FEATURES

q EASY-TO-USE COMPLETE LOG RATIO FUNCTION

q OUTPUT AMPLIFIERS FOR SCALING AND

SIGNAL LOSS INDICATION

q HIGH ACCURACY: 0.15% FSO Total Error Over

6 Decades

q WIDE INPUT DYNAMIC RANGE:

6 Decades, 1nA to 1mA

q LOW QUIESCENT CURRENT: 1.25mA

q SO-14 PACKAGE

Precision

LOGARITHMIC AND LOG RATIO AMPLIFIER

DESCRIPTION

The LOG102 is a versatile integrated circuit that computes

the logarithm or log ratio of an input current relative to a

reference current.

The LOG102 is tested over a wide dynamic range of input

signals. In log ratio applications, a signal current can be

generated by a photodiode, and a reference current from a

resistor in series with a precision external voltage reference.

In the block diagram shown below, A3 and A4 are identical,

uncommitted op amps that can be used for a variety of

functions, such as filtering, offsetting, adding gain or as a

comparator to detect loss of signal.

input current. It can be scaled with an output amplifier, either

A3 or A4.

Low dc offset voltage and temperature drift allow accurate

measurement of low-level signals over a wide environmental

temperature range. The LOG102 is specified over the tem-

perature range, 0

°C to +70°C, with operation over –40°C to

+85

°C.

APPLICATIONS

q ONET, OPTICAL POWER METERS

q LOG, LOG RATIO COMPUTATION:

Communication, Analytical, Medical, Industrial,

Test, General Instrumentation

q PHOTODIODE SIGNAL COMPRESSION AMP

q ANALOG SIGNAL COMPRESSION IN FRONT

OF A/D CONVERTER

q ABSORBANCE MEASUREMENT

q OPTICAL DENSITY MEASUREMENT

LOG102

www.ti.com

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of Texas Instruments

standard warranty. Production processing does not necessarily include

testing of all parameters.

Copyright © 2001-2005, Texas Instruments Incorporated

SBOS211B – DECEMBER 2001– REVISED APRIL 2005

A

2

A

3

A

1

I

1

Q

1

Q

2

VOUT3

A

4

VOUT4

+IN4

–IN4

–IN3

+IN3

V

LOG OUT

V

V

R

1

R

2

C

C

I

2

A4 can be used

as comparator for

signal loss detect.

V–

V+

614

1

911

4

7

12

8

3

5

10

GND

NOTE: U.S. Patent Pending.

LOG102

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

All trademarks are the property of their respective owners.

Not Recommended For New Designs