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MMA3202EGR2 Datasheet(PDF) 4 Page - Freescale Semiconductor, Inc |
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MMA3202EGR2 Datasheet(HTML) 4 Page - Freescale Semiconductor, Inc |
4 / 10 page Sensors 4 Freescale Semiconductor MMA3202KEG PRINCIPLE OF OPERATION The Freescale Semiconductor, Inc. accelerometer is a surface-micromachined integrated-circuit accelerometer. The device consists of a surface micromachined capacitive sensing cell (g-cell) and a CMOS signal conditioning ASIC contained in a single integrated circuit package. The sensing element is sealed hermetically at the wafer level using a bulk micromachined “cap'' wafer. The g-cell is a mechanical structure formed from semiconductor materials (polysilicon) using semiconductor processes (masking and etching). It can be modeled as a set of beams attached to a movable central mass that move between fixed beams. The movable beams can be deflected from their rest position by subjecting the system to an acceleration (Figure 3). As the beams attached to the central mass move, the distance from them to the fixed beams on one side will increase by the same amount that the distance to the fixed beams on the other side decreases. The change in distance is a measure of acceleration. The g-cell beams form two back-to-back capacitors (Figure 3). As the central mass moves with acceleration, the distance between the beams change and each capacitor's value will change, (C = NA ε/D). Where A is the area of the facing side of the beam, ε is the dielectric constant, D is the distance between the beams, and N is the number of beams. The X-Y device contains two structures at right angles to each other. The CMOS ASIC uses switched capacitor techniques to measure the g-cell capacitors and extract the acceleration data from the difference between the two capacitors. The ASIC also signal conditions and filters (switched capacitor) the signal, providing a high level output voltage that is ratiometric and proportional to acceleration. Figure 3. Simplified Transducer Physical Model SPECIAL FEATURES Filtering The Freescale Semiconductor, Inc. accelerometers contain an onboard 4-pole switched capacitor filter. A Bessel implementation is used because it provides a maximally flat delay response (linear phase) thus preserving pulse shape integrity. Because the filter is realized using switched capacitor techniques, there is no requirement for external passive components (resistors and capacitors) to set the cut- off frequency. Self-Test The sensor provides a self-test feature that allows the verification of the mechanical and electrical integrity of the accelerometer at any time before or after installation. This feature is critical in applications such as automotive airbag systems where system integrity must be ensured over the life of the vehicle. A fourth “plate'' is used in the g-cell as a self- test plate. When the user applies a logic high input to the self- test pin, a calibrated potential is applied across the self-test plate and the moveable plate. The resulting electrostatic force (Fe = 1/2 AV 2/d2) causes the center plate to deflect. The resultant deflection is measured by the accelerometer's control ASIC and a proportional output voltage results. This procedure assures that both the mechanical (g-cell) and electronic sections of the accelerometer are functioning. Ratiometricity Ratiometricity simply means that the output offset voltage and sensitivity will scale linearly with applied supply voltage. That is, as you increase supply voltage the sensitivity and offset increase linearly; as supply voltage decreases, offset and sensitivity decrease linearly. This is a key feature when interfacing to a microcontroller or an A/D converter because it provides system level cancellation of supply induced errors in the analog to digital conversion process. Status Freescale accelerometers include fault detection circuitry and a fault latch. The Status pin is an output from the fault latch, OR'd with self-test, and is set high whenever one (or more) of the following events occur: • Supply voltage falls below the Low Voltage Detect (LVD) voltage threshold • Clock oscillator falls below the clock monitor minimum frequency • Parity of the EPROM bits becomes odd in number. The fault latch can be reset by a rising edge on the self-test input pin, unless one (or more) of the fault conditions continues to exist. Acceleration |
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