Improved, Ultra Low Noise ±1 g Dual Axis Accelerometer with Digital Outputs MXD2020GL/HL MXD2020ML/NL FEATURES Resolution better than 1 milli-g Dual axis accelerometer fabricated on a monolithic CMOS IC On chip mixed mode signal processing No moving parts 50,000 g shock survival rating 17 Hz bandwidth expandable to >160 Hz 3.0V to 5.25V single supply continuous operation Continuous self test Independent axis programmability (special order) Compensated for Sensitivity over temperature Ultra low initial Zero-g Offset Sck (optional) Internal Oscillator C LK Tout Voltage R eference Vref C ontinous Self Test H eater C ontrol X axis LPF A/D Dout X LPF A/D Dout Y Factory Adjust Offset & Gain Y axis 2-AXIS SEN SOR APPLICATIONS Automotive – Vehicle Security/Vehicle stability control/ Headlight Angle Control/Tilt Sensing Security – Gas Line/Elevator/Fatigue Sensing Information Appliances – Computer Peripherals/PDA’s/Mouse Smart Pens/Cell Phones Gaming – Joystick/RF Interface/Menu Selection/Tilt Sensing GPS – electronic Compass tilt Correction Consumer – LCD projectors, pedometers, blood pressure Monitor, digital cameras GENERAL DESCRIPTION The MXD2020GL/HL/ML/NL is a low cost, dual axis accelerometer fabricated on a standard, submicron CMOS process. It is a complete sensing system with on-chip mixed mode signal processing. The MXD2020GL/HL/ML/NL measures acceleration with a full-scale range of ±1 g and a sensitivity of 20%/g. It can measure both dynamic acceleration (e.g. vibration) and static acceleration (e.g. gravity). The MXD2020GL/HL/ML/NL 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 of 50,000 g, leading to significantly lower failure rate and lower loss due to handling during assembly. 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. MEMSIC MXD2020GL/ML Tem perature Sensor Page 1 of 7 Vdd G nd Vda MXD2020GL/HL/ML/NL FUNCTIONAL BLOCK DIAGRAM The MXD2020GL/HL/ML/NL provides two digital outputs that are set to 50% duty cycle at zero g acceleration. The outputs are digital with duty cycles (ratio of pulse width to period) that are proportional to acceleration. The duty cycle outputs can be directly interfaced to a microprocessor. The typical noise floor is 0.2 mg/ Hz allowing signals below 1 milli-g to be resolved at 1 Hz bandwidth. The MXD2020GL/HL/ML/NL is packaged in a hermetically sealed LCC surface mount package (5 mm x 5 mm x 2 mm height) and is operational over a -40°C to 105°C(M/NL) and 0°C to 70°C(G/HL) temperature range. MEMSIC, Inc. 800 Turnpike St., Suite 202, North Andover, MA 01845 Tel: 978.738.0900 Fax: 978.738.0196 www.memsic.com 2002.08.29.1 MXD2020GL/HL/ML/NL SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0 g unless otherwise noted; VDD, VDA = 5.0V unless otherwise specified) Parameter SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2 Transverse Sensitivity3 SENSITIVITY Sensitivity, Digital Outputs at pins DOUTX and DOUTY4 Change over Temperature ZERO g BIAS LEVEL 0 g Offset4 0 g Duty Cycle4 Conditions Each Axis Min Best fit straight line X Sensor to Y Sensor DOUTX and DOUTY OUTPUTS Normal Output Range Current Rise/Fall Time Turn-On Time MXD2020M/NL Typ Max ±1.0 Units g 0.5 ±1.0 ±2.0 % of FS degrees % Each Axis 19 20 -10 21 19 +8 -25 +0.1 52 -0.1 48 20 21 % duty cycle/g +8 % +0.1 52 g % duty cycle mg/°C %/°C 0.4 mg/ Hz Each Axis -0.1 48 0.0 50 ±1.5 ±0.03 0.2 @3.0V-5.0V supply 12 17 1.21 4.6 1.25 5.0 2.4 2.5 0.1 Source @5.0V Supply @3.0V Supply Source or sink, @ 3.0V-5.0V supply 3.0 to 5.0V supply @5.0V Supply @3.0V Supply @ 5.0V @ 3.0V 0.0 50 ±1.5 ±0.03 0.4 0.2 12 17 1.29 5.4 1.21 4.6 1.25 5.0 1.29 5.4 V mV/°K 2.65 2.4 2.5 0.1 2.65 V mV/°C µA 100 @5.0V Supply, output rails to supply voltage @3.0V Supply, output rails to supply voltage POWER SUPPLY Operating Voltage Range Supply Current Supply Current4 TEMPERATURE RANGE Operating