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Motorola Sensor Device Data
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To see how the level translation works, let’s look at the
simplified schematic in Figure 5. Again assuming a common
mode voltage of 4.0 V, the voltage applied to pin 12 of U1D is
4.0 V, implying that pin 13 is also at 4.0 V. This leaves 4.0 V
– VOFFSET across R3, which is 3.5 V if VOFFSET is set to 0.5
V. Since no current flows into pin 13, the same current flows
through both R3 and R4. With both of these resistors set to the
same value, they have the same voltage drop, implying a 3.5
V drop across R4. Adding the voltages (0.5
+
3.5
+
3.5) yields
7.5 V at pin 14 of U1D. Similarly 4.0 V at pin 10 of U1C implies
4.0 V at pin 9, and the drop across R2 is 7.5 V – 4.0 V = 3.5
V. Again 3.5 V across R2 implies an equal drop across R1, and
the voltage at pin 8 is 4.0 V – 3.5 V = .5 V. For this DC output
voltage to be independent of the sensor’s common mode
voltage it is necessary to satisfy the condition that R4/R3 =
R2/R1. In Figure 4, VOFFSET is produced by R8 and
adjustment pot R9. R3’s value is adjusted such that the total
source impedance into pin 13 is approximately 1 k.
Figure 5. Simplified Sensor Specific Interface
B+
GND
U1A
MC33274
12
13
14
8
U1B
MC33274
R5
120*
3
2
4
1
R4 1 k
U1C
MC33274
10
9
6
5
7
2
3
4
1
XDCR1
MPX2000 SERIES
PRESSURE
SENSOR
OUTPUT
11
R6 7.5 k
+
–
+
–
+
–
–
+
*NOTE: FOR MPX2010, R5 = 75 OHMS
U1D
MC33274
R3
1 k
R1 2 k
R2 2 k
V
+8
OFFSET
Gain is approximately (R6/R5)(R1/R2
+
1), which is 125 for
the values shown in Figure 4. A gain of 125 is selected to
provide a 4 V span for the 32 mV of full scale sensor output that
is obtained with 8 V B
+
.
The resulting 0.5 V to 4.5 V output from U1C is preferable
to the 0.75 to 4.75 V range developed by the instrument
amplifier configuration in Figure 2. It also uses fewer parts.
This circuit does not have the instrument amplifier’s
propensity for oscillation and therefore does not require
compensation capacitor C3 that is shown in Figure 2. It also
requires one less resistor, which in addition to reducing
component count also reduces accumulated tolerances due
to resistor variations.
This circuit as well as the instrumentation amplifier interfaces
in Figures 2 and 3 is designed for direct connection to a
microcomputer A/D input. Using the MC68HC11 as an
example, the interface circuit output is connected to any of the
E ports, such as port E0 as shown in Figure 6. To get maximum
accuracy from the A/D conversion, VREFH is tied to 4.85 V and
VREFL is tied to 0.30 V by dividing down a 5 V reference with
1% resistors.
SINGLE SLOPE A/D CONVERTER
The 8 bit A/D converters that are commonly available on
chip in microcomputers are usually well suited to pressure
sensing applications. In applications that require more than 8
bits, the circuit in Figure 7 extends resolution to 11 bits with an
external analog–to–digital converter. It also provides an
interface to digital systems that do not have an internal A/D
function.
F
Freescale Semiconductor, Inc.
n
.