Texas Instruments SN74ALS138ADB

Overview

SN74ALS138ADB from Texas Instruments is a 3-line-to-8-line decoder/demultiplexer IC with active-low enable inputs (G2A, G2B) and one active-high enable input (G1), operating at 5 V supply, delivering propagation delays as low as 12 ns (tPHL, G2A→Y), and supporting TTL-compatible input thresholds (VIH = 2.0 V min, VIL = 0.8 V max). It serves as a core address decoder in microprocessor-based memory and I/O expansion systems.

For engineers reviewing the SN74ALS138ADB datasheet, SN74ALS138ADB pinout, SN74ALS138ADB application, or SN74ALS138ADB equivalent, key selection criteria include its dual-enable architecture for cascading, guaranteed 8-mA sink capability per output, -40°C to +85°C industrial temperature range, and SOP-16 package compatibility with automated assembly.

Technical Context

The SN74ALS138ADB implements combinational logic decoding using three binary select inputs (A, B, C) to activate exactly one of eight active-low outputs (Y0–Y7), conditioned by three enable signals: G1 (active-high) and G2A/G2B (both active-low, ORed internally as G2). Its internal gate structure ensures deterministic one-hot output behavior without race conditions.

Designed for TTL-family interoperability, it features ALS (Advanced Low-Power Schottky) process technology-achieving lower power than standard LS while maintaining compatible voltage thresholds and drive strength. Output stage design supports direct interfacing with TTL, LSTTL, and CMOS loads up to 15 pF with defined timing under RL = 2 kΩ.

Key Specifications

Pinout & Package

SOP-16 (Small Outline Package, 150 mil width), JEDEC MS-012AC, 1.27 mm pitch, surface-mount compatible with reflow soldering profiles.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with SN74LS138N and 74HC138PW, the SN74ALS138ADB uniquely balances TTL compatibility, 12 ns speed, and 3.8 mA supply current-making it optimal for industrial 5 V control systems where timing precision and legacy interoperability coexist.

Availability

SN74ALS138ADB is available at Aetrix Electronics and suitable for memory address decoding, I/O port expansion, interrupt vector selection, and LED display multiplexing requiring stable component supply across long-lifecycle industrial programs.

Supply support for SN74ALS138ADB 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 leader specializing in analog, embedded processing, and logic solutions, with over 50 years of innovation in industrial-grade logic families.

The SN74ALS138ADB belongs to TI's Advanced Low-Power Schottky (ALS) logic portfolio, engineered specifically for high-reliability 5 V digital systems demanding faster switching and lower power than standard LS devices.

FAQ

What is the maximum clock frequency supported by SN74ALS138ADB in demultiplexing mode?

The SN74ALS138ADB is a combinational logic device-not a clocked circuit-so it has no clock input or maximum clock frequency rating. Its performance is defined by propagation delay: tPHL = 12 ns (G2A→Y) and tPLH = 18 ns (A→Y) under CL = 15 pF. This enables reliable operation in address decode paths up to ~25 MHz, assuming setup/hold margins are maintained relative to the controlling microprocessor's bus timing.

Can SN74ALS138ADB drive LEDs directly without current-limiting resistors?

Yes, SN74ALS138ADB can drive standard 20 mA common-cathode LEDs directly when configured as a digit selector in multiplexed displays. Each output sinks up to 8 mA continuously (IOL = 8 mA), and VOL remains ≤0.5 V at that load. For 20 mA LEDs, a series resistor is still required to limit current-however, the SN74ALS138ADB's strong sink capability eliminates the need for external transistor buffers in most 8-digit display applications.

Is SN74ALS138ADB pin-compatible with SN74LS138N?

Yes, SN74ALS138ADB is pin-compatible with SN74LS138N in the same package variant (e.g., SOP-16 vs. PDIP-16), sharing identical pinout, terminal functions, and mechanical footprint. However, electrical differences-including propagation delay, supply current, and noise margins-require validation in timing-critical circuits before drop-in replacement.

Does SN74ALS138ADB support mixed-voltage operation (e.g., 3.3 V inputs with 5 V VCC)?

No, SN74ALS138ADB is a 5 V-only TTL-family device. Its input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and absolute maximum ratings (VIN ≤ 7 V) are specified for 5 V operation. Applying 3.3 V logic levels may result in marginal or unreliable recognition-level translation (e.g., SN74LVC1T45) is required for interfacing with 3.3 V microcontrollers.

What is the recommended decoupling strategy for SN74ALS138ADB in high-noise industrial environments?

Place a 0.1 µF X7R ceramic capacitor between VCC (pin 16) and GND (pin 15), located within 5 mm of the SN74ALS138ADB body. For boards with multiple SN74ALS138ADB units or high di/dt loads, add a bulk 4.7 µF tantalum capacitor per power rail segment. Avoid shared vias between decoupling caps and signal returns to prevent ground bounce coupling into enable or select inputs.