AN3956

Sensors

Freescale Semiconductor

11

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.

Figure 6. Altitude vs. Pressure

Table 3. Reference Points

Location

Altitude (m)

Altitude (ft)

Pressure (kPa)

Sea Level

0

0

101.3

Dead Sea (lowest surface on earth)

-396

-1300

106

Summit of Everest

10,058

+33,000

33

20000

15000

10000

5000

0

-5000

45

55

65

75

85

95

105

115

125

PRESSURE (kPA)