Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsLinear ICsLinear IC - Op-amp Log ratio amplifier SOIC 14

FEATURES

●

EASY-TO-USE COMPLETE LOG RATIO FUNCTION

● OUTPUT AMPLIFIERS FOR SCALING AND

SIGNAL LOSS INDICATION

● HIGH ACCURACY: 0.15% FSO Total Error Over

6 Decades

● WIDE INPUT DYNAMIC RANGE:

6 Decades, 1nA to 1mA

● LOW QUIESCENT CURRENT: 1.25mA

● 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.

The output signal at V

LOG OUT

is trimmed to 1V

per decade of

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

● ONET, OPTICAL POWER METERS

● LOG, LOG RATIO COMPUTATION:

Communication, Analytical, Medical, Industrial,

Test, General Instrumentation

● PHOTODIODE SIGNAL COMPRESSION AMP

● ANALOG SIGNAL COMPRESSION IN FRONT

OF A/D CONVERTER

● ABSORBANCE MEASUREMENT

● 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.

SBOS211B – DECEMBER 2001– REVISED APRIL 2005

VOUT3

VOUT4

+IN4

–IN4

–IN3+IN3V

LOG OUT

= LOG (I

)

OUT3

= G • LOG (I

), G = 1 +R

A4 can be used

as comparator for

signal loss detect.

V–

V+

614

911

GND

NOTE: U.S. Patent Pending.

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.

Summary of content (19 pages)

PAGE 1
LOG102 LOG 102 SBOS211B – DECEMBER 2001– REVISED APRIL 2005 Precision LOGARITHMIC AND LOG RATIO AMPLIFIER FEATURES DESCRIPTION ● EASY-TO-USE COMPLETE LOG RATIO FUNCTION The LOG102 is a versatile integrated circuit that computes the logarithm or log ratio of an input current relative to a reference current. ● OUTPUT AMPLIFIERS FOR SCALING AND SIGNAL LOSS INDICATION The LOG102 is tested over a wide dynamic range of input signals.
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PIN DESCRIPTION ABSOLUTE MAXIMUM RATINGS(1) Supply Voltage, V+ to V– .................................................................... 36V Top View SO Input Voltage ....................................................... V– (–0.5) to V+ (+0.5V) Input Current ................................................................................... ±10mA Output Short-Circuit(2) .............................................................. Continuous Operating Temperature ....................................
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ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = 0°C to +70°C. At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
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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 3 VOUT = 1V • Log I1 I2 0.9 Normalized Output Voltage (V) Normalized Output Voltage (V) 1 2 1 0 –1 –2 –3 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.001 0.01 0.1 1 10 100 1000 1 2 I Current Ratio, 1 I2 3 6 4 10 8 I Current Ratio, 1 I2 TOTAL ERROR vs INPUT CURRENT GAIN ERROR vs TEMPERATURE 0.35 60 0.30 50 40 30 70°C 0°C 0.
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TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
PAGE 6
APPLICATION INFORMATION R2 10kΩ The LOG102 is a true logarithmic amplifier that uses the base-emitter voltage relationship of bipolar transistors to compute the logarithm, or logarithmic ratio, of a current ratio. With two uncommitted on-chip operational amplifiers, the LOG102 provides design flexibility and simplicity. Figure 1 shows the basic connections required for operation of the LOG102 with a gain factor.
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Figure 5 shows a low-level current source using a series resistor. The low offset op-amp reduces the effect of the LOG102’s input offset voltage. V+ NEGATIVE INPUT CURRENTS The LOG102 will function only with positive input currents (conventional current flow into pins 1 and 14). In situations where negative input currents are needed, the circuits in Figures 6, 7, 8, and 9, may be used. V+ I1 = 2.5nA to 1mA REF3025 6 2.5V 1 5 VLOGOUT 1GΩ to 2.5kΩ LOG102 100kΩ I2 = 2.5nA 1MΩ +2.5mV +2.
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1.5kΩ 100kΩ 100kΩ +5V 10nA to 1mA +3.3V Back Bias +5V 1/2 OPA2335 +3.3V 1.5kΩ 1/2 OPA2335 Photodiode 1.5kΩ 100kΩ 100kΩ LOG102 10nA to 1mA Pin 1 or Pin 14 FIGURE 8. Precision Current Inverter/Current Source. DATA COMPRESSION In many applications the compressive effects of the logarithmic transfer function are useful. For example, a LOG102 preceding a 12-bit Analog-to-Digital (A/D) converter can produce the dynamic range equivalent to a 20-bit converter.
PAGE 9
INSIDE THE LOG102 also Using the base-emitter voltage relationship of matched bipolar transistors, the LOG102 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 R1 + R 2 R1 I1 = log I2 VOUT = VL (1) k = Boltzmann’s constant = 1.381 • 10–23 VOUT = or T = Absolute temperature in degrees Kelvin (9) (10) R1 + R 2 I n VT log 1 R1 I2 (11) q = Electron charge = 1.
PAGE 10
IREF = VREF VOUT = VREF – R1 R2 R3 • LOG ( IREF IPHOTO ) R3 R2 CC VLOGOUT 5 IREF 3 VMIN to VMAX I1 R1 Q1 1 Q2 A3 7 A/D A2 VREF A1 R2 4 IPHOTO I2 R3 14 LOG102 IMIN to IMAX 10 FIGURE 12. Technique for Using Full-Scale Reference Current such that V OUT Increases with Increasing Photodiode Current. ISHUNT +15V to +60V 500 Irx = 1µA to 1mA Receiver 5kΩ 5kΩ 10gbits/sec +5V APD INA168 SOT23-5 I to V Converter IOUT = 0.1 • ISHUNT 1 2 IOUT CC 1.
PAGE 11
DEFINITION OF TERMS ERRORS RTO AND RTI As with any transfer function, errors generated by the function itself may be Referred-to-Output (RTO) or Referred-toInput (RTI). In this respect, log amps have a unique property: TRANSFER FUNCTION The ideal transfer function is: VOUT = 1V • logI1/I2 (5) Figure 14 shows the graphical representation of the transfer over valid operating range for the LOG102.
PAGE 12
≈ 1.003 log 10 –6 10 –7 For the case of voltage inputs, the actual transfer function is + 0.004 + 0.003 (10) = 1.003 (1) + 0.004 + 0.0003 (11) = 1.0073V (12) VOUT (14) EOS1 V1 – IB1 ± R1 R1 = (1V) (1 ± ∆K ) log ± 2Nn ± VOS OUT E V2 OS 2 – IB 2 ± R2 R2 Since the ideal output is 1.000V, the error as a percent of reading is 0.0073 % error = • 100% = 0.73% 1 12 (13) (15) Where EOS1 E and OS2 are considered to be zero for large R1 R2 values of resistance from external input current sources.
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE OPTION ADDENDUM www.ti.
PAGE 15
PACKAGE MATERIALS INFORMATION www.ti.com 26-Jan-2013 TAPE AND REEL INFORMATION *All dimensions are nominal Device LOG102AIDR Package Package Pins Type Drawing SOIC D 14 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) 2500 330.0 16.4 Pack Materials-Page 1 6.5 B0 (mm) K0 (mm) P1 (mm) 9.0 2.1 8.0 W Pin1 (mm) Quadrant 16.
PAGE 16
PACKAGE MATERIALS INFORMATION www.ti.com 26-Jan-2013 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LOG102AIDR SOIC D 14 2500 367.0 367.0 38.
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