Texas Instruments LOG112AID

Overview

LOG112AID from Texas Instruments is a precision logarithmic amplifier IC that computes the logarithm or log ratio of input current I1 relative to reference current I2, delivering VLOGOUT = (0.5 V) × log₁₀(I1/I2). It features an on-chip 2.5 V voltage reference, ±4.5 V to ±18 V dual supply operation, 7.5-decade input dynamic range (100 pA to 3.5 mA), and 0.2% full-scale output (FSO) accuracy over five decades - used in absorbance measurement, photodiode signal compression, and analog front-end signal conditioning before A/D conversion.

For engineers reviewing the LOG112AID datasheet, LOG112AID pinout, LOG112AID application, or LOG112AID equivalent, key selection considerations include its SO-14 narrow-body package, verified log conformity error (0.13% over 7.5 decades), temperature-stable gain (0.005 %/°C), low input bias current (±5 pA), and compatibility with photodiode-based optical density systems requiring wide dynamic range and high DC precision.

Technical Context

The LOG112AID implements a true logarithmic transfer function using matched bipolar transistor pairs (Q1/Q2) and internal compensation to maintain 0.5 V/decade scaling across its full 7.5-decade input range. Its core architecture includes three integrated amplifiers (A1, A2, A3): A1 and A2 condition input currents and generate the log voltage, while A3 provides a programmable output scaling stage (VO3 = K × VLOGOUT, where K = 1 + R2/R1).

It integrates a bandgap-derived 2.5 V reference (±0.5% initial error, ±25 ppm/°C drift) usable with external resistors to set precise reference currents. Frequency compensation is externally applied via capacitor CC between pins 5 and 14, with value dependent on I1/I2 magnitude - enabling bandwidth optimization from 0.1 kHz (at 100 pA) to 45 kHz (at 1 mA).

Key Specifications

Pinout & Package

LOG112AID is housed in a 14-pin SOIC (narrow) package (TI package code D), with thermal resistance θ_JA = 110 °C/W. Pin functions are validated per TI SBOS246D datasheet Figure 1 and Package Configuration table.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar logarithmic amplifier applications.

Compared with ADL5513ACPZ-R7 and LOG2112AIDW, LOG112AID offers unique DC-precision logarithmic current-to-voltage conversion with verified 0.13% log conformity over 7.5 decades - making it the only viable option for high-accuracy, low-frequency optical density and absorbance systems requiring true DC ratio stability.

Availability

LOG112AID is available at Aetrix Electronics and suitable for absorbance measurement systems, photodiode front-ends, and analog signal compression circuits requiring stable component supply, long-term manufacturability, and guaranteed traceability.

Supply support for LOG112AID includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor company headquartered in Dallas, Texas, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and communications applications.

The LOG112AID belongs to TI's Precision Logarithmic Amplifier product line, designed specifically for high-accuracy DC current ratio measurement in analytical instrumentation, medical diagnostics, and optical sensing systems requiring wide dynamic range and minimal temperature-induced drift.

FAQ

What is the maximum input current the LOG112AID can handle while maintaining specified accuracy?

The LOG112AID maintains specified accuracy for input currents from 100 pA to 3.5 mA. Input currents above 3.5 mA increase nonlinearity; absolute maximum rating is ±10 mA. On ±5 V supplies, total input current (I1 + I2) must not exceed 4.5 mA due to internal compliance limits. For higher currents, increase supply voltage or apply a +1 V to +2.5 V common-mode voltage to VCM to reduce output error.

How does the LOG112AID achieve 0.5 V/decade scaling, and is it factory-trimmed?

The LOG112AID achieves 0.5 V/decade scaling via matched bipolar transistor pairs (Q1/Q2) and internal gain-setting resistors. This scale factor is factory-trimmed and specified as 0.5 V/decade with ±1% gain error over temperature. The trimming ensures consistent log output across units without requiring external calibration for ratio applications like absorbance measurement.

Can the LOG112AID operate from a single +5 V supply?

Yes, the LOG112AID can operate from a single +5 V supply using a charge-pump negative rail generator (e.g., TPS60402DBV) to create a –5 V rail, as shown in Figure 11 of the SBOS246D datasheet. Direct single-supply operation is not supported - dual supplies (±4.5 V to ±18 V) are required for full specification compliance, including input common-mode range and output swing.

What is the role of the VCM pin on the LOG112AID, and how should it be used?

The VCM pin on the LOG112AID accepts a common-mode voltage (0 V to +2.5 V) to bias the input transistors out of saturation at high input currents. Applying +1 V to +2.5 V to VCM reduces total output error from up to 25% to ~10% for 4.5 mA inputs. In photodiode applications, connect the photodiode cathode to VCM to apply reverse bias while maintaining log accuracy.

Does the LOG112AID include an internal voltage reference, and what are its specifications?

Yes, the LOG112AID includes an internal 2.5 V bandgap voltage reference (pin 7, VREF) with ±0.5% initial accuracy, ±25 ppm/°C temperature drift, ±10 ppm/V supply rejection, and ±600 ppm/mA load regulation. It delivers up to 16 mA short-circuit current and is intended for generating precision reference currents via external RREF - eliminating need for external references in most log-ratio designs.