DEMOAPEXSENSOR Freescale Semiconductor, DEMOAPEXSENSOR Datasheet - Page 13

• 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

Summarizing in
DEMOAPEXSENSOR can read a 0.000963 (kPa/ADC count) step for a 0 to 100kPa MPXM2102A sensor.
altitude. The conversion from pressure to altitude still has to use the exponential equation for a proper conversion. But the
approximate distances and the ADC count equivalent are shown here. These are approximate across the pressure range but as
written in the demo code, the pressure is determined first, the altitude second. The altitude is calculated via the pressure value
inserted into the exponential altitude-pressure formula.
Experimental Altimetry Section
value of pressure is not linear with vertical height as detailed in the graph of
enough, the end result can be treated as a straight line. The “Experimental” section of the Altimeter/Barometer tries this particular
method. Due to changing air currents and unstable smaller resolution, it may require outdoor demonstration to get better results.
shifts. This Experimental section shows how this method while being more accurate is not as stable as the exponential method
to see pressure. The resolution is reasonable for a short period of time before barometric changes in pressure change the
calibration.
Sensors
Freescale Semiconductor
Converting all this to altitude:
Reviewing
The preceding section on Altimetry has used the pressure sensor value to convert the pressure to an altitude reading. This
In this example the chart outlines the calibration routine.
Now the display will output the distance in smaller increments than 1 foot. Notice how it tends to jump around, and the “0 level”
The formulas earlier were shown to reference the pressure to altitude relationship.
• To calibrate the part, first place the board on the floor or table in front of you.
• Wait until the low pass filtered value is stable then press Enter. This saves the “0 level” barometric pressure.
• Following this, place the board about 3 feet above the “0 level”. Either this is at waist height if previously on the floor, or
above your head if previously on the table. When the value stabilizes, press Enter again.
Table 6.
Table
Distance
Table 5
Table 5.
0.5 m
1 m
3 ft
1 ft
6, the DEMOAPEXSENSOR has code written to interpret small pressure changes and convert these to
ADC Type
24-bit
18-bit
Pressure (mB)
0.05
0.11
0.09
0.03
15.6
Resolution (μV/ADC)
Pressure (kPa)
μ
V per 1 ADC count
0.244
0.005
0.009
0.003
0.011
μV
Figure
Voltage Change (μV)
0.000963 kPa per 1 ADC count
6. However if a curved line is magnified
0.81
1.78
1.46
0.49
Resolution (kPa/ADC)
0.0000151 kPa
ADC (counts)
11.4 so ~11
5.19 so ~5
9.35 so ~9
3.11 so ~3
AN3956
13