AN3956

Sensors

Freescale Semiconductor

14

Figure 7. APEX Short Distance Calibration Routine

DISSECTION OF THE ISSUES FACING THE PRESSURE READING

Note that with trying to resolve flowchart in Figure 7, there are issues as mentioned with the physical surroundings. A/C on/off

cycling along with airflow in the form of a breeze. Doors opening and closing change the pressure dynamic of a room as the air

flows in or out. This leads to the shifting in the zero level of the altitude as demonstrated in the flowchart.

This causes the scenario of leaving the APEX on the table and seeing that the 0 foot reading shifts up and down – due to the

many root causes in barometric pressure change. This is shown as an ‘experiment’ in the demo board. A straight linear pressure-

altitude can sometimes work, but generally the shifting pressure wreaks havoc on the result.

Multiple sensor systems can be used to possibly stabilize the result. For example, a multiple sensor system can use the

accelerometer to detect movement and re-zero altimeter readings, if pressure changes are not related to movement.

High Resolution Altimetry Customer Implementations

In the Altimetry screen shot as see in the Quick Start Guide, note the raw and the twice Low-Pass filtered values. These are

converted using the pressure equations 1 and 2 as seen on page 12 for pressure and altitude conversion. The APEX does not

have a calibration for the altitude as most customer implementations of high resolution pressure, zero a point and measure the

dynamic change in meters for a given amount of time. The following are examples of ways to implement a high resolution

altimeter with focus on the ‘dynamic’ changing pressure rather than a constant absolute altitude reading.

Altimeter Example 1:

A person sets a zero point before hiking a steep hill. At the top of the hill, the change in barometric pressure is related to meters

or feet. This is a measure of the dynamic change of pressure over a short distance. This is a targeted application versus the other

application of knowing the altitude from sea level at all times after one factory calibration.

Altimetry

Raw Value

Altitude

0x017c1d

2(LP) value

Pressure

0x017c1f

[Current Pressure –Zero Pressure Value]/3

[Current Pressure –Zero Pressure Value]/3

Filtered Value

3ft level lower P

Filtered Value

0ft level higher P

Difference in

Pressure

Resolution=

(Difference)/3ft

Calibration Section

Filtered Value

3ft level lower P

Filtered Value

0ft level higher P

Filtered Value

0ft level higher P

Difference in

Pressure

Difference in

Pressure

Resolution=

(Difference)/3ft

Resolution=

(Difference)/3ft

Calibration Section

Constantly Running

100ms Sample of raw

Pressure ? 24bit ADC

100ms Sample of raw

Pressure ? 24bit ADC

Twice Low-pass

filtered value ? 18bits

usable data

Twice Low-pass

filtered value ? 18bits

usable data

Current Pressure

value stored

Current Pressure

value stored