Range NOTES 1 Min 0.5 ±1.0 ±2.0 Based on 20%/g Continuous Voltage at DOUTX, DOUTY under Failure Max ±1.0 0 g Offset over Temperature NOISE PERFORMANCE Noise Density, rms 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 MXD2020G/HL Typ Hz 100 5.0 5.0 3.0 3.0 V V 0.1 0.1 4.9 2.9 0.1 0.1 100 90 3.0 2.7 3.2 100 100 40 3.8 4.7 0 4.9 2.9 V V µA 110 nS mS mS 5.25 4.4 5.4 V mA mA +105 °C 100 110 90 5.25 4.4 5.4 3.0 2.7 3.2 +70 -40 100 100 40 3.8 4.7 level specifications on this page will be met. Please contact the factory for specially trimmed devices for low supply voltage operation. Guaranteed by measurement of initial offset and sensitivity. 2 Alignment error is specified as the angle between the true and indicated axis of sensitivity. 3 Transverse sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors. 4 The device operates over a 3.0V to 5.25V supply range. Please note that sensitivity and zero g bias level will be slightly different at 3.0V operation. For devices to be operated at 3.0V in production, they can be trimmed at the factory specifically for this lower supply voltage operation, in which case the sensitivity and zero g bias MEMSIC MXD2020GL/ML/NL/HL Page 2 of 7 2002.08.29.1 ABSOLUTE MAXIMUM RATINGS* Supply Voltage (VDD, VDA) ………………...-0.5 to +7.0V Storage Temperature ……….…………-65°C to +150°C Acceleration ……………………………………..50,000 g *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. 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 THEORY OF OPERATION The MEMSIC device is a complete dual-axis acceleration measurement system fabricated on a monolithic CMOS IC process. The device operation is based on heat transfer by natural convection and operates like other accelerometers having a proof mass. The stationary element, or ‘proof mass’, in the MEMSIC sensor is a gas. Ordering Guide Model Package Style LCC - 8 MXD2020GL Digital Output Temperature Range 100 Hz 0 to 70°C MXD2020HL LCC - 8 400Hz 0 to 70°C MXD2020ML LCC - 8 100 Hz -40 to 105° MXD2020NL LCC - 8 400 Hz -40 to 105° Acceleration in any direction will disturb the temperature profile, due to free convection heat transfer, causing it to be asymmetrical. The temperature, and hence voltage output of the four thermopiles will then be different. The differential voltage at the thermopile outputs is directly proportional to the acceleration. There are two identical acceleration signal paths on the accelerometer, one to measure acceleration in the x-axis and one to measure acceleration in the y-axis. Please visit the MEMSIC website at www.memsic.com for a picture/graphic description of the free convection heat transfer principle. All parts are shipped in tape and reel packaging. Caution: ESD (electrostatic discharge) sensitive device. 8 7 M EM SIC 1 2 3 A single heat source, centered in the silicon chip is suspended across a cavity. Equally spaced aluminum/polysilicon thermopiles (groups of thermocouples) are located equidistantly on all four sides of the heat source (dual axis). Under zero acceleration, a temperature gradient is symmetrical about the heat source, so that the temperature is the same at all four thermopiles, causing them to output the same voltage. X +g 6 5 4 Y +g Top V iew 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 following the right-hand rule. MEMSIC MXD2020GL/ML/NL/HL Page 3 of 7 2002.08.29.1 MXD2020GL/HL/ML/NL PIN DESCRIPTIONS VDD – This is the supply input for the digital circuits and the sensor heater in the accelerometer. The DC voltage should be between 3.0 and 5.25 volts. Refer to the section on PCB layout and fabrication suggestions for guidance on external parts and connections recommended. VDA – This is the power supply input for the analog amplifiers in the accelerometer. VDA should always be connected to VDD. Refer to the section on PCB layout and fabrication