MPXM2102AS datasheet(468/670 Pages) MOTOROLA

468 / 670 page

AN1525

3–322

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 5. Two Op–Amp Signal–Conditioning Stage with Variable Gain and Negative Dc Level Shift Adjust

VIN2

VIN1

VREF

NODE 1

VCC

NODE 2

′

ROFF

To derive the general transfer function, nodal analysis and

superposition are used:

VO =

)

) 1 VIN2

–

)

VIN1

VREF +

OFF

(VIN2 – VCC) (14)

As before, defining the sensor’s differential output as

VSENSOR, defining VIN2 as S+ for pressure sensor

applications, and using the simplification that

R1 = R4

and

R2 = R3

obtains the following simplified transfer function:

VO =

)

) 1 (VSENSOR) + VREF

OFF

(S+ – VCC)

(15)

The gain is

G =

+ 1

(16)

To adjust the gain, refer to the guidelines presented in the

section on Two Op–Amp Gain Stage with Variable Gain.

VREF is the positive dc level shift, and the negative dc level

shift is:

V–shift =

OFF

(S+ – VCC)

(17)

The following guidelines will help design the circuitry for the

negative dc voltage level shift:

• To establish a stable negative dc level shift, VCC should be

regulated; otherwise, the amount of negative level shift will

vary as VCC varies.

• ROFF should be the only resistor varied to adjust the

negative level shift. Varying R4 will change the gain of the

two op–amp circuit and reduce the common mode rejec-

tion.

• To determine the value of ROFF:

1. Determine the amount of negative dc level shifting re-

quired (defined here as V–shift).

2. R4 already should have been determined to set the gain

for the desired signal–conditioned sensor output.

3. Although V–shift is dependent on S+, S+ changes only

slightly over the entire pressure range. With Motorola’s

MPX10 powered at a 5 V supply, S+ will have a value of

approximately 2.51 V at zero pressure and will increase

as high as 2.53 V at full–scale pressure. This error over

the full–scale pressure span of the device is negligible

when considering that many applications use an 8–bit

A/D converter to segment the pressure range. Using an

8–bit A/D, the 20 mV (0.02 V) error corresponds to only

1 bit of error over the entire pressure range (1 bit / 255

bits x 100% = 0.4% error).

4. ROFF is then calculated by the following equation:

ROFF =

S+ –V

–

shift

(18)

An alternative to using this equation is to use a

potentiometer for ROFF that has a resistance range on the

order of R4 (perhaps 1 to 5 times the value of R4). Use the

potentiometer to fine tune the negative dc level shift, while

monitoring the zero pressure offset output voltage, VO. As

before, if a fixed resistor is preferable, then measure the

potentiometer’s resistance and replace the potentiometer

with the closest 1% resistor value.

Important note: The common mode rejection of this amplifier

topology will be low and perhaps unacceptable in some

applications. (A SPICE model of this amplifier topology

showed the common mode rejection to be 28 dB.) However,

this circuit is presented as a solution for applications where

only two operational amplifiers are available and the common

mode rejection is not critical when considering the required

Freescale Semiconductor, Inc.

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Go to: www.freescale.com

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