71128 datasheet(3/7 Pages) VISHAY

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AN731

Vishay Siliconix

Document Number: 71128

28-Jan-00

www.vishay.com

S FaxBack 408-970-5600

To limit the common mode range of VX, the following equations

can be used:

1. To keep VX equal or greater than a minimum value, VXm=

1 V:

Vr2

Vxm(Vo2–Vr–aVc2)

) aVc2Vr

Vo2

) a(Vr–Vxm)–Vr

1(3.4–1.3–1.2

2.7)

) 1.2

2.7

1.3

3.4

) 1.2(1.3–1)–1.3

+ 1.249

(12)

2. To keep VX equal or less than a maximum value, VXm =

3 V:

Vr2

VxM(Vo2–Vr–aVc2)

) aVc1Vr

Vo2

) a(Vr–VxM–Vc2 ) Vc1)–Vr

2(3.4–1.3–1.2

2.7)

) 1.2

0.2

1.3

3.4

) 1.2(1.3–2–2.7 ) 0.2)–1.3

+ 1.13 V

(13)

So, Vr2 can be any value between 1.249 V and 1.3 V. Choose

Vr2 = 1.25 V.

Calculate m1 and then R2:

Vr–Vr2

Vo2

) a(Vr2–Vc2)–Vr

1.3–1.25

3.4

) 1.2(1.25–2.7)–1.3

+ 0.139

+ R2

å R2 + m1R1 + 0.139

22.1 k

+ 3.07 kW

(14)

Choose R2 = 3.01 k

W as a practical value

Calculate R3 and R4:

+ m2

m1 +

m1R4

å R3 + am1R4

(15)

Choose R4, then calculate R3. The value of R4 should be large

enough such that the current going through it will not be so

large as to cause excessive power dissipation under extreme

conditions. On the other hand, R4 should be small enough that

its current will not be overly sensitive to noise and op-amp bias

current.

If R4 is set at 22.1k, then R3 = 3.68 k, and we can choose R3

= 3.60 k as a practical value.

Express VO as a function of VC, using practical values of R’s:

+ m2

m1 +

R1R3

R2R4

+ 22.1kx3.68k

3.01kx22.1k

+ 1.223

+ 1

m1 )

1 Vr– 1

m1 )

a Vr2

) 1 Vr– R1

) a Vr2

+ 22.1k

3.01k

) 1 1.3– 22.1k

3.01k

) 1.223 1.25 + 0.1389

(16)

The final result: VO = aVC + b = 1.223 x VC + 0.1389

EXPERIMENTAL RESULTS

A circuit was built and tested (see Figure 5). The result is

tabulated in Table 1 and plotted in Figure 4:

TABLE 1

Measured

Calculated

Measured

Required

0.1

1.43

0.26

0.32

0.2

1.42

0.38

0.42

0.40

0.4

1.38

0.63

0.68

0.64

0.8

1.32

1.12

1.16

1.12

1.2

1.25

1.61

1.66

1.60

1.6

1.19

2.10

2.12

2.08

2.0

1.11

2.58

2.68

2.56

2.4

1.05

3.07

3.11

3.04

2.6

1.02

3.32

3.33

3.28

2.7

1.00

3.44

3.46

3.40

2.8

0.99

3.56

3.58

3.0

0.96

3.81

3.78

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.5

1.0

1.5

2.0

2.5

3.0

Control Voltage—VC (v)

FIGURE 4.

VX Measured

The measured values are very much in agreement with the

calculated and required values. A negligible error results from

the difference between an ideal op-amp and the actual circuit

with its finite offset voltage and bias current.

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