AN1979

Sensors

3

Freescale Semiconductor

Figure 3. Amplification Scheme

The range of the A/D converter is 0 to 255 counts. However,

the A/D values that the system can achieve are dependent on

the maximum and minimum system output values:

Count (64 kPa) = (0.03 – 0.0) / (5.0 – 0.0) x 255 = 2

Count (105 kPa) = (4.85 – 0.0) / (5.0 – 0.0) x 255 = 247

Total # counts = 247 – 2 = 245 counts.

The resolution of the system is determined by the

barometric pressure represented by each A/D count. As

calculated above, the system has a span of 247 counts to

represent a pressure from 64 kPa to 105 kPa. Therefore, the

resolution is:

Resolution = (System Pressure Range) /

Total # counts (5)

= (105 kPa — 64 kPa)/245 counts

= 0.17 kPa per A/D count

Microprocessor

To provide the signal processing for pressure values, a

microprocessor is needed. The MCU chosen for this

application is the MC68HC908QT4. This MCU is perfect

for appliance applications due to its low cost, small eight-pin

package, and other on-chip resources. The MC68HC908QT4

provide: a four-channel, eight-bit A/D, a 16-bit timer, a

trimmable internal timer, and in-system FLASH programming.

The central processing unit is based on the high

performance M68HC08 CPU core and it can address 64

Kbytes of memory space. The MC68HC908QT4 provides

4096 bytes of user FLASH and 128 bytes of random access

memory (RAM) for ease of software development and

maintenance. There are five bi-directional input/output lines

and one input line that are shared with other pin features.

The MCU is available in eight-pin as well as 16-pin

packages in both PDIP and SOIC. For this application, the

eight-pin PDIP was selected. The eight-pin PDIP was chosen

for a small package, eventually to be designed into

applications as the eight-pin SOIC. If added circuitry for

programming the microcontroller is added, a cyclone could be

used to program an SOIC on the PCB. If your design requires

software updates, consult the MC68HC908QT4 handbook for

adding this option.

IMPROVEMENTS

The resolution of this design is limited by the eight-bit A/D

converter on the microcontroller. Theoretically, the accuracy

achieved by this device should produce an output when

altitude change differs by about 41.54 feet (∆Z). This occurs at

approximately 1000 feet below sea level. Due to the

logarithmic relationship between pressure and elevation, the

accuracy of the results decreases as the device is elevated. At

12,000 feet above sea level, the device should recognize a

change of about 65.53 feet (∆Z) as shown in Table 3. A 10-bit,

12-bit or even a 16-bit A/D converter could be implemented in

order to increase the resolution of this reference design.

R8

10 Ω

R6

4.42 K

–

+

6

7

5

1

2

3

4

MPXM2102A

R7

10 Ω

R5

4.42 K

–

+

2

1

3

–

+

R11

1 K

R9

1 K

R12

412 K

R10

412 K

13

12

4

14

11

C5

0.1 µf

9

10

8

–

+