suggestions for guidance on external parts and connections recommended. Gnd – This is the ground pin for the accelerometer. DOUTX – This pin is the digital output of the x-axis acceleration sensor. It is factory programmable to 100 Hz 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 y-axis, the accelerometer can be programmed for non-equal sensitivities on the x- and y-axes. Contact the factory for additional information. DISCUSSION OF TILT APPLICATIONS AND 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 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 DOUTY – This pin is the digital output of the y-axis 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. 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. MEMSIC MXD2020GL/ML/NL/HL Page 4 of 7 +900 IC S M E gravity 00 M acceleration sensor. It is factory programmable to 100 Hz 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. Y Top View Figure 2: Accelerometer Position Relative to Gravity X-Axis X-Axis Orientatio n To Earth’s Surface (deg.) 90 85 80 70 60 45 30 20 10 5 0 X Output (g) Change per deg. of tilt (mg) Y-Axis Y Output (g) 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 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 2002.08.29.1 Pulse width Resolution: The accelerometer resolution is limited by noise. The output noise will vary with the measurement bandwidth. With the reduction of the bandwidth, by applying an external low pass filter, the output noise drops. Reduction of bandwidth will improve the signal to noise ratio and the 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. With a simple RC low pass filter, the rms noise is calculated as follows: Noise (mg rms) = Noise(mg/ Hz ) * ( Bandwidth( Hz) *1.6) The peak-to-peak noise is approximately equal to 6.6 times the rms value (for an average uncertainty of 0.1%). DIGITAL INTERFACE The MXD2020GL/HL/ML/NL 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 DUTY CYCLE DEFINITION The MXD2020GL/HL/ML/NL 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 20% 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 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. MEMSIC MXD2020GL/ML/NL/HL Page 5 of 7 Time period of the “on” pulse. Defined as T1. T2 T1 A (g)= (T1/T2 - 0.5)/20% 0g = 50% Duty Cycle T2= 2.5ms or 10ms (factory programmable) Figure 3: 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. T2 (ms) 2.5 2.5 2.5 10.0 10.0 10.0 MEMSIC Sample Rate 400 400 400 100 100 100 CounterClock Rate (MHz) 2.0 1.0 0.5 2.0 1.0 0.5 Counts Per T2 Cycle 5000 2500 1250 20000 10000 5000 Counts per g 1000 500 250 4000 2000 1000 Resolution (mg) 1.0 2.0 4.0 0.25 0.5 1.0 Table 2: Trade-Offs Between Microcontroller Counter Rate and T2 Period. 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 3.0V 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. 2002.08.29.1 V SUPPLY 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 4 for an example. DOUT MEMSIC Accel. C1 R VDA C2 VDD MEMSIC Accelerometer Figure 5: Power Supply Noise Rejection 10K AOUT 1uF Figure 4: Converting the digital output to an analog voltage POWER SUPPLY NOISE REJECTION Two capacitors and a resistor are recommended for best rejection of power supply noise (reference Figure 5 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. MEMSIC MXD2020GL/ML/NL/HL Page 6 of 7 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. 5. A metal ground plane should be added directly beneath the MEMSIC device. The size of the plane should be similar to the MEMSIC device’s footprint and be as thick as possible. 6. Vias can be added symmetrically around the ground plane. Vias increase thermal isolation of the device from the rest of the PCB. 2002.08.29.1 LCC-8 PACKAGE DRAWING Fig 6: Hermetically Sealed Package Outline MEMSIC MXD2020GL/ML/NL/HL Page 7 of 7 2002.08.29.1