MPXM2102AS datasheet(538/670 Pages) MOTOROLA

538 / 670 page

AN1668

3–392

Motorola Sensor Device Data

www.motorola.com/semiconductors

AMPLIFIER SELECTION AND

AMPLIFIER INDUCED ERRORS

A differential amplifier is needed to convert the differential

output of the MPX2010 sensor to a high level ground–

referenced (single–ended). The classic three–op amp instru-

mentation amplifier can be used. However, it requires additional

components (3 op–amps and possibly a split power supply). An

instrumentation topology shown in Figure 1 requires only a

single supply and only 2 op–amps and 1% resistors.

Figure 1. MPX2010 Amplifier Circuit

VREF

+VCC

R+S2

R+S1

–

U1B

VOUT_FS

+VCC

S+

PRESSURE SENSOR

U1A

The circuit uses a voltage divider R+S1 and R+S2 to provide

the reference (level shift), U1A and U1B are non–inverting

amplifiers arranged in a differential configuration with gain

resistors R1, R2, R3 and R4. Note that U1B is the main gain

stage and it has the most gain. It is recommended to place a

0.015

µF capacitor in it’s feedback loop (in parallel with R4) to

reduce noise. The amplifier output can be characterized with

the equation below:

(4)

Gain

+ R4

) 1

(5)

Voffset

+ VREF R2·R1

R1·R3

* VSCM R2·R4

R1·R3

* 1

(6)

Vout

+ (S)* S*) Gain ) Voffset

(7)

where (S

)* S*) + Sensor differential output

) Sensor offset

Equation 4 is the differential gain of the amplifier and

equation 5 is the resulting offset voltage of the amplifier.

The above equations assume that the amplifier is close to

ideal (high AOL , low input offset voltage and low input offset

bias currents). Since an ideal op–amp is hard to come by, the

customer should select an op–amp based on cost and perfor-

mance. Below are some points to keep in mind when selecting

an op–amp and designing the amplifier circuit.

Note that the ratio R2*R4/R1*R3 controls the system offset

as well as the common mode error of the amplifier. Mis-

matches in these resistors will result in an offset and common

mode error which appear as offset. It is therefore recom-

mended to use 1% metal film resistors to reduce these errors.

Also, Vref source impedance should be minimized in compari-

son with R1 in order to reduce common mode error.

Amplifier input offset and input bias currents can induce

errors. For example, an input offset (Vio) of the amplifier can

become significant when the closed–loop gain of the amplifier

is increased. Furthermore, there is also a temperature coeffi-

cient of the input voltage offset which contribute an additional

error across temperature. If the input bias current of the ampli-

fier is not taken into account in the design, it can also become

a source of error. A technique to reduce this error is to match

the impedance the source impedance of what the op–amp

input pins sees.

It is important to note that high performance op–amps are

more expensive. An MC33272 op–amp has a low input offset

and low input bias current which is suitable for the two–op amp

amplifier design. We can see that there is a tradeoff between

accuracy and cost when designing a solution with the

MPX2010.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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