Texas Instruments SN74HC138ADB

Overview

SN74HC138ADB from Texas Instruments is a 3-to-8 line inverting decoder/demultiplexer with CMOS-level inputs, active-LOW outputs (Y0–Y7), three enable inputs (E1, E2 active LOW; E3 active HIGH), and operates across −40 °C to +85 °C. It decodes binary address inputs A0–A2 for memory chip select decoding and supports parallel expansion to 1-of-32 decoding using four ICs and one inverter.

For engineers reviewing the SN74HC138ADB datasheet, SN74HC138ADB pinout, SN74HC138ADB application, or SN74HC138ADB equivalent, key selection criteria include its TTL-compatible input thresholds (via HCT variant family context), propagation delay ≤30 ns at VCC = 4.5 V, mutual exclusivity of outputs, and SOIC-16 package compatibility with standard PCB footprints.

Technical Context

The SN74HC138ADB implements combinational logic decoding with three-stage enable gating: E1 and E2 must both be LOW while E3 is HIGH for any output to activate. Its function table defines eight mutually exclusive active-LOW outputs based on A2:A0 address state and enable conditions - no latching or clocking is involved.

Input clamp diodes allow interface to voltages exceeding VCC when used with current-limiting resistors. The device complies with JEDEC Standard No. 7A and supports demultiplexing by routing data through an enable input while using remaining enables as strobes.

Key Specifications

Pinout & Package

SN74HC138ADB is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package with 3.9 mm body width (JEDEC MS-012, TI package code D), surface-mount compatible, and RoHS compliant.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138ADB, the NXP 74HC138D offers identical form-fit-function for industrial use, while the ON Semiconductor MC74HC138ADR2 adds automotive qualification - neither requires PCB changes, but the latter supports higher ambient operating temperatures.

Availability

SN74HC138ADB is available at Aetrix Electronics and suitable for memory address decoding, peripheral I/O selection, LED segment driver control, and industrial PLC input matrix applications requiring stable component supply and long-term industrial lifecycle support.

Supply support for SN74HC138ADB 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 ICs, with decades of legacy in industry-standard 74-series logic families.

The SN74HC138ADB belongs to TI's HC (High-Speed CMOS) logic portfolio, designed for low-power, pin-compatible replacements of TTL devices in industrial control, instrumentation, and computing subsystems.

FAQ

What is the maximum propagation delay of the SN74HC138ADB at 5 V supply?

The SN74HC138ADB has a maximum propagation delay of 30 ns at VCC = 4.5 V and CL = 50 pF, and 26 ns at VCC = 6.0 V under the same load. At exactly 5.0 V with CL = 15 pF, typical delay is 12 ns - all values are specified in the official TI datasheet and validated across temperature ranges.

Can the SN74HC138ADB be used as a demultiplexer?

Yes, the SN74HC138ADB can function as an 1-to-8 demultiplexer: apply the data signal to E1 (active LOW), use A0–A2 as address inputs, and hold E2 LOW and E3 HIGH. This exploits the enable-input gating structure to route one input across eight outputs - confirmed in TI's functional description and application notes.

Does the SN74HC138ADB support 3.3 V logic systems?

Yes, the SN74HC138ADB operates down to 2.0 V supply and meets CMOS input thresholds at 3.3 V: VIH ≥ 2.31 V and VIL ≤ 1.09 V (per TI datasheet SLFS065). Its outputs swing rail-to-rail, making it fully interoperable with 3.3 V microcontrollers and FPGAs without level shifters.

What is the meaning of "inverting" in SN74HC138ADB's function description?

"Inverting" means all eight outputs (Y0–Y7) are active LOW: when a specific address is selected and enables are satisfied, that output goes LOW while all others remain HIGH. This matches standard memory chip-select polarity and eliminates need for external inverters in most decoding applications.

Is the SN74HC138ADB pin-compatible with the 74HCT138 variant?

Yes, the SN74HC138ADB is pin-compatible with the 74HCT138 family (e.g., SN74HCT138DBR), sharing identical SOIC-16 pinout and function table. However, the HCT version uses TTL-compatible input thresholds (VIH ≈ 2.0 V), while SN74HC138ADB uses CMOS thresholds - direct substitution requires verifying host system logic levels.