MPXM2102AS datasheet(472/670 Pages) MOTOROLA

472 / 670 page

AN1536

3–326

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 1 shows the pressure sensor (XDCR) connected to

the analog switches of the 74HC4053 which feeds the

differential signal to the first stage of op–amps. An A/D

conversion is performed on the two op–amp output signals,

Vout1 and Vout2. The difference (Vout1 – Vout2) is com-

puted and stored in microcontroller memory. The analog

switch commutates (op–amp connections switch from Y0 and

Z0 to Y1 and Z1), reversing the sensor output signals to the two

op–amps, and another conversion is performed. This value is

then also stored in the microcontroller memory. To summarize,

via software, the following computation takes place:

Step 1: Vfirst = Vout1 – Vout2

Step 2: Vsecond = Vout2 – Vout1

Step 3: Vresult = (Vfirst + Vsecond) / 2

Again, because any op–amp offset will remain the same

polarity regardless of sensor output polarity, this routine will

effectively cancel any amplifier offset. Any offset the sensor

may introduce is compensated for by software routines that

are invoked when the initial system calibration is done.

The single slope A/D provides 11 or more unsigned bits of

resolution. This capability provides a water pressure

resolution to at least 0.05 psi. This translates to a boat speed

resolution of 0.1 mph over the entire speed range.

Figure 2 describes the pressure versus voltage transfer

function of the first op–amp stage.

Figure 1. X–ducer, Instrument Amplifier and Analog Switch

–

33078

–

33078

MPXM2202GS

74HC4053

–

616

VOUT1

VOUT2

DENOTES ANALOG GROUND

DENOTES LOGIC GROUND

XDCR INPUT REVERSE CONTROL

10 k

22 pF

316 k

10 k

Figure 2. Instrument Amplifier Transfer Function

PSI

PRESSURE IN

U3–1 (U3–7)

U3–7 (U3–1)

Figure 3 details the analog circuitry, microcontroller’s timer

capture registers and I/O port which comprise the single slope

A/D. The microcontroller’s 16–bit free running counter is also

employed, but not shown in the figure.

Comparators U6A, U6B and U6D of the LM139A are used

to provide the A/D function. Constant current source, U7,

resistors R13 and R14 and diode D2 provide a linear voltage

ramp to the inverting inputs of U6, with about 470 microamps

charge current to capacitor C8, with transistor Q1 in the off

state. C8 will charge to 5 volts in about 5 milliseconds at the

given current. Q1 is turned on to provide a discharge path for

C8 when required. The circuit is designed such that when the

voltage to the inverting inputs of the comparators exceeds the

voltage to the noninverting comparators, each comparator

output will trip from a logic 1 to a logic 0.

One A/D conversion consists of the following steps:

(1) setting the pressure sensor output polarity (via software

and the analog switches of U4) to the amplifier inputs of the

MC33078 (U3), (2) reading the value of the free running

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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