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AN3956 参数 Datasheet PDF下载

AN3956图片预览
型号: AN3956
PDF下载: 下载PDF文件 查看货源
内容描述: DEMOAPEXSENSOR用法海拔压力试验( APEX )传感器板 [DEMOAPEXSENSOR Usage Altitude Pressure EXperimental (APEX) Sensor Board]
分类和应用: 传感器
文件页数/大小: 20 页 / 392 K
品牌: FREESCALE [ Freescale ]
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Hardware Issues to Consider when Designing such a Circuit  
ADC Resolution: Since the MPXM2102A is an analog part, the ADC resolution is important to get the resolution needed for  
stable measurement. The ADC has to be greater than 16-bits to gain 3 foot intervals in measurement.  
Noise: Noise on the board has to be minimized. The digital and analog supplies and grounds must be separated. The  
MPXM2102A on the DEMOAPEXSENSOR has its own dedicated shunt voltage reference after the 5 V regulator. The external  
ADC also has its own digital and analog 5 V lines separated. The reference voltage on the ADC has its own shunt reference to  
provide a stable value to compare ADC results. The board is 4 layers with a ground and Vdd plane to minimize parasitic  
capacitance that creates noise. Analog 5 V has its own separated section from the digital 5 V in the power layer of the layout.  
This minimizes the noise on both 5 V digital and 5 V analog sections.  
Air Flow: The pressure sensor should have a cover when trying to measure a sub 1 m altitude measurement. Either the unit  
should be housed in a box, or in a customer application and should have a method to introduce a laminar flow of air. This will  
reduce A/C and breeze effects on the pressure sensor. Most applications of such, include a housing perforated with small holes.  
This prevents a strong breeze from affecting the sensor, by filtering out the drastic pressure changes. DEMOAPEXSENSOR, as  
a demo board, has no such housing.  
Software Considerations  
Filtering: There are high frequency noise and sensor fluctuations that have to be handled. In the software, two low pass filters  
are used to minimize fluctuations in the ADC values. This makes the conversions slower, but the values are more stable. Low  
pass filters are better than using a running average. A running average is a filter that does not discriminate noise outliers as well,  
since it is included in the result.  
Calibration Routines: As seen in the experimental section of the APEX board, it is possible to improve results with various  
software calibrations. A simple example is done in the APEX code, but could be further worked with a possibility of multiple  
sensors to take this further with motion combined. It will be explained in the short distance model of altimetry in this application  
note.  
Altimetry Background Information (refer to AN3914)  
Altimetry utilizes absolute pressure sensors. An absolute sensor measures the deflection of the surrounding barometric  
pressure with reference to a known pressure (usually a vacuum). This allows it to compare the air pressure at sea level  
(101.3kPa) to the vacuum to gain an absolute pressure result. At a different elevation, the barometric (surrounding) pressure can  
be compared again to the vacuum for that absolute pressure result. Since both readings were taken against the same reference,  
they can be compared against each other.  
Barometric pressure does not have a linear relationship with altitude. As altitude increases, the pressure decreases. Common  
reference points are given in Table 3.  
Table 3. Reference Points  
Location  
Altitude (m)  
Altitude (ft)  
0
Pressure (kPa)  
Sea Level  
0
101.3  
106  
33  
Dead Sea (lowest surface on earth)  
Summit of Everest  
-396  
-1300  
10,058  
+33,000  
20000  
15000  
10000  
5000  
0
-5000  
45  
55  
65  
75  
85  
95  
105  
115  
125  
PRESSURE (kPA)  
Figure 6. Altitude vs. Pressure  
AN3956  
Sensors  
Freescale Semiconductor  
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