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Motorola Sensor Device Data
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Prepared by: Warren Schultz
Discrete Applications Engineering
INTRODUCTION
Bar Graph displays are noted for their ability to very quickly
convey a relative sense of how much of something is present.
They are particularly useful in process monitoring applications
where quick communication of a relative value is more
important than providing specific data.
Designing bar graph pressure gauges based upon
semiconductor pressure sensors is relatively straightforward.
The sensors can be interfaced to bar graph display drive IC’s,
microcomputers and MC33161 voltage monitors. Design
examples for all three types are included.
BAR GRAPH DISPLAY DRIVER
Interfacing semiconductor pressure sensors to a bar graph
display IC such as an LM3914 is very similar to microcomputer
interface. The same 0.5 to 4.5 V analog signal that a
microcomputer’s A/D converter wants to see is also quite
suitable for driving an LM3914. In Figure 1, this interface is
provided by dual op amp U2 and several resistors.
The op amp interface amplifies and level shifts the sensor’s
output. To see how this amplifier works, simplify it by
grounding the output of voltage divider R3, R5. If the common
mode voltage at pins 2 and 4 of the sensor is 4.0 V, then pin
2 of U2A and pin 6 of U2B are also at 4.0 V. This puts 4.0 V
across R6. Assuming that the current in R4 is equal to the
current in R6, 323
μ
A
100 ohms produces a 32 mV drop
across R4 which adds to the 4.0 V at pin 2. The output voltage
at pin 1 of U2A is, therefore, 4.032 V. This puts 4.032 – 4.0 V
across R2, producing 43
μ
A. The same current flowing
through R1 again produces a voltage drop of 4.0 V, which sets
the output at zero. Substituting a divider output greater than
zero into this calculation reveals that the zero pressure output
voltage is equal to the output voltage of divider R3, R5. For this
DC output voltage to be independent of the sensor’s common
mode voltage, it is necessary to satisfy the condition that
R1/R2 = R6/R4.
Gain can be determined by assuming a differential output
at the sensor and going through the same calculation. To do
this assume 100 mV of differential output, which puts pin 2 of
U2A at 3.95 V, and pin 6 of U2B at 4.05 V. Therefore, 3.95 V is
applied to R6, generating 319
μ
A. This current flowing through R4
produces 31.9 mV, placing pin 1 of U2A at 3950 mV + 31.9 mV
= 3982 mV. The voltage across R2 is then 4050 mV – 3982 mV
= 68 mV, which produces a current of 91
μ
A that flows into R1.
The output voltage is then 4.05 V + (91
μ
A
93.1k) = 12.5 V.
Dividing 12.5 V by the 100 mV input yields a gain of 125, which
provides a 4.0 V span for 32 mV of full scale sensor output.
Setting divider R3, R5 at 0.5 V results in a 0.5 V to 4.5 V
output that is easily tied to an LM3914. The block diagram that
appears in Figure 2 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 V
zero pressure output voltage. Returning to Figure 1, this is
accomplished by using the zero pressure offset voltage that
is generated at the output of divider R3, R5.
Again looking at Figure 1, full scale 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 R7, R9, and
adjustment pot R8.
Eight volt regulated power is supplied by an MC78L08. 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 R7, R8, and R9 to ground. In this design it is
nominally (4.5 V/4.9 k)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%).
This circuit can be simplified by substituting an MPX5100
integrated sensor for the MPX2100 and the op amp interface.
The resulting schematic is shown in Figure 3. In this case zero
reference for the bar graph is provided by dividing down the
5 V regulator with R4, R1 and adjustment pot R6. The voltage
at the wiper of R6 is adjusted to match the sensor’s zero
pressure offset voltage. It is connected to RLO to zero the bar
graph.
SEMICONDUCTOR APPLICATION NOTE
REV 1
F
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