AN3956

Sensors

Freescale Semiconductor

10

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 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.

Characteristic

Symbol

Min

Typ

Max

Units

Differential Pressure Range

0

—

100

kPa

Supply Voltage

—

10

16

Supply Current

—

6.0

—

mAdc

Full Scale Span

38.5

40

41.5

mV

Offset

MPX2102D Series

MPX2102A Series

MPXM2102D/G Series

MPXM2102A Series

-1.0

-2.0

-1.0

-2.0

—

—

—

—

1.0

2.0

1.0

2.0

mV

mV

Sensitivity

ΔV/ΔΡ

—

0.4

—

mV/kPa

Linearity

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

Pressure Hysteresis (0 to 100 kPa)

—

—

±0.1

—

Temperature Hysteresis (-40°C to +125°C)

—

—

±0.5

—

Temperature Coefficient of Full Scale Span

-2.0

—

2.0

Temperature Coefficient of Offset

-1.0

—

1.0

mV

Input Impedance

1000

—

2500

W

Output Impedance

1400

—

3000

W

Response Time (10% to 90%)

—

1.0

—

ms

Warm-Up Time

—

—

20

—

ms

Offset Stability

—

—

±0.5

—