3–326
Motorola Sensor Device Data
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Figure 1 shows the pressure sensor (XDCR) connected to
the analog switches of the 74HC4053 which feeds the
differential signal to the first stage of op–amps. An A/D
conversion is performed on the two op–amp output signals,
Vout1and Vout2. The difference (Vout1 – Vout2) is com-
puted and stored in microcontroller memory. The analog
switch commutates (op–amp connections switch from Y0 and
Z0 to Y1 and Z1), reversing the sensor output signals to the two
op–amps, and another conversion is performed. This value is
then also stored in the microcontroller memory. To summarize,
via software, the following computation takes place:
Step 1: Vfirst = Vout1 – Vout2
Step 2: Vsecond = Vout2 – Vout1
Step 3: Vresult = (Vfirst + Vsecond) / 2
Again, because any op–amp offset will remain the same
polarity regardless of sensor output polarity, this routine will
effectively cancel any amplifier offset. Any offset the sensor
may introduce is compensated for by software routines that
are invoked when the initial system calibration is done.
The single slope A/D provides 11 or more unsigned bits of
resolution. This capability provides a water pressure
resolution to at least 0.05 psi. This translates to a boat speed
resolution of 0.1 mph over the entire speed range.
Figure 2 describes the pressure versus voltage transfer
function of the first op–amp stage.
Figure 1. X–ducer, Instrument Amplifier and Analog Switch
+
–
33078
–
+
33078
MPXM2202GS
74HC4053
Y0
Y1
Z0
Z1
3
2
+
4
1
2
1
5
3
11
12
13
9
10
+8
–
8
7
6
16
+5
15
4
5
3
2
6
7
1
8
4
+8
VOUT1
VOUT2
DENOTES ANALOG GROUND
DENOTES LOGIC GROUND
XDCR INPUT REVERSE CONTROL
10 k
10 k
22 pF
22 pF
316 k
316 k
10 k
Figure 2. Instrument Amplifier Transfer Function
8
6
4
2
0
0
10
20
30
PSI
PRESSURE IN
U3–1 (U3–7)
V
U3–7 (U3–1)
Figure 3 details the analog circuitry, microcontroller’s timer
capture registers and I/O port which comprise the single slope
A/D. The microcontroller’s 16–bit free running counter is also
employed, but not shown in the figure.
Comparators U6A, U6B and U6D of the LM139A are used
to provide the A/D function. Constant current source, U7,
resistors R13 and R14 and diode D2 provide a linear voltage
ramp to the inverting inputs of U6, with about 470 microamps
charge current to capacitor C8, with transistor Q1 in the off
state. C8 will charge to 5 volts in about 5 milliseconds at the
given current. Q1 is turned on to provide a discharge path for
C8 when required. The circuit is designed such that when the
voltage to the inverting inputs of the comparators exceeds the
voltage to the noninverting comparators, each comparator
output will trip from a logic 1 to a logic 0.
One A/D conversion consists of the following steps:
(1) setting the pressure sensor output polarity (via software
and the analog switches of U4) to the amplifier inputs of the
MC33078 (U3), (2) reading the value of the free running
F
Freescale Semiconductor, Inc.
n
.