MPXM2102AS datasheet(384/670 Pages) MOTOROLA

384 / 670 page

AN1309

3–238

Motorola Sensor Device Data

www.motorola.com/semiconductors

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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