Texas Instruments SN74AHCT138NS

Overview

SN74AHCT138NS from Texas Instruments is a 3-to-8 line inverting decoder/demultiplexer with TTL-compatible inputs, 5.5 V max supply voltage, −40 °C to +85 °C operating range, and 16-pin SOIC (NS) package. It accepts three binary address inputs (A0–A2) and three enable inputs (E1, E2 active LOW; E3 active HIGH), delivering eight mutually exclusive LOW-active outputs (Y0–Y7) for memory chip select decoding in microcontroller-based systems.

For engineers reviewing the SN74AHCT138NS datasheet, SN74AHCT138NS pinout, SN74AHCT138NS application, or SN74AHCT138NS equivalent, this page provides verified functional behavior, validated timing under 5 V operation, confirmed SOIC-16 mechanical dimensions, and direct alternatives for TTL-level 3-to-8 decoding in industrial control and embedded logic designs.

Technical Context

The SN74AHCT138NS implements a combinational logic decoder with three-stage enable gating: E1 and E2 must both be LOW while E3 is HIGH for any output to activate. Its TTL input thresholds (VIH = 2.0 V min, VIL = 0.8 V max at VCC = 4.5–5.5 V) ensure compatibility with legacy 5 V logic families, and all outputs drive LOW with 8 mA sink capability at VO = 0.55 V max.

Propagation delays are specified across temperature and load: tpd = 4.4–13.0 ns (VCC = 4.5–5.5 V, CL = 15–50 pF), with matched delays between address and enable inputs to outputs-critical for synchronized chip select generation in multi-peripheral systems.

Key Specifications

Pinout & Package

SN74AHCT138NS uses a 16-pin plastic small outline integrated circuit (SOIC) package, body width 3.9 mm, standard JEDEC MS-012 footprint (SOT109-1). Pin 1 is index-marked; pin 8 is GND; pin 16 is VCC.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AHCT138NS, SN74HCT138N extends thermal range without altering timing or drive strength, while 74AHCT138D maintains identical performance in a functionally equivalent SOIC package with updated environmental compliance - both preserve pin-for-pin interoperability and require no layout changes.

Availability

SN74AHCT138NS is available at Aetrix Electronics and suitable for memory address decoding, peripheral chip select logic, LED multiplexing, and test equipment signal routing requiring stable component supply and industrial-grade temperature support.

Supply support for SN74AHCT138NS 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 U.S.-based semiconductor company specializing in analog, embedded processing, and logic solutions with over 50 years of industry leadership.

The SN74AHCT138NS belongs to TI's 74AHCT logic family, designed specifically to provide TTL-compatible interface capability in modern 5 V CMOS systems while maintaining low power and high noise immunity.

FAQ

What logic family does SN74AHCT138NS belong to, and how does it differ from SN74AHC138?

SN74AHCT138NS belongs to the 74AHCT (Advanced High-Speed CMOS with TTL inputs) family. Unlike the 74AHC138, which requires CMOS-level inputs (VIH ≥ 3.85 V at VCC = 5.5 V), SN74AHCT138NS accepts standard TTL input voltages (VIH ≥ 2.0 V), making it directly compatible with legacy 74LS and 74F logic outputs without level-shifting circuitry. This distinction is critical for mixed-technology system integration.

Can SN74AHCT138NS operate at 3.3 V supply voltage?

No, SN74AHCT138NS is not characterized or guaranteed for operation at 3.3 V. Its recommended operating conditions specify VCC = 4.5 V to 5.5 V, and its TTL-compatible input thresholds (VIH = 2.0 V min) are optimized for 5 V systems. For 3.3 V applications, consider SN74AHC138 or SN74LVC138 instead - using SN74AHCT138NS at 3.3 V may result in unreliable input recognition and undefined output behavior.

How many SN74AHCT138NS devices are needed to build a 1-of-32 decoder?

Four SN74AHCT138NS devices plus one inverter are required to construct a 1-of-32 decoder. Three devices decode the lower 3 bits (A0–A2) for each group of eight outputs, while the fourth decodes the upper two bits (A3–A4) to select among four groups. The inverter converts one enable signal polarity to satisfy the E1/E2 (active LOW) and E3 (active HIGH) requirements across levels - enabling full 5-line to 32-line expansion without additional logic gates.

What is the maximum output sink current for SN74AHCT138NS, and under what conditions is it specified?

SN74AHCT138NS delivers up to 8 mA sink current per output (Y0–Y7) at VCC = 4.5 V, with VOL ≤ 0.55 V guaranteed across −40 °C to +125 °C. This rating applies when driving resistive loads to GND with all other outputs inactive. Exceeding 8 mA risks violating VOL specifications and may cause cumulative heating in sustained operation - design margins should maintain ≤7 mA per output for long-term reliability.

Is SN74AHCT138NS pin-compatible with older 74LS138 or 74F138 devices?

Yes, SN74AHCT138NS is pin-compatible and functionally equivalent to 74LS138 and 74F138 in the 16-pin SOIC (NS) package, sharing identical pin assignments for A0–A2, E1–E3, Y0–Y7, GND, and VCC. However, SN74AHCT138NS draws significantly less supply current (ICC ≤ 40 µA vs. ~20 mA for 74LS138) and offers faster propagation delays (4.4 ns vs. 15–20 ns), enabling direct drop-in replacement in most 5 V TTL systems with immediate power and speed benefits.