MPXM2102AS datasheet(418/670 Pages) MOTOROLA

418 / 670 page

AN1318

3–272

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

U1B

MC33274

120*

R4 1 k

U1C

MC33274

XDCR1

MPX2000 SERIES

PRESSURE

SENSOR

OUTPUT

R6 7.5 k

–

*NOTE: FOR MPX2010, R5 = 75 OHMS

U1D

MC33274

1 k

R1 2 k

R2 2 k

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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