MPXM2102AS datasheet(437/670 Pages) MOTOROLA

437 / 670 page

AN1326

3–291

Motorola Sensor Device Data

www.motorola.com/semiconductors

This gain is then used to determine the appropriate resistor

values and offset voltage for the amplifier circuit defined by the

transfer function shown below.

Vout +*

) 1

* D

) V

off

∆V is the differential output of the sensor.

The gain of 187 can be implemented with:

R1 ≈ R3 = 121 Ω

R2 ≈ R4 = 22.6 k Ω.

Choosing Roffset1tobe1kΩandRoffset2to be 2.5 k Ω,Vout

is 0.5 V at the presumed maximum barometric pressure of

30.5 in–Hg. The maximum pressure output voltage can be

trimmed to a value other than 0.5 V, if desired via Roffset1. In

addition, the trimmable offset resistor is incorporated to

provide offset calibration if significant offset drift results from

large weather fluctuations.

The circuit shown in Figure 3 employs an MC33272

(low–cost, low–drift) dual operational amplifier IC. In order to

control large supply voltage fluctuations, an 8 Vdc regulator,

MC78L08ACP, is used. This design permits use of a battery

for excitation.

Microcontroller Interface

The low cost of MCU devices has allowed for their use as

a signal processing tool in many applications. The MCU used

in this application, the MC68HC11, demonstrates the power of

incorporating intelligence into such systems. The on–chip

resources of the MC68HC11 include: an 8 channel, 8–bit A/D,

a 16–bit timer, an SPI (Serial Peripheral Interface –

synchronous), and SCI (Serial Communications Interface –

asynchronous), and a maximum of 40 I/O lines. This device is

available in several package configurations and product

variations which include additional RAM, EEPROM, and/or

I/O capability. The software used in this application was

developed using the MC68HC11 EVB development system.

The following software algorithm outlines the steps used to

perform the desired digital processing. This system will

convert the voltage at the A/D input into a digital value, convert

this measurement into inches of mercury, and output this data

serially to an LCD display interface (through the on–board

SPI). This process is outlined in greater detail below:

1. Set up and enable A/D converter and SPI interface.

2. Initialize memory locations, initialize variables.

3. Make A/D conversion, store result.

4. Convert digital value to inches of mercury.

5. Determine if conversion is in system range.

6a. Convert pressure into decimal display digits.

6b. Otherwise, display range error message.

7. Output result via SPI to LCD driver device.

The signal conditioned sensor output signal is connected to

pin PE5 (Port E–A/D Input pin). The MCU communicates to

the LCD display interface via the SPI protocol. A listing of the

assembly language source code to implement these tasks is

included in the appendix. In addition, the software can be

downloaded directly from the Motorola MCU Freeware

Bulletin Board (in the MCU directory). Further information is

included at the beginning of the appendix.

GROUND

OUT

–

+12 V

MC78L08ACP

VS = 8 V

U2B

MC33272

Vout

5.1 V

ZENER

22.6 k

Ω

0.33

µF

0.33

µF

MPXM2102AS

Roffset1

1 k

Ω

Roffset2

2.5 k

Ω

22.6 k W

121

Ω

U2A

MC33272

121

Ω

S –

S+

Figure 3. Signal Conditioning Circuit

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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