltage Sensitivity VOLTAGE REFERENCE VRef Change over Temperature Current Drive Capability SELF TEST Continuous Voltage at DOUTX, DOUTY under Failure Continuous Voltage at DOUTX, DOUTY under Failure DOUTX and DOUTY OUTPUTS Normal Output Range Current Rise/Fall Time POWER SUPPLY Operating Voltage Range Supply Current Supply Current6,7 TEMPERATURE RANGE Operating Range NOTES Conditions Each Axis Best fit straight line Min ±10 Typ Max Units g % of FS degrees % 1.0 ±1.0 ±2.0 1.80 -47 100µA typical. While the sensitivity of this axis has been programmed at the factory to be the same as the sensitivity for the y-axis, the accelerometer can be programmed for non-equal sensitivities on the x- and y-axes. Contact the factory for additional information. DOUTY – This pin is the digital output of the y-axis Package Characteristics Package θJA θJC LCC-8 110°C/W 22°C/W 8 1 2 3 M E M S IC 7 Device Weight < 1 gram X +g 6 5 4 Y +g T op V i ew Note: The MEMSIC logo’s arrow indicates the +X sensing direction of the device. The +Y sensing direction is rotated 90° away from the +X direction. Small circle indicates pin one(1). Pin Description: LCC-8 Package Pin Name Description 1 TOUT Temperature (Analog Voltage) 2 DOUTY Y-Axis Acceleration Digital Signal 3 Gnd Ground 4 VDA Analog Supply Voltage 5 DOUTX X-Axis Acceleration Digital Signal 6 Vref 2.5V Reference 7 Sck Optional External Clock 8 VDD Digital Supply Voltage Ordering Guide Model Package Style D2002AL D2002BL LCC-8 SMD* LCC-8 SMD* Digital Output 100 Hz 400 Hz *LCC parts are shipped in tape and reel packaging. acceleration sensor. It is factory programmable to 100 Hz 5/15/2004 MEMSIC MXD2002A/B Rev.E Page 3 of 8 or 400 Hz. The user should ensure the load impedance is sufficiently high as to not source/sink >100µA typical. While the sensitivity of this axis has been programmed at the factory to be the same as the sensitivity for the x-axis, the accelerometer can be programmed for non-equal sensitivities on the x- and y-axes. Contact the factory for additional information. TOUT – This pin is the buffered output of the temperature sensor. The analog voltage at TOUT is an indication of the die temperature. This voltage is useful as a differential measurement of temperature from ambient and not as an absolute measurement of temperature. After correlating the voltage at TOUT to 25°C ambient, the change in this voltage due to changes in the ambient temperature can be used to compensate for the change over temperature of the accelerometer offset and sensitivity. Please refer to the section on Compensation for the Change in Sensitivity Over Temperature for more information. Sck – The standard product is delivered with an internal clock option (800kHz). This pin should be grounded when operating with the internal clock. An external clock option can be special ordered from the factory allowing the user to input a clock signal between 400kHz And 1.6MHz Vref – A reference voltage is available from this pin. It is set at 2.50V typical and has 100µA of drive capability. COMPENSATION FOR THE CHANGE IN SENSITIVITY OVER TEMPERATURE All thermal accelerometers display the same sensitivity change with temperature. The sensitivity change depends on variations in heat transfer that are governed by the laws of physics. Manufacturing variations do not influence the sensitivity change, so there are no unit to unit differences in sensitivity change. The sensitivity change is governed by the following equation (and shown in Figure 1 in °C): Si x Ti2.67 = Sf x Tf2.67 where Si is the sensitivity at any initial temperature Ti, and Sf is the sensitivity at any other final temperature Tf with the temperature values in °K and the sensitivity expressed in % duty cycle/g. In gaming applications where the game or controller is typically used in a constant temperature environment, sensitivity might not need to be compensated in hardware or software. The compensation for this effect could be done instinctively by the game player. For applications where sensitivity changes of a few percent are acceptable, the above equation can be approximated with a linear function. Using a