Texas Instruments CD74AC138M

Overview

CD74AC138M from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 package, operating from 1.5 V to 5.5 V with ±24 mA output drive, balanced propagation delays, and three enable inputs for cascading. It targets high-speed memory decoding and data-routing systems requiring sub-11 ns propagation at 5 V.

For engineers reviewing the CD74AC138M datasheet, CD74AC138M pinout, CD74AC138M application, or CD74AC138M equivalent, key selection criteria include supply voltage range (1.5–5.5 V), output drive strength (±24 mA), propagation delay (2.8–11 ns depending on VCC), enable-input logic configuration (G1 active-high, G2A/G2B active-low), and SOIC-16 thermal performance (RθJA = 106.6 °C/W).

Technical Context

The CD74AC138M implements positive-logic 3-to-8 decoding with three independent strobe inputs (G1, G2A, G2B) that must satisfy G1 = HIGH and G2A = G2B = LOW for any output activation. Its CMOS architecture delivers balanced tPLH/tPHL delays across all eight Y0–Y7 outputs.

Each output drives ±24 mA at VCC = 4.5–5.5 V, supports fanout to 15 F-type devices, and features SCR-latchup-resistant design with ≥2 kV HBM ESD rating. Input thresholds scale with VCC (VIH = 0.7×VCC, VIL = 0.3×VCC), ensuring noise immunity at 30% of supply.

Key Specifications

Pinout & Package

CD74AC138M uses a 16-pin SOIC (D) package measuring 9.9 mm × 6.0 mm (body: 9.9 mm × 3.9 mm), with standard JEDEC MS-012AC footprint and gull-wing leads. Thermal resistance is RθJA = 106.6 °C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AC138N, CD74AC138M offers identical functionality and qualification but differs in orderable part marking and tape-and-reel packaging (M96 vs N). Versus 74LVC138PW, CD74AC138M provides wider voltage range, higher temperature grade, and SOIC mechanical compatibility - critical for legacy or mixed-supply industrial boards.

Availability

CD74AC138M is available at Aetrix Electronics and suitable for memory decoding, peripheral selection, bus isolation, and digital signal routing requiring stable component supply across industrial, automotive, and test equipment programs.

Supply support for CD74AC138M 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.

The CDx4AC138 product line was designed for high-speed memory-decoding and data-routing applications demanding short propagation delays, wide supply voltage tolerance, and robust noise immunity in industrial and aerospace systems.

FAQ

What is the maximum propagation delay of the CD74AC138M at 3.3 V operation?

The CD74AC138M exhibits a maximum propagation delay of 14 ns (tPHL/tPLH) at VCC = 3.3 V ± 0.3 V, CL = 50 pF, and TA = –40°C to +85°C. This value is measured from any address or enable input to any Y output and ensures reliable timing in 3.3 V microprocessor bus interfaces. The CD74AC138M maintains balanced delays across all eight outputs, preventing skew-related setup violations.

Does the CD74AC138M support 5 V logic systems?

Yes, the CD74AC138M fully supports 5 V logic systems with guaranteed operation at VCC = 5 V ± 0.5 V. Its absolute maximum rating extends to 6 V, and its output drive capability remains ±24 mA at 4.5–5.5 V. Input thresholds scale with supply (VIH = 3.85 V min at 5.5 V), ensuring compatibility with standard TTL and CMOS 5 V families without level shifting. The CD74AC138M is widely deployed in legacy 5 V industrial controllers.

How many enable inputs does the CD74AC138M have, and what are their logic polarities?

The CD74AC138M has three enable inputs: G1 (active-high), G2A (active-low), and G2B (active-low). All three must satisfy G1 = HIGH and G2A = G2B = LOW for any Y output to activate. This triple-enable structure allows flexible cascading - for example, connecting G1 of a second CD74AC138M to Y7 of the first enables 4-to-16 decoding. The CD74AC138M datasheet confirms this behavior in Table 6-1 (Function Table).

Is the CD74AC138M pin-compatible with the CD74HC138?

No, the CD74AC138M is not pin-compatible with the CD74HC138. While both are 16-pin SOIC decoders with identical pin numbering (A–C, G1–G2B, Y0–Y7, VCC, GND), the CD74HC138 uses different enable logic: G1 is active-high, but G2 is a single active-low input combining G2A/G2B functionality. The CD74AC138M requires two separate active-low strobes (G2A and G2B), making direct substitution electrically invalid without board-level redesign.

What is the thermal resistance (RθJA) of the CD74AC138M in SOIC package?

The CD74AC138M in SOIC (D) package has a junction-to-ambient thermal resistance (RθJA) of 106.6 °C/W, as specified in Section 4.4 of the official TI datasheet (SCHS328C). This value assumes standard JEDEC 2-layer board conditions. For thermally demanding applications, designers should verify PCB copper area and airflow, as RθJA degrades significantly with reduced copper or no airflow. The CD74AC138M's 110 pF power dissipation capacitance also influences self-heating under high-frequency switching.