DEMOAPEXSENSOR Altitude Measurement
There are two sensors that can be used for altitude measurement. The MPL115A can be used as an approximate altimeter,
but on this demo board, the MPXM2102A pressure sensor has been used to get a high resolution for altitude measurement. The
MPXM2102A is an absolute pressure sensor. It measures the pressure on its port in relation to a vacuum sealed reference. The
specs given on the data sheet do not easily translate into creating a product that resolves 1m of height difference. The
DEMOAPEXSENSOR in this sense explores how a sensor can be amplified and put through a 24-bit delta sigma ADC to pursue
that type of application.
There is more than one section in the software to determine the vertical altitude. The code has more than one method to try
to improve the resolution the MPXM2102A. One method aims at a sub 1 foot increment, but this is not as stable as the 1m
increment method. Again, this is a experimental board and the method used to measure the MPXM2102A for altitude is subject
to many outside influences. These are not limited to, but include; sudden pressure changes, gusts of wind, A/C unit on/off cycling,
temperature, and exposure to high brightness light. Note the value is setup for dynamic change. The altitude absolute height is
not calibrated. This is typical of end application requiring a starting point.
Table 2. Operating Characteristics (VS = 10 VDC, TA = 25°C unless otherwise noted, P1 > P2)
Characteristic
Symbol
Min
Typ
Max
Units
Differential Pressure Range
Supply Voltage
Supply Current
Full Scale Span
Offset
POP
0
—
100
kPa
VS
IO
—
—
10
6.0
40
16
—
VDC
mAdc
mV
VFSS
38.5
41.5
MPX2102D Series
MPX2102A Series
MPXM2102D/G Series
MPXM2102A Series
VOFF
VOFF
-1.0
-2.0
-1.0
-2.0
—
—
—
—
1.0
2.0
1.0
2.0
mV
mV
Sensitivity
Linearity
ΔV/ΔΡ
—
0.4
—
mV/kPa
MPX2102D Series
MPX2102A Series
MPXM2102D/G Series
MPXM2102A Series
—
—
—
—
-0.6
-1.0
-0.6
-1.0
—
—
—
—
0.4
1.0
0.4
1.0
%VFSS
%VFSS
—
—
—
—
±0.1
±0.5
—
—
—
%VFSS
%VFSS
%VFSS
mV
Pressure Hysteresis (0 to 100 kPa)
Temperature Hysteresis (-40°C to +125°C)
Temperature Coefficient of Full Scale Span
TCVFSS
TCVOFF
ZIN
-2.0
-1.0
1000
1400
—
2.0
Temperature Coefficient of Offset
Input Impedance
—
1.0
—
2500
3000
—
W
Output Impedance
ZOUT
tR
—
W
Response Time (10% to 90%)
1.0
ms
Warm-Up Time
Offset Stability
—
—
—
—
20
—
—
ms
±0.5
%VFSS
Table 2 shows the typical operating characteristics of the MPXM2102A. The supply voltage is ratiometric, but at 10 V supply,
the FSS is 40 mV for 100 kPa of applied absolute pressure. The DEMOAPEXSENSOR does not have a full 10 V supply. The
part is biased instead by a 5 V supply. This cuts the FSS to 20 mV for 100 kPa. Since the output of an analog sensor is ratiometric
to the input voltage, it is crucial to have a stable supply voltage. In the case of amplifying and analyzing small pressure changes,
the input voltage must be stable to remove input noise. To achieve this, a shunt voltage reference connected to the 5 V regulator
provides a low noise, stable voltage of 4.096 V for the MPX2102A. This is the IC, D8 on the demo schematic, and is typically
used for data-acquisition systems. Thus, the span is really only 16.38 mV at 4.096 V, for a pressure range of 0 to 100 kPa.
MPXM2102A is on a separate shunt such that no other connected IC will induce additional noise on the shared line.
AN3956
Sensors
Freescale Semiconductor
10
FREESCALE [ Freescale ]
