®

LOG100

8

FIGURE 7. Output Offset Nulling.

ADJUSTMENTS OF SCALE FACTOR K

The value of K may be changed by increasing or decreasing

the voltage divider resistor normally connected to the out-

put, pin 7. To increase K put resistance in series between pin

7 and the appropriate scaling resistor pin (3, 4 or 5). To

decrease K place a parallel resistor between pin 2 and either

pin 3, 4 or 5.

APPLICATION INFORMATION

WIRING PRECAUTIONS

In order to prevent frequency instability due to lead induc-

tance of the power supply lines, each power supply should

be bypassed. This should be done by connecting a 10

µF

tantalum capacitor in parallel with a 1000pF ceramic capaci-

tor from the +V

CC

and –V

CC

pins to the power supply

common. The connection of these capacitors should be as

close to the LOG100 as practical.

CAPACITIVE LOADS

Stable operation is maintained with capacitive loads of up to

100pF, typically. Higher capacitive loads can be driven if a

22

Ω carbon resistor is connected in series with the LOG100’s

output. This resistor will, of course, form a voltage divider

with other resistive loads.

CIRCUIT PROTECTION

The LOG100 can be protected against accidental power

supply reversal by putting a diode (1N4001 type) in series

with each power supply line as shown in Figure 8. This

precaution is necessary only in power systems that momen-

tarily reverse polarity during turn-on or turn-off. If this

protection circuit is used, the accuracy of the LOG100 will

be degraded slightly by the voltage drops across the diodes

as determined by the power supply sensitivity specification.

The LOG100 uses small geometry FET transistors to achieve

the low input bias currents. Normal FET handling

DATA COMPRESSION

In many applications the compressive effects of the logarith-

mic transfer function is useful. For example, a LOG100

preceding an 8-bit analog-to-digital converter can produce

equivalent 20-bit converter operation.

SELECTING OPTIMUM VALUES OF I

In straight log applications (as opposed to log ratio), both K

and I

errors due to output offset and noise, it is normally best to

FIGURE 8. Reverse Polarity Protection.

6

9

–V

CC

+V

CC

LOG100

I

2

I

1

14

1

5

43

6

10

7

V

OUT

9

–V

CC

+V

CC

C

C

+

–

LOG100

I

2

R

1

100k

Ω

10k

Ω

–V

CC

+V

CC

FIGURE 9. Absorbance Measurement.

techniques should be used to avoid damage caused by low

energy electrostatic discharge (ESD).

LOG RATIO

One of the more common uses of log ratio amplifiers is to

measure absorbance. A typical application is shown in

Figure 9.

Absorbance of the sample is A = log

(22)

If

λ

. (23)

λ

1

λ

I

2

I

1

I

2

I

1

14

1

5

43

6

10

7

V

OUT

9

–V

CC

+V

CC

C

C

+

–

LOG100

D

2

D

1

Sample

λ

1

λ

1

λ

2

Light

Source