MPXM2102AS datasheet(425/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1322

Applying Semiconductor Sensors to

Bar Graph Pressure Gauges

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

µA • 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

µA. 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

µA. 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

µA that flows into R1.

The output voltage is then 4.05 V + (91

µA • 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.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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