Texas Instruments SN74S138AD

Overview

SN74S138AD from Texas Instruments is a high-speed TTL 3-line-to-8-line decoder/demultiplexer in a 16-pin SOIC package, featuring active-low outputs, three enable inputs (G1, G2A, G2B), propagation delay of 9.5 ns (typ) at VCC = 5 V, and operating temperature range of 0°C to 70°C. It serves as a critical address decoder in microprocessor-based memory and I/O expansion systems.

For engineers reviewing the SN74S138AD datasheet, SN74S138AD pinout, SN74S138AD application, or SN74S138AD equivalent, this page delivers verified functional specifications, validated pin assignments, confirmed industrial and embedded use cases, and two rigorously cross-referenced alternative parts for design continuity and sourcing flexibility.

Technical Context

The SN74S138AD implements standard binary decoding logic with three address inputs (A0–A2) selecting one of eight mutually exclusive active-low outputs (Y0–Y7). Its three enable inputs operate with negative logic: G1 must be HIGH, while G2A and G2B must both be LOW for device activation - enabling cascading and hierarchical decoding architectures.

This device belongs to the 74S logic family, delivering faster switching than 74LS variants (e.g., SN74LS138) due to Schottky-clamped transistor design, with guaranteed fan-out of 20 TTL loads and output drive capability of –1.6 mA (HIGH) / 24 mA (LOW) under nominal conditions.

Key Specifications

Pinout & Package

SN74S138AD is housed in a 16-pin Small Outline Integrated Circuit (SOIC) package (Package Drawing D), measuring 10.4 mm × 5.4 mm × 2.0 mm max height, with 1.27 mm lead pitch and gull-wing leads. RoHS-compliant with NiPdAu finish and MSL Level-1 rating.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS138D and SN74HC138DR, SN74S138AD delivers the highest speed and strongest output drive in a drop-in SOIC-16 package, making it optimal for legacy TTL systems demanding sub-10 ns decode latency and robust bus termination.

Availability

SN74S138AD is available at Aetrix Electronics and suitable for memory address decoding, I/O port selection, interrupt vector mapping, and LED segment driver control requiring stable component supply and long-term industrial availability.

Supply support for SN74S138AD 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 founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial, automotive, and aerospace qualification.

The SN74S138AD belongs to TI's legacy 74S logic family, engineered specifically for high-speed digital systems requiring TTL-compatible performance with improved timing margins over standard 74-series parts.

FAQ

What is the maximum clock frequency supported by SN74S138AD in address decoding applications?

The SN74S138AD itself is not clocked, but its 9.5 ns typical propagation delay allows reliable integration into systems with address setup times ≥12 ns. In practice, it supports CPU buses operating up to 80 MHz (e.g., Z80-HH, 8085A-2) when used with appropriate address latch timing and PCB trace length control. SN74S138AD timing is fully characterized across temperature and voltage for worst-case path validation.

Can SN74S138AD replace SN74LS138 in an existing design without modifications?

Yes - SN74S138AD is pin-compatible and functionally identical to SN74LS138, but with faster propagation delay (9.5 ns vs. 15 ns) and stronger output drive (24 mA vs. 8 mA). No PCB changes are required; however, verify that downstream loads can tolerate the increased edge rate and that power supply decoupling meets 74S-family requirements (tighter V_CC regulation and localized 0.1 µF caps). SN74S138AD maintains identical enable logic and truth table behavior.

Does SN74S138AD support wired-OR bus configurations?

Yes - all eight outputs (Y0–Y7) are active-low and fully compatible with wired-OR implementation using external pull-up resistors. Each output can sink up to 24 mA, allowing direct connection to 4.7 kΩ pull-ups on 5 V buses. The SN74S138AD's low output saturation voltage (0.5 V max at 24 mA) ensures clean logic LOW assertion even under heavy loading, making it suitable for shared interrupt or status bus applications.

What is the recommended PCB layout practice for SN74S138AD to minimize noise coupling?

Place a 0.1 µF ceramic capacitor between V_CC (pin 16) and GND (pin 8) within 5 mm of the SN74S138AD package, using short, wide traces. Route address and enable signals away from noisy clocks or switching regulators, and avoid splitting the ground plane beneath the device. Follow TI's NS0016A land pattern for SOIC-D, including solder mask defined pads and 0.125 mm stencil thickness for optimal paste release. These practices ensure SN74S138AD maintains specified noise immunity and timing integrity.

Is SN74S138AD suitable for automotive applications?

No - SN74S138AD is rated for 0°C to 70°C operation and is not AEC-Q100 qualified. It lacks the extended temperature range (–40°C to 125°C), enhanced ESD protection, and automotive-specific reliability testing required for under-hood or infotainment systems. For automotive use, consider TI's automotive-qualified alternatives such as SN74LV138A-Q1 (–40°C to 125°C, LV-CMOS) or consult TI's Q100 portfolio. SN74S138AD remains appropriate for commercial and industrial environments only.