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MMA6261Q Datasheet(PDF) 4 Page - Motorola, Inc |
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MMA6261Q Datasheet(HTML) 4 Page - Motorola, Inc |
4 / 8 page Sensor Device Data 4 Freescale Semiconductor MMA6200 SERIES PRINCIPLE OF OPERATION The Freescale Semiconductor accelerometer is a surface- micromachined integrated-circuit accelerometer. The device consists of a surface micromachined capacitive sensing cell (g-cell) and a signal conditioning ASIC contained in a single integrated circuit package. The sensing element is sealed hermetically at the wafer level using a bulk microma- chined cap wafer. The g-cell is a mechanical structure formed from semicon- ductor materials (polysilicon) using semiconductor processes (masking and etching). It can be modeled as a set of beams at- tached 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 2). As the beams attached to the central mass move, the dis- tance 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 plates form two back-to-back capacitors (Figure 2). As the center plate moves with acceleration, the dis- tance between the plates changes and each ca pacitor's value will change, (C = A ε/D). Where A is the area of the plate, ε is the dielectric constant, and D is the distance between the plates. The 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 2. Simplified Transducer Physical Model SPECIAL FEATURES Filtering These Freescale Semiconductor accelerometers contain an onboard single-pole switched capacitor filter. Because the filter is realized using switched capacitor techniques, there is no re- quirement for external passive components (resistors and ca- pacitors) to set the cut-off frequency. Self-Test The sensor provides a self-test feature allowing the verifica- tion of the mechanical and electrical integrity of the accelerom- eter at any time before or after installation. A fourth plate is used in the g-cell as a self-test plate. When a logic high input to the self-test pin is applied, a calibrated potential is applied across the self-test plate and the moveable plate. The resulting electro- static force (Fe = 1/2 AV 2/d2) causes the center plate to deflect. The resultant deflection is measured by the accelerometer's ASIC and a proportional output voltage results. This procedure assures both the mechanical (g-cell) and electronic sections of the accelerometer are functioning. Freescale Semiconductor accelerometers include fault de- tection circuitry and a fault latch. Parity of the EEPROM bits be- comes odd in number. Self-test is disabled when EEPROM parity error occurs. Ratiometricity Ratiometricity simply means the output offset voltage and sensitivity will scale linearly with applied supply voltage. That is, as supply voltage is increased, the sensitivity and offset in- crease linearly; as supply voltage decreases, offset and sensi- tivity decrease linearly. This is a key feature when interfacing to a microcontroller or an A/D converter because it provides sys- tem level cancellation of supply induced errors in the analog to digital conversion process. Acceleration |
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