Texas Instruments CD74AC138E

Overview

CD74AC138E from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in PDIP-16 package, operating across 1.5V–5.5V supply range with ±24mA output drive, 2.8ns typical propagation delay at 5V, and ESD rating of ±2000V HBM. It serves as a high-speed memory address decoder in industrial control systems requiring low-power, noise-immune logic translation.

For engineers reviewing the CD74AC138E datasheet, CD74AC138E pinout, CD74AC138E application, or CD74AC138E equivalent, this page delivers verified functional modes, real-world timing behavior at 1.5V/3.3V/5V, thermal resistance (RθJA = 67°C/W), and validated alternative options for memory decoding and data routing designs.

Technical Context

The CD74AC138E implements positive-logic 3-to-8 decoding with three active-low enable inputs (G1, G2A, G2B) that must all be asserted low for any output activation. Its Schmitt-trigger inputs provide balanced noise immunity at 30% of VCC, enabling reliable operation in electrically noisy environments.

Propagation delays are tightly balanced across all eight outputs (Y0–Y7), with tPLH/tPHL ≤10ns at 5V and CL = 50pF. The device supports cascading via enable inputs and features 3-state outputs-driven actively high/low when enabled, high-impedance when disabled-making it suitable for bus-oriented data routing.

Key Specifications

Pinout & Package

CD74AC138E uses a 16-pin Plastic Dual In-line Package (PDIP-N), body size 19.3mm × 6.35mm, with through-hole mounting and standard 0.3-inch row spacing. Thermal performance is optimized for natural convection cooling in legacy industrial PCB layouts.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AC138N and CD74HC138E, CD74AC138E delivers superior extended-temperature reliability (–55°C to +125°C), faster propagation (11ns vs 21ns), and stronger output drive (±24mA vs ±5.2mA), making it optimal for mission-critical industrial and automotive decoding where timing margin and ruggedness are non-negotiable.

Availability

CD74AC138E is available at Aetrix Electronics and suitable for industrial PLCs, automotive body control modules, legacy instrumentation backplanes, and medical diagnostic equipment requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74AC138E 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 90 years of innovation in high-reliability components for industrial, automotive, and aerospace markets.

CD74AC138E belongs to TI's Advanced CMOS (AC) logic family, engineered for high-speed memory decoding and data-routing systems demanding low power, wide voltage operation, and robust noise immunity in harsh environments.

FAQ

What is the maximum operating frequency supported by CD74AC138E?

The CD74AC138E does not specify a maximum clock frequency directly, but its propagation delay (tPLH/tPHL ≤11ns at 5V) implies reliable operation up to ~45MHz in decode-enable cycles. For continuous data demultiplexing, system-level timing analysis must account for setup/hold times of downstream devices. The CD74AC138E itself remains functional across its full 1.5V–5.5V supply and –55°C to +125°C temperature range.

Does CD74AC138E support mixed-voltage interfacing, such as 3.3V inputs driving a 5V-powered device?

Yes, CD74AC138E supports mixed-voltage operation: its input thresholds scale with VCC (VIH = 0.7×VCC, VIL = 0.3×VCC), so 3.3V logic inputs are fully compatible when VCC = 5V. The device meets VIH minimum of 3.85V at VCC = 5.5V, ensuring robust recognition of 3.3V-high signals. No level-shifting circuitry is needed for this common interface scenario.

How does the three-enable architecture (G1, G2A, G2B) improve system design versus dual-enable decoders?

The three-enable structure of CD74AC138E allows independent control of decode activation across hierarchical address spaces-for example, using G1 for major subsystem selection and G2A/G2B for minor bank selection. This eliminates external AND gates in cascaded configurations and enables precise power gating: all outputs enter high-Z simultaneously when any enable is deasserted, reducing leakage in standby states.

Is thermal pad connection required for CD74AC138E in PDIP package?

No, the CD74AC138E in PDIP (N) package has no thermal pad; that feature applies only to the WQFN (BQB) variant. The PDIP version relies on leads and PCB copper for heat dissipation. Its RθJA of 67°C/W assumes standard JEDEC test board conditions-designers should verify junction temperature using Pd = ICC × VCC + Σ(VOL × IOL) under worst-case loading.

Can CD74AC138E be used as a 1-of-8 data demultiplexer, and what are the key timing considerations?

Yes, CD74AC138E functions as a 1-of-8 demultiplexer when data is applied to an enable input (e.g., G1) and address bits to A/B/C. Critical timing parameters include tPLZ/tPHZ (enable-to-output disable delay ≤10ns at 5V) and tPZL/tPZH (enable-to-output enable delay ≤10ns). These ensure clean data routing without glitches during channel switching in real-time signal distribution systems.