MPXM2102AS datasheet(469/670 Pages) MOTOROLA

469 / 670 page

AN1525

3–323

Motorola Sensor Device Data

www.motorola.com/semiconductors

system performance. Adding a third op–amp to the circuit for

the negative dc level shifting capability (as shown in the next

section) is a solution that provides good common mode

rejection, but at the expense of adding an additional op–amp.

THE THREE OP–AMP GAIN STAGE

FOR NEGATIVE DC LEVEL SHIFTING

This circuit adds a third op–amp to the output of the two

op–amp gain block (see Figure 6). This op–amp has a dual

function in the overall amplifier circuit:

• Its non–inverting configuration provides gain via the ratio

of R6 and R5.

• It has negative dc voltage level shifting capability typically

created by a resistor divider at V–shift, as discussed in the

section on Application to Pressure Sensor Circuits. Al-

though this configuration requires a third op–amp for the

negative dc level shift, it has no intrinsic error nor low com-

mon mode rejection associated with the negative level shift

(as does the previous two op–amp stage). Depending on

the application’s accuracy requirement, this may be a more

desirable configuration for providing the negative dc level

shift.

First, use the same simplifications 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 )

)

) 1 V

SENSOR

+ VREF –

V–shift

(20)

The gain is

G = 1 )

)

) 1

(21)

VREF is the positive dc level shift (offset), and V–shift is the

negative dc level shift.

Figure 6. Three Op–Amp Gain Stage with Variable Gain and Negative Dc Level Shift

VIN2

VIN1

VREF

VCC

′

″

V–SHIFT

The transfer function for this stage will be similar to the

chosen two op–amp gain stage configuration (either the fixed

gain with positive dc level shift circuit or the variable gain with

positive dc level shift circuit) with additional terms for the

negative level shift and gain. As an example, the variable–gain

two op–amp gain circuit is used here. All of the design

considerations and explanations for the variable gain two

op–amp circuit apply.

The transfer function may be derived with nodal analysis

and superposition.

VO = 1 )

)

) 1 VIN2

–

)

VIN1

REF

–

V–shift

(19)

The preceding simplifications have been performed in the

previous sections, but by examining Equation 20, notice that

the third op–amp’s gain term also amplifies the positive and

negative dc voltage level shifts, VREF and V–shift. If R6and R5

are chosen to make an arbitrary contribution to the overall

system gain, designing an appropriate amount of positive and

negative dc level shift can be difficult. To simplify the transfer

function, set R5 = R6, and the following equation for VO results:

VO = 2

)

) 1 V

SENSOR )

REF

– V–shift

(22)

Now the third op–amp’s contribution to the overall system

gain is a factor of two. When designing the overall system gain

and the positive dc level shift, use the following guidelines:

Freescale Semiconductor, Inc.

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