Calibration Section
Filtered Value
3ft level lower P
Constantly Running
100ms Sample of raw
Pressure ? 24bit ADC
Difference in
Pressure
Resolution=
(Difference)/3ft
Filtered Value
0ft level higher P
Twice Low-pass
filtered value ? 18bits
usable data
Current Pressure
value stored
Altitude in ft =
[Current Pressure –Zero Pressure Value]/3
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
Altimetry
Raw Value
0x017c1d
Altitude
459m
2(LP) value
0x017c1f
Pressure
95.365 kPa
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.
AN3956
Sensors
Freescale Semiconductor
14
FREESCALE [ Freescale ]
