3–291
Motorola Sensor Device Data
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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.
V
out
R
2
R
1
1
*
V
V
off
V is the differential output of the sensor.
The gain of 187 can be implemented with:
R1
≈
R3 = 121
R2
≈
R4 = 22.6 k
.
Choosing Roffset1 to be 1 k
and Roffset2 to be 2.5 k
,
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.
IN
GROUND
OUT
–
+
3
1
4
–
+
+12 V
U1
MC78L08ACP
VS = 8 V
U2B
MC33272
Vout
5.1 V
ZENER
R4
22.6 k
C1
0.33
μ
F
C2
0.33
μ
F
MPXM2102AS
Roffset1
1 k
Roffset2
2.5 k
R2
22.6 k W
R1
121
U2A
MC33272
R3
121
1
S –
S+
1
2
2
1
2
1
2
1
2
Figure 3. Signal Conditioning Circuit
2
1
2
F
Freescale Semiconductor, Inc.
n
.