MPXM2102AS datasheet(492/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1556

Designing Sensor Performance

Specifications for MCU-based Systems

Prepared by: Eric Jacobsen and Jeff Baum

Sensor Systems Engineering Group

Motorola Sensor Products Division

Phoenix, AZ

INTRODUCTION

When designing a circuit for a sensor system, it is desirable

to use fixed–value components in the design. This makes the

system easier and cheaper to produce in high volume. The

alternatives to using fixed–value circuitry are very expensive

and usually impractical: laser–trimming resistances, manually

calibrating potentiometers, or measuring and selecting

specific component values are all very labor–intensive

processes. However, every sensor has device–to–device

variations in offset output voltage, full–scale output voltage,

dynamic output voltage range (difference between the

full–scale output voltage and zero–scale output voltage which

is commonly referred to as the span), etc. Moreover, these

same parameters also vary with temperature — e.g.,

temperature coefficient of offset (TCVoff) and temperature

coefficient of full–scale span (TCVFSS). To further complicate

this situation, the fixed–value circuit in which a sensor is

applied also has variation — e.g., the voltage or current

regulator and resistors all have a specified tolerance.

Since today’s unamplified solid–state sensors typically

have an output voltage on the order of tens of millivolts

(Motorola’s basic 10 kPa pressure sensor, MPX10, has a

typical full–scale span of 58 mV, when powered with a 5 V

supply), a major part of the fixed–value circuitry is a gain stage

that amplifies the signal to a level that is large enough for

additional processing. Typically, this additional processing is

digitization of the amplified analog sensor signal by a

microcontroller’s A/D converter. To obtain the best signal

resolution with an A/D, the sensor’s amplified dynamic output

voltage range should fill as much of the A/D window (difference

between the A/D’s high and low reference voltages) as

possible without extending beyond the high and low reference

voltages (i.e., the zero–pressure offset voltage must be

greater than or equal to the low reference voltage, and the

full–scale output voltage must be less than or equal to the high

reference voltage). In any case, the device–to–device,

temperature, and circuit variations create a design dilemma:

with a fixed–value amplifier circuit, the gain as well as any dc

level shift incorporated in the amplifier design are fixed. If the

variation of any of the aforementioned sensor parameters is

too large, the amplified sensor output may saturate the

amplifier near either its high or low supply rail or may extend

beyond either the high or low reference voltages of the A/D

converter. In either case, error (non–linearity) results in the

system. To avoid this scenario, the solution is to design a

fixed–value circuit that optimizes performance (signal

resolution) while taking into account all possible types of

variation that may cause the sensor output to vary. In other

words, the goal of this fixed–value sensor system is to attain

the best performance possible while ensuring through design,

regardless of any system variation, that the sensor’s amplified

output will ALWAYS be within the saturation levels of the

amplifier and the high and low reference voltages of an A/D

converter.

The implication of ensuring that the sensor’s amplified

output is always unsaturated and within the high and low

reference voltages of the A/D is that an accurate software

calibration of the sensor’s output is possible. By sampling the

sensor’s output voltage at a couple of points at room

temperature (zero and full–scale output, for example), all the

room temperature device–to–device and circuit variations are

nullified. Obviously, temperature variations will create error in

the system (sensor’s output voltage will drift with changing

temperature), but, by design, the sensor’s output voltage will

remain within the A/D’s valid range.

This paper discusses a methodology that optimizes a

sensor

system’s

performance

while

considering

device–to–device, temperature, and circuit variations that can

create variation in the amplified sensor output. The

methodology starts with a desired performance and some

established parameters and then considers each type of

variation in a worst case analysis to determine if the desired

performance is attainable. While this paper discusses this

methodology for pressure sensors and a specific amplifier

topology, the methodology is applicable to low–level,

differential–voltage output sensors and amplifier circuits in

general. Two specific examples are presented that apply this

methodology. The first example uses Motorola’s MPX10

pressure sensor, and the second example uses Motorola’s

MPX2010 pressure sensor. Both sensors have a full–scale

rated pressure of 10 kPa; the difference between the devices

is the MPX2010 has on–chip calibration and temperature

compensation

circuitry

calibrate

and

temperature

compensate the zero–pressure offset voltage and span. The

comparison of these two devices will emphasize how

dramatically device–to–device and temperature variations, if

not compensated, can affect a system’s overall performance.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

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