AN3956

Sensors

Freescale Semiconductor

13

Summarizing in Table 5

DEMOAPEXSENSOR can read a 0.000963 (kPa/ADC count) step for a 0 to 100kPa MPXM2102A sensor.

The formulas earlier were shown to reference the pressure to altitude relationship.

Converting all this to altitude:

Reviewing Table 6, the DEMOAPEXSENSOR has code written to interpret small pressure changes and convert these to

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

The preceding section on Altimetry has used the pressure sensor value to convert the pressure to an altitude reading. This

value of pressure is not linear with vertical height as detailed in the graph of Figure 6. However if a curved line is magnified

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.

In this example the chart outlines the calibration routine.

• 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.

Now the display will output the distance in smaller increments than 1 foot. Notice how it tends to jump around, and the “0 level”

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.

Table 5.

ADC Type

Resolution (V/ADC)

Resolution (kPa/ADC)

24-bit

0.244 V

0.0000151 kPa

18-bit

15.6 V per 1 ADC count

0.000963 kPa per 1 ADC count

Table 6.

Distance

Pressure (mB)

Pressure (kPa)

Voltage Change (V)

ADC (counts)

0.5 m

0.05

0.005

0.81

5.19 so ~5

1 m

0.11

0.011

1.78

11.4 so ~11

3 ft

0.09

0.009

1.46

9.35 so ~9

1 ft

0.03

0.003

0.49

3.11 so ~3