www.ti.com

LOG104

5

SBOS243C

17

15

13

11

9

7

5

3

1

–1

Input Current (I

LOG CONFORMITY vs INPUT CURRENT

+85

°C

+75

°C

–40

°C to +25°C

100pA

1nA

10nA

100nA

1

µA10µA 100µA1mA

LOG CONFORMITY vs TEMPERATURE

350

300

250

200

150

100

50

0

–40 –30 –20 –10 0

10 20

30 4050 6070 8090

Temperature (

°C)

7 Decades

(100pA to 1mA)

6 Decades

(1nA to 1mA)

5 Decades

(1nA to 100

µA)

8

1

6

3

4

5

V–

V+

10

µF

LOG104

1000pF

10

µF

1000pF

I

1

I

2

V

OUT

C

C

FIGURE 1. Basic Connections of the LOG104.

APPLICATION INFORMATION

The LOG104 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 shows the basic connections required for operation

of the LOG104. 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. Connecting the capacitors as close to the LOG104

as possible will contribute to noise reduction as well.

INPUT CURRENT RANGE

To maintain specified accuracy, the input current range of the

LOG104 should be limited from 100pA to 3.5mA. Input currents

outside of this range may compromise LOG104 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 logging transistor.

On

±5V supplies, the total input current (I

4.5mA. Due to compliance issues internal to the LOG104, to

accommodate larger total input currents, supplies should be

increased.

Currents smaller than 100pA 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+

8

1

4

6

3

V

OUT

5

V–

V+

C

C

LOG104

GND