5

Application Information

Figures 4 and 5 show the equivalent circuits for the current

source and linearization diodes in the CA3280. The current

through the linearization network is approximately equal to the

programming current. There are several advantages to driving

these diodes with a current source. First, only the offset current

from the biasing network flows through the input resistor.

Second, another input is provided to extend the gain control

dynamic range. And third, the input is truly differential and can

accept signals within the common mode range of the CA3280.

Typical Applications

The structure of the variable operational amplifier eliminates

the need for matched resistor networks in differential to single

ended converters, as shown in Figure 6. A matched resistor

network requires ratio matching of 0.01% or trimming for 80dB

of common-mode rejection. The CA3280, with its excellent

common mode rejection ratio, is capable of converting a small

(

±25mV) differential input signal to a single-ended output

without the need for a matched resistor network.

Figure 7 shows the CA3280 in a typical gain control application.

Gain control can be performed with the amplifier bias current

With the CA3280 operating into a 5k

Ω resistor, the gain is 80.

The need for external buffers can be eliminated by the use of

low value load resistors, but the resulting increase in the

required amplifier bias current reduces the input impedance

FIGURE 3A. EFFECTS OF DIODE LINEARIZATION, WITH DIODE PROGRAMMING TERMINAL ACTIVE

FIGURE 3B. WITH DIODE PROGRAMMING TERMINAL CUTOFF

FIGURE 3. CA3280 TRANSFER CHARACTERISTICS

1/2

13

CA3280

1

14

4

3

15

16

15V

-15V

1k

Ω

V+

10k

Ω

10k

Ω

V-

Horizontal = 1V/Div.

1/2

12

CA3280

8

11

6

4

10

9

15V

1k

Ω

V+

10k

Ω

10k

Ω

-15V

V-

Horizontal = 25mV/Div.

CA3280, CA3280A