MPXM2102AS datasheet(467/670 Pages) MOTOROLA

467 / 670 page

AN1525

3–321

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 3. A Resistor Divider to Create VREF

VREF

TO U1

VCC

RREF1

RREF2

THE TWO OP–AMP GAIN STAGE WITH

VARIABLE GAIN

Varying the gain of the two op–amp stage is desirable for

fine–tuning the sensor’s signal–conditioned output span.

However, to adjust the gain in the two op–amp gain circuit in

Figure 2 and to simultaneously preserve the common mode

rejection, two resistors must be adjusted. To adjust the gain,

it is more desirable to change one resistor. By adding an

additional feedback resistor, RG, the gain can be adjusted with

this one resistor while preserving the common mode rejection.

Figure 4 shows the two op–amp gain stage with the added

resistor, RG.

Figure 4. Two Operational–Amplifier Gain Stage with

Variable Gain

VIN2

VIN1

VREF

NODE 1

VCC

NODE 2

VO′

As with the two op–amp gain stage, nodal analysis and

superposition are used to derive the general transfer function

for the variable gain stage.

VO =

)

) 1 VIN2

–

)

VIN1

VREF

(11)

This general transfer function also is quite cumbersome

and is susceptible to producing poor common mode rejection

without additional constraints on the resistor values. To obtain

good common mode rejection, use a similar simplification as

before; that is, set

R1 = R4

and

R2 = R3

Defining the voltage differential between VIN2 and VIN1 as

VSENSOR, the simplified transfer function is

VO =

)

) 1 (VSENSOR) + VREF (12)

Thus, the gain is

G =

+ 1

(13)

and VREF is the positive dc level shift (offset).

Use the following guidelines when determining the value for

RG:

• By examining the gain equation, RG’s resistance should

be comparable to R4’s resistance. This will allow fine tun-

ing of the gain established by R4 and R3. If RG is too large

(e.g., RG approaches ∞), it will have a negligible effect on

the gain. If RG is too small (e.g., RG approaches zero), the

RG term will dominate the gain expression, thus prohibit-

ing fine adjustment of the gain established via the ratio of

R4 and R3.

• Use a potentiometer for RG that has a resistance range on

the order of R4 (perhaps with a maximum resistance equal

to the value of R4). If a fixed resistor is preferable to a poten-

tiometer, use the potentiometer to adjust the gain, measure

the potentiometer’s resistance, and replace the potentiom-

eter with the closest 1% resistor value.

• To maintain good common mode rejection while varying

the gain, RG should be the only resistor that is varied. RG

equally modifies both of the resistor ratios which need to be

well–matched for good common mode rejection, thus pre-

serving the common mode rejection.

THE TWO OP–AMP GAIN STAGE WITH VARIABLE

GAIN AND NEGATIVE DC LEVEL SHIFT

The last two op–amp circuits both incorporate positive dc

level shift capability. Recall that a positive dc level shift is

required to keep the operational amplifiers from saturating

near the low rail of the supply or to keep the zero pressure

offset above (or equal to) the low reference voltage of an A/D.

This two op–amp stage incorporates an additional resistor,

ROFF, to provide a negative dc level shift. A negative dc level

shift is useful when the zero pressure offset voltage of the

sensor is too high. In this case, the user may be required to

level shift the zero pressure offset voltage down (toward zero

volts). Now, for a specified amount of gain, the full–scale

pressure output voltage does not saturate the amplifier at the

high rail of the voltage supply, nor is it greater than the A/D’s

high reference voltage. Figure 5 shows the schematic for this

amplifier circuit.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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