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MMA7456LR2 Datasheet(PDF) 8 Page - Freescale Semiconductor, Inc |
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MMA7456LR2 Datasheet(HTML) 8 Page - Freescale Semiconductor, Inc |
8 / 35 page Sensors Freescale Semiconductor 8 MMA7456L Note: The response time is between 10% of full scale VDD input voltage and 90% of the final operating output voltage. *The bandwidth for detecting interrupts in level and pulse is 600Hz which is changed from measurement mode. PRINCIPLE OF OPERATION The Freescale accelerometer is a surface-micromachined integrated-circuit accelerometer. The device consists of a surface mi- cromachined capacitive sensing cell (g-cell) and a signal conditioning ASIC contained in a single 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 accel- eration. The g-cell beams form two back-to-back capacitors (Figure 3). As the center beam moves with acceleration, the distance between the beams changes and each capacitor's value will change, (C = A ε/D). Where A is the area of the beam, ε is the di- electric constant, and D is the distance between the beams. The ASIC uses switched capacitor techniques to measure the g-cell capacitors and extract the acceleration data from the differ- ence between the two capacitors. The ASIC also signal conditions and filters (switched capacitor) the signal, providing a digital output that is proportional to acceleration. Figure 3. Simplified Transducer Physical Model Table 4. Function Parameters for Detection –40°C < TA < 85°C, 2.4 V < AVDD < 3.6 V, unless otherwise specified Characteristic Symbol Min Typ Max Unit Level Detection Detection Threshold Range 0 — FS g Pulse Detection Pulse detection range (Adjustable range) Time step for pulse detection Threshold range for pulses Detection levels for threshold Latency timer (Adjustable range) Time Window (Adjustable range) Bandwidth for detecting interrupt* Time step for latency timer and time window 0.5 — 0 — 1 1 — — — 0.5 — 127 — — 600 1 127 — FS — 150 250 — — ms ms g Counts ms ms Hz ms Acceleration |
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