linear approximation, an external circuit that provides a gain adjustment of –0.9%/°C would keep the sensitivity within 10% of its room temperature value over a 0°C to +50°C range. For applications that demand high performance, a low cost micro-controller can be used to implement the above equation. A reference design using a Microchip MCU (p/n 16F873/04-SO) and MEMSIC developed firmware is available by contacting the factory. With this reference design, the sensitivity variation over the full temperature range (-40°C to +105°C) can be kept below 3%. Please visit the MEMSIC web site at www.memsic.com for reference design information on circuits and programs including look up tables for easily incorporating sensitivity compensation. DISCUSSION OF TILT APPLICATIONS AND MINIMUM RESOLUTION Tilt Applications: One of the most popular applications of the MEMSIC accelerometer product line is in tilt/inclination measurement. An accelerometer uses the force of gravity as an input to determine the inclination angle of an object. A MEMSIC accelerometer is most sensitive to changes in position, or tilt, when the accelerometer’s sensitive axis is perpendicular to the force of gravity, or parallel to the Earth’s surface. Similarly, when the accelerometer’s axis is parallel to the force of gravity (perpendicular to the Earth’s surface), it is least sensitive to changes in tilt. Table 1 and Figure 2 help to illustrate the output changes in the X- and Y-axes as the unit is tilted from +90° to 0°. Notice that when one axis has a small change in output per degree of tilt (in mg), the second axis has a large change in output per degree of tilt. The complementary nature of these two signals permits low cost accurate tilt sensing to be achieved with the MEMSIC device (reference application note AN-00MX-007). X +90 0 M E M SIC 2.0 Sensitivity (normalized) 1.5 1.0 0.5 0.0 -40 -20 0 20 40 60 80 100 Temperature (C) gravit y 00 Y T op V iew Figure 1: Thermal Accelerometer Sensitivity Figure 2: Accelerometer Position Relative to Gravity MEMSIC MXD2002A/B Rev.E Page 4 of 8 5/15/2004 X-Axis X-Axis Orientation To Earth’s Surface (deg.) 90 85 80 70 60 45 30 20 10 5 0 Y-Axis Y Output (g ) Change per deg. of tilt (mg) 17.45 17.37 17.16 16.35 15.04 12.23 8.59 5.86 2.88 1.37 0.15 X Output (g ) Change per deg. of tilt (mg) 1.000 0.15 0.000 0.996 1.37 0.087 0.985 2.88 0.174 0.940 5.86 0.342 0.866 8.59 0.500 0.707 12.23 0.707 0.500 15.04 0.866 0.342 16.35 0.940 0.174 17.16 0.985 0.087 17.37 0.996 0.000 17.45 1.000 Table 1: Changes in Tilt for X- and Y-Axes DUTY CYCLE DEFINITION The MXD2002A/B has two PWM duty cycle outputs (x,y). The acceleration is proportional to the ratio T1/T2. The zero g output is set to 50% duty cycle and the sensitivity scale factor is set to 2% duty cycle change per g. These nominal values are affected by the initial tolerance of the device including zero g offset error and sensitivity error. This device is offered from the factory programmed to either a 10ms period (100 Hz) or a 2.5ms period (400Hz). T1 T2 (Period) Duty Cycle Pulse width Length of the “on” portion of the cycle. Length of the total cycle. Ratio of the “0n” time (T1) of the cycle to the total cycle (T2). Defined as T1/T2. Time period of the “on” pulse. Defined as T1. T2 Minimum Resolution: Accelerometers can be used in a wide variety of low g applications such as tilt and orientation. The device noise floor will vary with the measurement bandwidth. With the reduction of the bandwidth the noise floor drops. This will improve the signal to noise ratio of the measurement and resolution. The output noise scales directly with the square root of the measurement bandwidth. The maximum amplitude of the noise, its peak- to- peak value, approximately defines the worst case resolution of the measurement. The peak-topeak noise is approximately equal to 6.6 times as the rms value (with an average uncertainty of .1%). The maximum noise for 1.0Hz bandwidth will be 1 mg/ Hz . If the bandwidth is increased to 10 Hz for example: 3.162 mg is the maximum rms noise and 20.87mg is the maximum peak -to-peak noise. DIGITAL INTERFACE The