Texas Instruments SN74HCT138ANS

Overview

SN74HCT138ANS from Texas Instruments is a 3-to-8 line inverting decoder/demultiplexer with TTL-compatible inputs, active-LOW outputs (Y0–Y7), three enable inputs (E1, E2 active LOW; E3 active HIGH), and SOIC-16 packaging. It operates from 4.5 V to 5.5 V across –40 °C to +85 °C and supports memory chip select decoding and parallel expansion to 1-of-32 using four units and one inverter.

For engineers reviewing the SN74HCT138ANS datasheet, SN74HCT138ANS pinout, SN74HCT138ANS application, or SN74HCT138ANS equivalent, key selection criteria include TTL-level input compatibility, propagation delay ≤53 ns at 5 V, active-LOW output drive capability (4 mA sink), demultiplexing functionality via enable-as-data routing, and SOIC-16 thermal performance up to +85 °C ambient.

Technical Context

The SN74HCT138ANS implements combinational logic decoding of three binary address inputs (A0–A2) into eight mutually exclusive active-LOW outputs, controlled by three independent enables: two active-LOW (E1, E2) and one active-HIGH (E3). All outputs remain HIGH unless E1 = E2 = LOW and E3 = HIGH - enabling precise hierarchical enable control for cascaded decoding.

Its TTL-compatible input thresholds (VIH = 2.0 V min, VIL = 0.8 V max at VCC = 4.5–5.5 V) ensure direct interfacing with legacy 5 V TTL logic families without level shifting. The device features integrated input clamp diodes, allowing safe interface to voltages exceeding VCC when used with current-limiting resistors.

Key Specifications

Pinout & Package

SN74HCT138ANS is supplied in a 16-pin SOIC (Small Outline Integrated Circuit) package (SO-16, body width 3.9 mm, JEDEC MS-012 variant), with gull-wing leads and surface-mount footprint.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138N, SN74HCT138ANS offers guaranteed TTL-level input compatibility for legacy 5 V systems, while 74HCT138D provides extended temperature range and alternate supply chain assurance - both require no PCB changes but differ in input threshold behavior and thermal rating.

Availability

SN74HCT138ANS is available at Aetrix Electronics and suitable for industrial control panels, test equipment signal routing, and memory-mapped peripheral interfaces requiring stable component supply and long-lifecycle support.

Supply support for SN74HCT138ANS 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 for industrial, automotive, and communications markets.

The SN74HCT138ANS belongs to TI's 74HCT logic family, designed specifically to provide TTL-compatible input thresholds and 5 V CMOS output performance for seamless integration into legacy and mixed-voltage digital systems.

FAQ

What is the maximum propagation delay of the SN74HCT138ANS under industrial temperature conditions?

The SN74HCT138ANS has a maximum propagation delay of 53 ns (E3 → Yn) over the full –40 °C to +125 °C range at VCC = 4.5 V, as specified in Table 7 of the TI datasheet. At the industrial grade range (–40 °C to +85 °C), the max delay is 44 ns under identical conditions. This ensures reliable timing margin in real-time control and synchronous bus decoding applications using SN74HCT138ANS.

Can SN74HCT138ANS be used as a demultiplexer, and how is it configured?

Yes, SN74HCT138ANS can operate as a 1-to-8 demultiplexer. Configure it by using one active-LOW enable input (e.g., E1) as the data input, holding the other two enables (E2 = LOW, E3 = HIGH) static to activate the decoder, and applying address bits A0–A2 to select the output channel. This configuration routes a single input signal to one of eight outputs - a capability explicitly confirmed in the SN74HCT138ANS functional description and supported by its truth table.

Does SN74HCT138ANS support 3.3 V logic inputs?

No, SN74HCT138ANS does not reliably accept 3.3 V logic inputs as valid HIGH signals. Its VIH minimum is 2.0 V at VCC = 4.5–5.5 V, but 3.3 V CMOS outputs (typically VOH ≈ 3.3 V) fall below the recommended 2.0 V threshold margin for robust noise immunity. For 3.3 V–5 V interfacing, SN74HC138N or level-shifting circuitry is recommended instead of SN74HCT138ANS.

What package type is used for SN74HCT138ANS, and is it RoHS compliant?

SN74HCT138ANS uses a 16-pin SOIC (SO-16) package with 3.9 mm body width and gull-wing leads (JEDEC MS-012AC). It is RoHS compliant and lead-free, meeting TI's standard environmental specifications for industrial-grade logic devices. The "NS" suffix in SN74HCT138ANS explicitly denotes the green, lead-free SOIC packaging per TI's part numbering convention.

How many enable inputs does SN74HCT138ANS have, and what are their active states?

SN74HCT138ANS has three enable inputs: E1 and E2 are active-LOW, and E3 is active-HIGH. All three must satisfy their respective logic states (E1 = LOW, E2 = LOW, E3 = HIGH) for decoding to occur; otherwise, all outputs remain HIGH. This triple-enable structure enables flexible hierarchical decoding and cascading - a defining feature confirmed in the SN74HCT138ANS functional diagram and truth table.