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LOG102

6

SBOS211A

14

I

1

I

2

1

10

8

11

3

12

7

4

5

6

V+

9

V–

10

µF

Amplifier A

C

C

10

µF

LOG102

V

OUT

R

2

V

LOGOUT

V

R

1

1000pF

1000pF

Unused amplifiers should

have positive inputs grounded

and negative inputs tied to

their respective outputs.

FIGURE 1. Basic Connections with Output Gain Factor of the

LOG102.

APPLICATION INFORMATION

The LOG102 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.

With two uncommitted on-chip operational amplifiers, the

LOG102 provides design flexibility and simplicity.

Figure 1 shows the basic connections required for operation

of the LOG102 with a gain factor. 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 capaci-

tor, as shown in Figure 1. Connecting the capacitors as close

to the LOG102 as possible will contribute to noise reduction

as well.

INPUT CURRENT RANGE

To maintain specified accuracy, the input current range of the

LOG102 should be limited from 1nA to 1mA. Input currents

outside of this range may compromise LOG102 performance.

Input currents larger than 1mA result in increased nonlinearity.

An absolute maximum input current rating of 10mA is included

to prevent excessive power dissipation that may damage the

logging transistor.

On

±5V supplies the total input current (I

1.1mA. Due to compliance issues internal to the LOG102, to

accommodate larger total input currents, supplies should be

increased.

Currents smaller than 1nA will result in increased errors due

The input bias currents may be compensated for, as shown in

Figure 2. The input stages of the amplifiers have FET inputs,

with input bias current doubling every 10

°C, which makes the

nulling technique shown practical only where the temperature

is fairly stable.

FIGURE 2. Bias Current Nulling.

V–

R

> 1M

Ω

I

2

I

1

R

10k

Ω

R

1

1M

Ω

R

2

10k

Ω

V+

14

1

9

10

5

V

OUT

6

V–

V+

C

C

LOG102

2N2905

I

REF

R

REF

2N2905

+15V

–15V

I

6V

R

REF

3.6k

Ω

6V

IN834

FIGURE 3. Temperature Compensated Current Source.

SETTING THE REFERENCE CURRENT

which the other is compared.

(1)

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:

A

1

+

R

2

R

1

+5V

R

3

V

R

I

REF

–

V

OS

14

FIGURE 4. T Network for Reference Current.

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.