5–7

IL300

Table 1. R5 selection

The last step in the design is selecting the LED current limiting

resistor (R4). The output of the operational amplifier is targeted

to be 50% of the Vcc, or 2.5 V. With an LED quiescent current of

tional output voltage, R4 can be calculated.

.

The circuit was constructed with an LM201 differential opera-

tional amplifier using the resistors selected. The amplifier was

compensated with a 100 pF capacitor connected between pins

1 and 8.

The DC transfer charateristics are shown in Figure 19. The

amplifier was designed to have a gain of 0.6 and was mea-

sured to be 0.6036. Greater accurracy can be achieved by

adding a balancing circuit, and potentiometer in the input

divider, or at R5. The circuit shows exceptionally good gain lin-

earity with an RMS error of only 0.0133% over the input voltage

range of 4 V–6 V in a servo mode; see Figure 20.

Figure 19. Transfer gain

Bins

Min.

Max.

K3

Typ.

R5

Resistor

K

Ω

1%

K

Ω

A

0.560

0.623

0.59

50.85

51.1

B

0.623

0.693

0.66

45.45

45.3

C

0.693

0.769

0.73

41.1

41.2

D

0.769

0.855

0.81

37.04

37.4

E

0.855

0.950

0.93

32.26

32.4

F

0.950

1.056

1.00

30.00

30.0

G

1.056

1.175

1.11

27.03

27.0

H

1.175

1.304

1.24

24.19

24.0

I

1.304

1.449

1.37

21.90

22.0

J

1.449

1.610

1.53

19.61

19.4

R4

V

opamp

V

F

–

I

Fq

--------------------------------

2.5V

1.3V

–

12mA

------------------------------

100

Ω

=

=

=

6.0

5.5

5.0

4.5

4.0

2.25

2.50

2.75

3.00

3.25

3.50

3.75

Vin - Input Voltage - V

Vout = 14.4 mV + 0.6036 x Vin

LM 201

Ta = 25

°C

Figure 20. Linearity error vs. input voltage

The AC characteristics are also quite impressive offering a -3

dB bandwidth of 100 KHz, with a -45

° phase shift at 80 KHz as

shown in Figure 21.

Figure 21. Amplitude and phase power supply control

The same procedure can be used to design isolation amplifiers

that accept biploar signals referenced to ground. These amplifi-

ers circuit configurations are shown in Figure 22. In order for the

amplifier to respond to a signal that swings above and below

ground, the LED must be prebiased from a separate source by

using a voltage reference source (Vref1). In these designs, R3

can be determined by the following equation.

6.0

5.5

5.0

4.5

4.0

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

Vin - Input Voltage - V

LM201

10 3

10 4

10 5

10 6

-8

-6

-4

-2

0

2

-180

-135

-90

-45

0

45

dB

PHASE

F - Frequency - Hz

R3

V

ref1

I

P1

-------------

V

ref1

K1I

Fq

---------------

=

=