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MXA2500EL Datasheet(PDF) 6 Page - List of Unclassifed Manufacturers |
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MXA2500EL Datasheet(HTML) 6 Page - List of Unclassifed Manufacturers |
6 / 9 page 29/7/2003 A OUTX R C A OUTY R C A OUTX Filtered Output A OUTY Filtered Output Figure 3: High Pass Filter Low Pass Filter: An external low pass filter is useful in low frequency applications such as tilt or inclination. The low pass filter limits the noise floor and improves the resolution of the accelerometer. The low pass filter shown in Figure 4 has a –3dB breakpoint given by the equation: RC f π 2 1 = . For the 200 Hz ratiometric output device filter, C=0.1 µF and R=8kΩ, ±5%, 1/8W. MEMSIC MXA2500E Rev C Page 6 of 9 A OUTX R C A OUTY R C A OUTX Filtered Output A OUTY Filtered Output Figure 4: Low Pass Filter 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 3V 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 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 simple circuit can be used to equally compensate all sensors. For most applications, the compensating circuit does not require adjustment for individual units. A simple compensating network comprising two operational amplifiers and a few resistors and capacitors provides increasing gain with increasing frequency (see Figure 5). The circuit shown is for an absolute output accelerometer operating at 5 V supply. It provides a DC gain of X2, so the offset at the output is 2.5V and the sensitivity is doubled. The 14.3 K Ω and the 5.9KΩ resistors along with the non-polarized 0.82 µF capacitors tune the gain of the network to compensate for the output attenuation at the higher frequencies. The resistors and the capacitors provide noise reduction and stability. Figure 5: Frequency Response Extension Circuit The accelerometer response (bottom trace), the network response (top trace) and the compensated response (middle trace) are shown in Figure 6. The amplitude remains above –3db beyond 100 Hz, and there is useable signal well after this frequency. 8.06K 5.9K 160K 1.5uF - + UA 0.01uF 8.06K 14.3K 5.9K 0.01uF 0.047uF 1.5uF - + UB 0.047uF 14.3K 0.0022uF Aout X or Y Freq. Comp. Output |
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