LOG102
LOG102
SBOS211A – MARCH 2002
Precision
LOGARITHMIC AND LOG RATIO AMPLIFIER
FEATURES
DESCRIPTION
The LOG102 is a versatile integrated circuit that computes
the logarithm or log ratio of an input current relative to a
reference current.
●
EASY-TO-USE COMPLETE LOG RATIO FUNCTION
● OUTPUT AMPLIFIERS FOR SCALING AND
SIGNAL LOSS INDICATION
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.
● HIGH ACCURACY: 0.15% FSO Total Error Over
6 Decades
● WIDE INPUT DYNAMIC RANGE:
6 Decades, 1nA to 1mA
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.
● LOW QUIESCENT CURRENT: 1.25mA
● SO-14 PACKAGE
The output signal at VLOG OUT is trimmed to 1V per decade of
input current. It can be scaled with an output amplifier, either
A3 or A4.
APPLICATIONS
● ONET, OPTICAL POWER METERS
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.
● LOG, LOG RATIO COMPUTATION:
Communication, Analytical, Medical, Industrial,
Test, General Instrumentation
● PHOTODIODE SIGNAL COMPRESSION AMP
NOTE: U.S. Patent Pending.
● ANALOG SIGNAL COMPRESSION IN FRONT
OF A/D CONVERTER
● ABSORBANCE MEASUREMENT
● OPTICAL DENSITY MEASUREMENT
R1
R2
VLOG OUT = LOG (I1/I2)
VOUT3 = G • LOG (I1/I2), G = 1 +R2/R1
CC
I2
V+
6
VLOG OUT
+IN3
3
–IN3
5
4
14
I1
1
Q1
Q2
7
A3
VOUT3
+IN4
A2
A1
12
8
A4
VOUT4
A4 can be used
as comparator for
signal loss detect.
9
10
11
–IN4
GND
V–
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.
PRODUCTION DATA information is current as of publication date.
Copyright © 2001, Texas Instruments Incorporated
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
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