MXD2002A/B is easily interfaced with low cost microcontrollers. For the digital output accelerometer, one digital input port is required to read one accelerometer output. For the analog output accelerometer, many low cost microcontrollers are available today that feature integrated a/d (analog to digital converters) with resolutions ranging from 8 to 12 bits. In many applications the microcontroller provides an effective approach for the temperature compensation of the sensitivity and the zero g offset. Specific code set, reference designs, and applications notes are available from the factory. The following parameters must be considered in a digital interface: Resolution: smallest detectable change in input acceleration Bandwidth: detectable accelerations in a given period of time Acquisition Time: the duration of the measurement of the acceleration signal MEMSIC MXD2002A/B Rev.E Page 5 of 8 T1 A (g)= (T1/T2 - 0.5)/2% 0g = 50% Duty Cycle T2= 2.5ms or 10ms (factory programmable) Figure 4: Typical output Duty C ycle CHOOSING T2 AND COUNTER FREQUENCY DESIGN TRADE-OFFS The noise level is one determinant of accelerometer resolution. The second relates to the measurement resolution of the counter when decoding the duty cycle output. The actual resolution of the acceleration signal is limited by the time resolution of the counting devices used to decode the duty cycle. The faster the counter clock, the higher the resolution of the duty cycle and the shorter the T2 period can be for a given resolution. Table 2 shows some of the trade-offs. It is important to note that this is the resolution due to the microprocessors’ counter. It is probable that the accelerometer’s noise floor may set the lower limit on the resolution. CounterResoCounts Clock MEMSIC lution Counts Per T2 Rate Sample (mg) per g Cycle (MHz) Rate T2 (ms) 2.5 400 2.0 5000 100 10 2.5 400 1.0 2500 50 20 2.5 400 0.5 1250 25 40 10.0 100 2.0 20000 400 2.5 10.0 100 1.0 10000 200 5.0 10.0 100 0.5 5000 100 10 Table 2: Trade-Offs Between Microcontroller Counter Rate and T2 Period. 5/15/2004 USING THE ACCELEROMETER IN VERY LOW POWER APPLICATIONS (BATTERY OPERATION) In applications with power limitations, power cycling can be used to extend the battery operating life. One important consideration when power cycling is that the accelerometer turn on time limits the frequency bandwidth of the accelerations to be measured. For example, operating at 2.7V the turn on time is 40mS. To double the operating time, a particular application may cycle power ON for 40mS, then OFF for 40mS, resulting in a measurement period of 80mS, or a frequency of 12.5Hz. With a frequency of measurements of 12.5Hz, accelerations changes as high as 6.25Hz can be detected. Power cycling can be used effectively in many inclinometry applications, where inclination changes can be slow and infrequent. COMPENSATION FOR ZERO G OFFSET CHANGE OVER TEMPERATURE The compensation of offset is performed with the following equation: Aoc = A + ( a + b * T + c * T * T) where Aoc is the offset compensated acceleration, A is the uncompensated acceleration, T is temperature and a, b, c are constants characteristic to each accelerometer. Computer programs are used to determine these constants. The constants can be read from and written to the MCU EEPROM via the RS-232. The constants a,b,c are normally stored in the MCU EEPROM. To determine the values of the constants, each accelerometer is taken to three different temperatures, preferably evenly spread across the desired temperature span. The zero g bias (A0, A1 and A2) and the temperatures (T0, T1 and T2) are recorded at each temperature. The data collected (A0, T0, A1, T1, A2, T2) is used in a quadratic interpolation (or LaGrange polynomial) to determine a, b and c as follows: r0 = A0 / ( (T0-T1)*(T0-T2) ) r1 = A1 / ( (T1-T0)*(T1-T2) ) r2 = A2 / ( (T2-T0)*(T2-T1) ) a = r0 * T1 * T2 + r1 * T0 * T2 + r2 * T0 * T1 b = - r0 * (T1+T2) – r1 * (T0+T2) – r2 *(T0+T1) c = r0 + r1 + r2 In many cases a computer is used to control the temperature, communicate with the MCU, and to calculate the constants. After calculating the constants, the