LOG114

LOG114 SBOS301 − MAY 2004 Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER FEATURES D EASY-TO-USE COMPLETE FUNCTION D TWO OUTPUT, WIDE BW SCALING D WIDE INPUT DYNAMIC RANGE: D 2.5V REFERENCE D EXCELLENT GAIN STABILITY OVER D D D D TEMPERATURE LOW QUIESCENT CURRENT: 10mA DUAL OR SINGLE SUPPLY: +5V, +5V PACKAGE: Small QFN-16 (4mm x 4mm) SPECIFIED TEMPERATURE RANGE: −5°C to +75°C 8 Decades, 100pA to 10mA AMPLIFIERS DESCRIPTION The LOG114 computes the logarithm or log-ratio of an input current or voltage relative to a reference current or voltage. High precision is ensured over a wide dynamic range of input signals on either bipolar or single supply. Special temperature drift compensation circuitry is included on-chip. In log-ratio applications, a signal current can come from a photodiode, and a reference current from a resistor in series with a precision internal voltage refernce. APPLICATIONS D LOG, LOG-RATIO FUNCTION FOR TEST, www.DataSheet4U.com D D D D GENERAL INSTRUMENTATION PHOTODIODE SIGNAL COMPRESSION AMP ANALOG SIGNAL COMPRESSION IN FRONT OF ANALOG-TO-DIGITAL CONVERTER (ADC) ABSORBANCE MEASUREMENT OPTICAL DENSITY MEASUREMENT Q1 I1 4 VCM IN 5 I1 and I2 are current inputs from a photodiode or other current source Q2 I2 3 I REF RREF 16 VREF 2.5V 1 VREF GND REF A2 A1 R5 R6 VLOG OUT 9 10 − IN4 +IN4 11 R1 R2 A4 VO4 12 A3 13 R3 R4 A5 +IN5 15 VO5 VLOG = 0.375 × log(I1/I2) V04 = 0.375 × K × log(I1/I2) K = 1 + R6/R5 8 V+ 6 V− 7 Com 14 − IN5 Please be a...