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Motorola Sensor Device Data
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DESIGN CONSIDERATIONS
In this type of application the design challenge is how to take
a relatively small DC coupled differential signal and produce
a ground referenced output that is suitable for driving
microcomputer A/D inputs. A user friendly interface circuit that
will do this job is shown in Figure 3. It uses one quad op amp
and several resistors to amplify and level shift the sensor’s
output. Most of the amplification is done in U2D which is
configured as a differential amplifier. It is isolated from the
sensor’s positive output by U2B. The purpose of U2B is to
prevent feedback current that flows through R3 and R4 from
flowing into the sensor. At zero pressure the voltage from pin
2 to pin 4 on the sensor is zero volts. For example with the
common mode voltage at 2.5 volts, the zero pressure output
voltage at pin 14 of U2D is then 2.5 volts, since any other
voltage would be coupled back to pin 13 via R3 and create a
nonzero bias across U2D’s differential inputs. This 2.5 volt
zero pressure DC output voltage is then level translated to the
desired zero pressure offset voltage (VOFFSET) by U2C and
U2A. To see how the level translation works, assume 0.5 volts
at (VOFFSET). With 2.5 volts at pin 10, pin 9 is also at 2.5 volts.
This leaves 2.5 – 0.5 = 2.0 volts across R7. Since no current
flows into pin 9, the same current flows through R6, producing
2.0 volts across R6 also. Adding the voltages (0.5 + 2.0 +2.0)
yields 4.5 volts at pin 8. Similarly 2.5 volts at pin 3 implies 2.5
volts at pin 2, and the drop across R2 is 4.5 V – 2.5 V = 2.0
volts. Again 2.0 volts across R2 implies an equal drop across
R1, and the voltage at pin 1 is 2.5 V – 2.0 V = 0.5 volts. For this
DC output voltage to be independent of the sensor’s common
mode voltage it is necessary to satisfy the condition that
R6/R7 = R2/R1.
Gain is close but not exactly equal to R3/R4(R1/R2+1),
which predicts 200.0 for the values shown in Figure 3. A more
exact calculation can be performed by doing a nodal analysis,
which yields 199.9. Cascading the gains of U2D and U2A
using standard op amp gain equations does not give an exact
result, because the sensor’s negative going differential signal
at pin 4 subtracts from the DC level that is amplified by U2A.
The resulting 0.5 V to 4.5 V output from U2A is directly
compatible with microprocessor A/D inputs. Tying this output
to an LM3914 for a bar graph readout is also very straight
forward. The block diagram that appears in Figure 4 shows the
LM3914’s internal architecture. Since the lower resistor in the
input comparator chain is pinned out at RLO, it is a simple
matter to tie this pin to a voltage that is approximately equal
to the interface circuit’s 0.5 volt zero pressure output voltage.
In Figure 2, this is accomplished by dividing down the 5.0 volt
regulator’s output voltage through R13 and adjustment pot
R11. The voltage generated at R11’s wiper is the offset voltage
identified as VOFFSET in Figure 3. Its source impedance is
chosen to keep the total input impedance to U3C at
approximately 1K. The wiper of R11 is also fed into RLO for
zeroing the bar graph.
The full scale measurement is set by adjusting the upper
comparator’s reference voltage to match the sensor’s output
at full pressure. An internal regulator on the LM3914 sets this
voltage with the aid of resistors R2, R3, and adjustment pot R1
that are shown in Figure 2.
Five volt regulated power is supplied by an MC78L05. The
LED’s are powered directly from LM3914 outputs, which are
set up as current sources. Output current to each LED is
approximately 10 times the reference current that flows from
pin 7 through R3, R1, and R2 to ground. In this design it is
nominally (4.5 V/4.9K)10 = 9.2 mA.
Over a zero to 50
°
C temperature range combined accuracy
for the sensor, interface and driver IC are +/– 10%. Given a
10 segment display total accuracy for the bar graph readout
is approximately +/– (10 kPa +10%).
APPLICATION
Using the analog output to provide pressure information to
a microcomputer is very straightforward. The output voltage
range, which goes from 0.5 volts at zero pressure to 4.5 volts
at full scale, is designed to make optimum use of
microcomputer A/D inputs. A direct connection from the
evaluation board analog output to an A/D input is all that is
required. Using the MC68HC11 as an example, the output is
connected to any of the E ports, such as port E0 as shown in
Figure 5. To get maximum accuracy from the A/D conversion,
VREFH is tied to 4.85 volts and VREFL is tied to 0.3 volts by
dividing down a 5.0 volt reference with 1% resistors.
CONCLUSION
Perhaps the most noteworthy aspect to the bar graph
pressure gauge described here is the ease with which it can
be designed. The interface between an MPX2000 series
sensor and LM3914 bar graph display driver consists of one
quad op amp and a few resistors. The result is a simple and
inexpensive circuit that is capable of measuring pressure,
vacuum, or differential pressure with an output that is directly
compatible to a microprocessor.
F
Freescale Semiconductor, Inc.
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