Texas Instruments SN74AHCT138RGYR

Overview

SN74AHCT138RGYR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin VQFN package (4 mm × 3.5 mm), designed for high-speed memory decoding and data-routing applications. It features TTL-voltage-compatible inputs, three enable inputs (G1 active-high, G2A/G2B active-low), propagation delays as low as 1 ns at 5 V with 15 pF load, and operates from 1.5 V to 5.5 V across −40°C to +85°C.

For engineers reviewing the SN74AHCT138RGYR datasheet, SN74AHCT138RGYR pinout, SN74AHCT138RGYR application, or SN74AHCT138RGYR equivalent, key selection criteria include its 16-pin VQFN thermal performance (RθJA = 39°C/W), latch-up immunity (>250 mA), ±2000-V HBM ESD rating, and compatibility with legacy TTL logic levels while operating from a single 5-V supply.

Technical Context

The SN74AHCT138RGYR implements positive-logic 3-to-8 decoding with three independent enable controls that support cascading without external inverters-enabling 24-line decoding natively and 32-line decoding with only one external inverter. Its internal architecture routes binary-select inputs (A, B, C) to eight active-low outputs (Y0–Y7) only when all enables are asserted per the function table.

It uses advanced AHCT CMOS technology to achieve TTL-level input thresholds (VIH = 2 V min, VIL = 0.8 V max) while delivering rail-to-rail output swing (VOH ≥ 4.4 V at IOH = −50 mA, VOL ≤ 0.1 V at IOL = 50 mA) and fast switching (tPLH/tPHL ≤ 12 ns at CL = 15 pF, VCC = 5 V).

Key Specifications

Pinout & Package

VQFN-16 (RGY) package with 4 mm × 3.5 mm body size, 0.5-mm pitch, and exposed thermal pad for enhanced power dissipation. Pin 1 marked by top-side dot; pin numbering follows standard counter-clockwise convention from top-left corner.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT138DR and 74LV138PW, the SN74AHCT138RGYR delivers superior thermal performance in a compact VQFN package while retaining full 5-V TTL compatibility and the unique active-high G1 enable-making it optimal for high-density, high-speed industrial control boards requiring robust legacy interface support.

Availability

SN74AHCT138RGYR is available at Aetrix Electronics and suitable for industrial control systems, memory subsystems, and embedded peripheral management requiring stable component supply, RoHS-compliant packaging, and long-term production continuity.

Supply support for SN74AHCT138RGYR 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 50 years of innovation in high-reliability logic interfaces and timing products.

The SN74AHCT138RGYR belongs to TI's AHCT logic family, engineered specifically for high-speed memory decoding and data-routing applications where TTL compatibility, low propagation delay, and robust ESD/latch-up performance are critical.

FAQ

What is the maximum clock frequency supported by SN74AHCT138RGYR in demultiplexer mode?

The SN74AHCT138RGYR is not a clocked device and has no defined clock frequency. Its operation is purely combinational: output state changes within 12 ns (max) after input transitions at 5 V with 15-pF load. For reliable 8:1 data routing, ensure input setup/hold times exceed 3 ns and avoid violating the 20 ns/V input transition rate limit. The SN74AHCT138RGYR supports effective data rates up to ~40 MHz in synchronous demux applications.

Does SN74AHCT138RGYR require external pull-up resistors on its Y0–Y7 outputs?

No, SN74AHCT138RGYR features push-pull CMOS outputs capable of actively driving HIGH or LOW without external pull-ups. Each output sources up to 8 mA (IOH) and sinks up to 8 mA (IOL) at rated voltage. Pull-ups are unnecessary unless interfacing with open-drain buses or implementing wired-OR logic-scenarios not supported by the SN74AHCT138RGYR's output structure.

Can SN74AHCT138RGYR operate reliably at 3.3 V supply voltage?

Yes, SN74AHCT138RGYR is fully specified from 1.5 V to 5.5 V, including 3.3 V. At VCC = 3.3 V, VIH remains 2.0 V (min) and VIL stays 0.8 V (max), ensuring compatibility with 3.3-V LVTTL and LVCMOS logic. Output drive is reduced (IOH/IOL ≈ ±6 mA), but propagation delay increases only modestly (tPLH/tPHL ≤ 14 ns at CL = 15 pF), maintaining functional integrity in mixed-voltage systems.

How does the thermal pad on the SN74AHCT138RGYR package affect PCB layout?

The exposed thermal pad on the SN74AHCT138RGYR (RGY) must be soldered to a dedicated PCB copper pour connected to GND for optimal thermal performance (RθJA = 39°C/W). Use at least 9 thermal vias (0.3-mm diameter, spaced ≤1 mm apart) under the pad to conduct heat to inner or bottom-layer planes. Omitting pad connection raises junction temperature by >25°C at full output loading, risking parametric drift or reliability degradation.

Is SN74AHCT138RGYR pin-compatible with older SN74LS138 devices?

No, SN74AHCT138RGYR is not pin-compatible with SN74LS138. While both are 16-pin 3-to-8 decoders with identical pin functions (A–C, G1/G2A/G2B, Y0–Y7, VCC, GND), the SN74LS138 uses a 16-pin PDIP (N) or SOIC (D) package with 0.3-inch or 7.5-mm width, whereas SN74AHCT138RGYR uses a 4 mm × 3.5 mm VQFN (RGY) with 0.5-mm pitch and thermal pad. Footprint, soldering, and thermal design differ fundamentally.