Datasheet

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LOG112

LOG2112

SBOS246D – JUNE 2002 – REVISED APRIL 2005

www.ti.com

DESCRIPTION

The LOG112 and LOG2112 are versatile integrated circuits

that compute the logarithm or log ratio of an input current

relative to a reference current. V

LOGOUT

of the LOG112 and

LOG2112 are trimmed to 0.5V per decade of input current,

ensuring high precision over a wide dynamic range of input

signals.

The LOG112 and LOG2112 features a 2.5V voltage refer-

ence that may be used to generate a precision current

reference using an external resistor.

Low DC offset voltage and temperature drift allow accurate

measurement of low-level signals over the specified tem-

perature range of –5°C to +75°C.

FEATURES

● EASY-TO-USE COMPLETE FUNCTION

● OUTPUT SCALING AMPLIFIER

● ON-CHIP 2.5V VOLTAGE REFERENCE

● HIGH ACCURACY: 0.2% FSO Over 5 Decades

● WIDE INPUT DYNAMIC RANGE:

7.5 Decades, 100pA to 3.5mA

● LOW QUIESCENT CURRENT: 1.75mA

● WIDE SUPPLY RANGE:

±4.5V to ±18V

● PACKAGES: SO-14 (narrow) and SO-16

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.

Precision

LOGARITHMIC AND LOG RATIO AMPLIFIERS

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.

APPLICATIONS

● LOG, LOG RATIO:

Communication, Analytical, Medical, Industrial,

Test, General Instrumentation

● PHOTODIODE SIGNAL COMPRESSION AMP

● ANALOG SIGNAL COMPRESSION IN FRONT

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

● ABSORBANCE MEASUREMENT

● OPTICAL DENSITY MEASUREMENT

REF – GND

NOTE: Internal resistors are used to compensate gain change over temperature.

The V

pin is internally connected to GND in the LOG2112.

V+

GND

LOG112

LOGOUT

= (0.5V)LOG (I

)

= K (0.5V)LOG (I

), K = 1 + R

REF

+IN3

LOGOUT

–IN3

V–

REF

All trademarks are the property of their respective owners.

Summary of content (20 pages)

PAGE 1
LOG LOG112 LOG2112 2112 LOG 112 SBOS246D – JUNE 2002 – REVISED APRIL 2005 Precision LOGARITHMIC AND LOG RATIO AMPLIFIERS FEATURES DESCRIPTION ● EASY-TO-USE COMPLETE FUNCTION The LOG112 and LOG2112 are versatile integrated circuits that compute the logarithm or log ratio of an input current relative to a reference current. VLOGOUT of the LOG112 and LOG2112 are trimmed to 0.5V per decade of input current, ensuring high precision over a wide dynamic range of input signals.
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ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Supply Voltage, V+ to V– .................................................................. ±18V Inputs ................................................................................................. ±18V Input Current ................................................................................... ±10mA Output Short-Circuit Current(2) ................................................ Continuous Operating Temperature ......................
PAGE 3
ELECTRICAL CHARACTERISTICS Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. At TA = +25°C, VS = ±5V, and ROUT = 10kΩ, unless otherwise noted.
PAGE 4
ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = –5°C to +75°C. At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted.
PAGE 5
TYPICAL CHARACTERISTICS At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. ONE CYCLE OF NORMALIZED TRANSFER FUNCTION NORMALIZED TRANSFER FUNCTION 0.50 VLOGOUT = (0.5V)LOG (I1/I2) 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0 0.0001 0.001 0.01 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.
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TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = ±5V, and RL = 10kΩ, unless otherwise noted. 1M Select CC for I1 min. and I2 max. Values below 2pF may be ignored.
PAGE 7
INPUT CURRENT RANGE APPLICATION INFORMATION The LOG112 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. Figure 1 and Figure 2 show the basic connections required for operation of the LOG112 and LOG2112.
PAGE 8
Figure 5 shows a low-level current source using a series resistor. The low offset op amp reduces the effect of the LOG112 and LOG2112’s input offset voltage. VREF I1 = 2.5nA to 1mA FREQUENCY COMPENSATION Frequency compensation for the LOG112 is obtained by connecting a capacitor between pins 5 and 14. Frequency compensation for the LOG2112 is obtained by connecting a capacitor between pins 2 and 5, or 15 and 12.
PAGE 9
VOLTAGE INPUTS age to VCM of at least +1V and up to 2.5V, brings the log transistors out of saturation and reduces output error to approximately 10%. To avoid forward biasing a photodiode, return the cathode to the VCM pin, as shown in Figure 9. To reverse bias the photodiode, apply a more positive voltage to the cathode than the anode. The LOG112 and LOG2112 give the best performances with current inputs.
PAGE 10
DATA COMPRESSION MEASURING AVALANCHE PHOTODIODE CURRENT In many applications, the compressive effects of the logarithmic transfer function are useful. For example, a LOG112 preceding a 12-bit A/D converter can produce the dynamic range equivalent to a 20-bit converter. The wide dynamic range of the LOG112 and LOG2112 is useful for measuring avalanche photodiode current (APD), as shown in Figure 12.
PAGE 11
INSIDE THE LOG112 Using the base-emitter voltage relationship of matched bipolar transistors, the LOG112 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 also VOUT = VL (10) I  VOUT = (0.5V)LOG  1   I2  k = Boltzmann’s constant = 1.381 • 10–23 or T = Absolute temperature in degrees Kelvin (9) R1 + R2 I n VT log 1 R1 I2 VOUT = (1) R1 + R2 R1 (11) q = Electron charge = 1.
PAGE 12
ERRORS RTO AND RTI The individual component of error is: ∆K = gain error (0.10%, typ), as specified in the specification table. As with any transfer function, errors generated by the function may be Referred-to-Output (RTO) or Referred-to-Input (RTI). In this respect, log amps have a unique property: given some error voltage at the log amp’s output, that error corresponds to a constant percent of the input regardless of the actual input level.
PAGE 13
PACKAGE OPTION ADDENDUM www.ti.
PAGE 14
PACKAGE MATERIALS INFORMATION www.ti.com 14-Jul-2012 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant LOG112AIDR SOIC D 14 2500 330.0 16.4 6.5 9.0 2.1 8.0 16.0 Q1 LOG2112AIDWR SOIC DW 16 1000 330.0 16.4 10.75 10.7 2.7 12.0 16.
PAGE 15
PACKAGE MATERIALS INFORMATION www.ti.com 14-Jul-2012 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LOG112AIDR LOG2112AIDWR SOIC D 14 2500 367.0 367.0 38.0 SOIC DW 16 1000 367.0 367.0 38.
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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.