computer downloads the constants to EEPROM. For a more detail discussion of temperature compensation reference MEMSIC application note #AN-00MX-002 Microcontroller MEMSIC Accel Ax Ay T I/O I/O A/D Figure 5: Zero g Offset Temperature Compensation Circuit COMPENSATION FOR EXTENDING THE FREQUENCY RESPONSE The response of the thermal accelerometer is a function of the internal gas physical properties, the natural convection mechanism and the sensor electronics. Since the gas properties of MEMSIC's mass produced accelerometer are uniform, a digital filter can be used to equally compensate all sensors. The compensating filter does not require adjustment for individual accelerometers. The function of the compensating filter is to apply gain in proportion with the acceleration changes. The faster the acceleration changes occur, the higher the gain that the filter applies. For analog output accelerometers, the compensating filter can be implemented with a circuit involving two op-amps and some resistors and capacitors. For digital output accelerometers, a digital filter is necessary. In applications where high frequency accelerations need to be measured, a DSP (digital signal processor) may be necessary to implement the digital filter. DSP IC’s and development tools are readily available from major IC manufacturers. However, if the bandwidth requirement is relatively low (i.e. 100Hz), it is possible to implement a digital frequency compensating filter with an 8 bit microcontroller. The microcontroller will likely have to be capable of operating at relatively high clock frequencies (20MHz). CONVERTING THE DIGITAL OUTPUT TO AN ANALOG OUTPUT The PWM output can be easily converted into an analog output by integration. A simple RC filter can do the conversion. Note that that the impedance of the circuit following the integrator must be much higher than the impedance of the RC filter. Reference figure 6 for an example. 10K DOUT MEMSIC Accel. AOUT 1uF Figure 6: Converting the digital output to an analog voltage MEMSIC MXD2002A/B Rev.E Page 6 of 8 5/15/2004 TEMPERATURE OUTPUT NOISE REDUCTION It is recommended that a simple RC low pass filter is used when measuring the temperature output. Temperature output is typically a very slow changing signal, so a very low frequency filter eliminates erroneous readings that may result from the presence of higher frequency noise. A simple filter is shown in Figure 8. 8.2K 5. 6. A metal ground plane should be added directly beneath the MEMSIC device. The size of the ground plane should be similar to the MEMSIC device’s footprint and as thick as possible. Vias can be added symmetrically around the ground plane. Vias increase thermal isolation of the device from the rest of the PCB TOUT MEMSIC Accel. Filtered TOUT 0.1uF Figure 8: Temperature Output Noise Reduction POWER SUPPLY NOISE REJECTION Two capacitors and a resistor are recommended for best rejection of power supply noise (reference Figure 9 below). The capacitors should be located as close as possible to the device supply pins (VDA, VDD). The capacitor lead length should be as short as possible, and surface mount capacitors are preferred. For typical applications, capacitors C1 and C2 can be ceramic 0.1 µF, and the resistor R can be 10 Ω. In 5V applications where power consumption is not a concern, maximum supply noise rejection can be obtained by significantly increasing the values of C1, C2 and R. For example, C1 = C2 = 0.47 µF and R = 270 Ω will virtually eliminate power supply noise effects. V SUPPLY C1 R C2 VDA VDD MEMSIC Accelerometer Figure 9: Power Supply Noise Rejection PCB LAYOUT AND FABRICATION SUGGESTIONS 1. The Sck pin should be grounded to minimize noise. 2. Liberal use of ceramic bypass capacitors is recommended. 3. Robust low inductance ground wiring should be used. 4. Care should be taken to ensure there is “thermal symmetry” on the PCB immediately surrounding the MEMSIC device and that there is no significant heat source nearby. MEMSIC MXD2002A/B Rev.E Page 7 of 8 5/15/2004 Package Drawing Fig 10: Hermetically Sealed Package Outline MEMSIC MXD2002A/B Rev.E Page 8 of 8 5/15/2004