DEMOAPEXSENSOR Freescale Semiconductor, DEMOAPEXSENSOR Datasheet - Page 11

• Manufacturer Part Number: DEMOAPEXSENSOR
• Description: BOARD DESIGN ALTITUDE PRESSURE
• Manufacturer: Freescale Semiconductor

Specifications of DEMOAPEXSENSOR

• Sensor Type: Accelerometer, Pressure, Water Level, Altimeter/Barometer
• Sensing Range: Multiple Ranges
• Interface: I²C, SPI, Analog
• Sensitivity: Multiple
• Voltage - Supply: 10 V
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: MPX2102, MPXV5004G, MPL115A2, MMA7361L
• Acceleration: 6 g
• Sensing Axis: Triple Axis
• Output Type: Digital
• Interface Type: I2C, SPI
• Operating Voltage: 16 V
• Operating Current: 10 mA
• Leaded Process Compatible: Yes
• Peak Reflow Compatible (260 C): Yes
• Rohs Compliant: Yes
• Silicon Manufacturer: Freescale
• Silicon Core Number: MPXM2102A, MPXV5004G, MPL115A2, MMA7361L
• Kit Application Type: Sensing - Sampling Kit / Related Components
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: MPL115A, MPXM102A, MPXV5004G

Hardware Issues to Consider when Designing such a Circuit
stable measurement. The ADC has to be greater than 16-bits to gain 3 foot intervals in measurement.
MPXM2102A on the DEMOAPEXSENSOR has its own dedicated shunt voltage reference after the 5V regulator. The external
ADC also has its own digital and analog 5V 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 5V has its own separated section from the digital 5V in the power layer of the layout. This
minimizes the noise on both 5V digital and 5V analog sections.
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
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.
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)
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.
reference points are given in
Sensors
Freescale Semiconductor
ADC Resolution: Since the MPXM2102A is an analog part, the ADC resolution is important to get the resolution needed for
Noise: Noise on the board has to be minimized. The digital and analog supplies and grounds must be separated. The
Air Flow: The pressure sensor should have a cover when trying to measure a sub 1 m altitude measurement. Either the unit
Filtering: There are high frequency noise and sensor fluctuations that have to be handled. In the software, two low pass filters
Calibration Routines: As seen in the experimental section of the APEX board, it is possible to improve results with various
Altimetry utilizes absolute pressure sensors. An absolute sensor measures the deflection of the surrounding barometric
Barometric pressure does not have a linear relationship with altitude. As altitude increases, the pressure decreases. Common
Table 3. Reference Points
Sea Level
Dead Sea (lowest surface on earth)
Summit of Everest
Location
Table
20000
15000
10000
-5000
5000
0
45
3.
55
Figure 6. Altitude vs. Pressure
65
75
PRESSURE (kPA)
Altitude (m)
10,058
-396
85
0
95
105
Altitude (ft)
115
+33,000
-1300
0
125
Pressure (kPa)
101.3
106
33
AN3956
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