Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsLinear ICsLinear IC - Op-amp Log ratio amplifier SOIC 8

FEATURES

●

EASY-TO-USE COMPLETE CORE FUNCTION

● HIGH ACCURACY: 0.01% FSO Over 5 Decades

● WIDE INPUT DYNAMIC RANGE:

7.5 Decades, 100pA to 3.5mA

● LOW QUIESCENT CURRENT: 1mA

● WIDE SUPPLY RANGE: ±4.5V to ±18V

Precision

LOGARITHMIC AND LOG RATIO AMPLIFIER

DESCRIPTION

The LOG104

is a versatile integrated circuit that computes

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

reference current.

The LOG104 is tested over a wide dynamic range of input

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

from a photodiode, and a reference current from a resistor in

series with a precision external reference.

The output signal at V

OUT

is trimmed to 0.5V per decade of

input current, allowing seven decades of input current, dy-

namic range.

Low DC offset voltage and temperature drift allow accurate

measurement of low-level signals over a wide environmental

temperature range. The LOG104 is specified over the tem-

perature range –5°C to +75°C, with operation over –40°C to

+85°C.

APPLICATIONS

● LOG, LOG RATIO COMPUTATION:

Communication, Analytical, Medical, Industrial,

Test, General Instrumentation

● PHOTODIODE SIGNAL COMPRESSION AMP

● ANALOG SIGNAL COMPRESSION IN FRONT

OF ANALOG-TO-DIGITAL(A/D) CONVERTER

LOG104

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.

SBOS243C – MAY 2002 – REVISED APRIL 2005

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.

Note: Protected under US Patent #6,667,650; other patents pending.

V–

OUT

V+

GND

LOG104

OUT

= 0.5 LOG (I

)

All trademarks are the property of their respective owners.

Summary of content (14 pages)

PAGE 1
LOG104 LOG 104 SBOS243C – MAY 2002 – REVISED APRIL 2005 Precision LOGARITHMIC AND LOG RATIO AMPLIFIER FEATURES DESCRIPTION ● EASY-TO-USE COMPLETE CORE FUNCTION The LOG104 is a versatile integrated circuit that computes the logarithm or log ratio of an input current relative to a reference current. ● HIGH ACCURACY: 0.01% FSO Over 5 Decades ● WIDE INPUT DYNAMIC RANGE: 7.5 Decades, 100pA to 3.5mA ● LOW QUIESCENT CURRENT: 1mA The LOG104 is tested over a wide dynamic range of input signals.
PAGE 2
ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Supply Voltage, V+ to V– .................................................................... 36V Input Voltage ....................................................... V– (–0.5) to V+ (+0.5V) Input Current ................................................................................... ±10mA Output Short-Circuit(2) .............................................................. Continuous Operating Temperature ...............................
PAGE 3
ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted. LOG104AID PARAMETER TOTAL Initial ERROR(4)(5) vs Temperature vs Supply CONDITION MIN MAX UNITS ±75 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±20 ±3.0 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.1 ±0.25 ±0.1 ±0.
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TYPICAL CHARACTERISTICS At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted. ONE CYCLE OF NORMALIZED TRANSFER FUNCTION NORMALIZED TRANSFER FUNCTION 2.0 VOUT = 0.5V LOG (I1/I2) Normalized Output Voltage (V) Normalized Output Voltage (V) 1.5 0.50 1.0 0.5 0.0 –0.5 –1.0 –1.5 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 –2.0 0.0001 0.001 0.01 0.
PAGE 5
TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
PAGE 6
SETTING THE REFERENCE CURRENT V+ V+ When the LOG104 is used to compute logarithms, either I1 or I2 can be held constant and becomes the reference current to which the other is compared. I1 = 2.5nA to 1mA REF3025 4 2.5V 3 1 VOUT 1GΩ to 2.5kΩ VOUT is expressed as: LOG104 100kΩ I2 = 2.5nA VOUT = (0.5V) • log (I1/I2) 1MΩ (1) IREF can be derived from an external current source (such as shown in Figure 3), or it may be derived from a voltage source with one or more resistors.
PAGE 7
VOLTAGE INPUTS V+ The LOG104 gives the best performance with current inputs. Voltage inputs may be handled directly with series resistors, but the dynamic input range is limited to approximately three decades of input voltage by voltage noise and offsets. The transfer function of Equation (13) applies to this configuration. I1 VOUT LOG104 λ 1´ λ1 Light λ 1 Source TLV271 or 1 OPA2335 2 +3.3V(1) 3 D1 Sample I2 +5V 4 1 8 6 5 D2 1/2 OPA2335 CC 1.5kΩ V– 1.5kΩ +5V FIGURE 9.
PAGE 8
INSIDE THE LOG104 also Using the base-emitter voltage relationship of matched bipolar transistors, the LOG104 establishes a logarithmic function of input current ratios. Beginning with the base-emitter voltage defined as: VBE = VT ln IC IS where : VT = kT q VOUT = VL VOUT = (1) k = Boltzmann’s constant = 1.381 • 10–23 (9) R1 + R 2 I n VT log 1 R1 I2 VOUT = 0.5V • log or T = Absolute temperature in degrees Kelvin R1 + R 2 R1 (10) I1 I2 (11) q = Electron charge = 1.
PAGE 9
TOTAL ERROR MEASURING AVALANCHE PHOTODIODE CURRENT The total error is the deviation (expressed in mV) of the actual output from the ideal output of VOUT = 0.5V • log(I1/I2). The wide dynamic range of the LOG104 is useful for measuring avalanche photodiode current (APD), as shown in Figure 13. Thus, VOUT(ACTUAL) = VOUT(IDEAL) ± Total Error. (6) It represents the sum of all the individual components of error normally associated with the log amp when operated in the current input mode.
PAGE 10
INDIVIDUAL ERROR COMPONENTS Example: what is the error when The ideal transfer function with current input is: VOUT = (0.5V) • log I1 I2 (8) VOUT = (0.5 ± 0.0015) log The actual transfer function with the major components of error is: VOUT = (0.5V) (1 ± ∆K ) log I1 – IB1 ± Nm ± VOS O I2 – IB 2 (9) (11) Since the ideal output is 0.5V, the error as a percent of reading is % error = 0.5055 • 100% = 1.1% 0.
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PACKAGE OPTION ADDENDUM www.ti.
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IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.