LOG112, 2112

7

SBOS246C

www.ti.com

FIGURE 1. Basic Connections of the LOG112.

APPLICATION INFORMATION

The LOG112 is a true logarithmic amplifier that uses the

base-emitter voltage relationship of bipolar transistors to

compute the logarithm, or logarithmic ratio of a current ratio.

Figure 1 and Figure 2 show the basic connections required

for operation of the LOG112 and LOG2112. In order to

reduce the influence of lead inductance of power-supply

lines, it is recommended that each supply be bypassed with

a 10

µF tantalum capacitor in parallel with a 1000pF ceramic

capacitor, as shown in Figure 1 and Figure 2. Connecting

the capacitors as close to the LOG112 and LOG2112 as

possible will contribute to noise reduction as well.

INPUT CURRENT RANGE

To maintain specified accuracy, the input current range of the

LOG112 and LOG2112 should be limited from 100pA to

3.5mA. Input currents outside of this range may compromise

the LOG112 performance. Input currents larger than 3.5mA

result in increased nonlinearity. An absolute maximum input

current rating of 10mA is included to prevent excessive power

dissipation that may damage the input transistor.

On

±5V supplies, the total input current (I

4.5mA. Due to compliance issues internal to the LOG112 and

LOG2112, to accommodate larger total input currents, supplies

should be increased.

SETTING THE REFERENCE CURRENT

When the LOG112 and LOG2112 are used to compute loga-

reference current to which the other is compared.

(1)

that shown in Figure 3), or it may be derived from a voltage

source with one or more resistors. When a single resistor is

10nA and +2.5V is used:

(2)

14

1

5

8

6

910

13

V–

V+

10

µF

LOG112

1000pF

10

µF

1000pF

I

1

I

2

V

LOGOUT

V

REF

11

V

REF – GND

V

CM – IN

C

C

15

16

5

6

11

8

V–

V+

10

µF

LOG2112

1000pF

10

µF

1000pF

I

1B

I

2B

1

2

I

2A

I

1A

V

LOGOUTA

9

V

REF

12

V

LOGOUTB

C

CB

C

CA

FIGURE 2. Basic Connections of the LOG2112.

FIGURE 3. Temperature Compensated Current Source.

2N2905

I

REF

R

REF

2N2905

+15V

–15V

I

6V

R

REF

3.6k

Ω

6V

IN834

FIGURE 4. T Network for Reference Current.

A

1

+

R

2

R

1

+5V

R

3

V

R

I

REF

–

V

OS

1

A voltage divider may be used to reduce the value of the

resistor, as shown in Figure 4. When using this method, one

must consider the possible errors caused by the amplifier’s

value of 100mV.