Texas Instruments SN74AHCT138D

Overview

SN74AHCT138D from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 package, designed for high-speed memory decoding and data-routing applications. It features TTL-voltage-compatible inputs (VIH = 2 V, VIL = 0.8 V), three enable inputs (G1 active-high, G2A/G2B active-low), and delivers propagation delays as low as 1 ns at 5 V with 15 pF load. It operates across −40°C to +85°C and supports 1.5–5.5 V supply range.

For engineers reviewing the SN74AHCT138D datasheet, SN74AHCT138D pinout, SN74AHCT138D application, or SN74AHCT138D equivalent, key selection criteria include its triple-enable architecture for cascaded decoding, ±8 mA output drive capability, latch-up immunity >250 mA, ESD protection per JESD22 (±2000 V HBM), and compatibility with legacy TTL logic levels in mixed-voltage systems.

Technical Context

The SN74AHCT138D implements positive-logic 3-to-8 decoding with active-low outputs (Y0–Y7) and three independent enable controls-enabling flexible hierarchical decoding without external inverters. Its functional table defines eight decoded outputs based on binary-select inputs A/B/C and the logical AND of G1 (high) and G2A/G2B (both low).

Designed specifically for minimizing system decoding delay in high-performance memory systems, the device achieves sub-12 ns typical propagation delay (tPLH/tPHL) under 5 V/15 pF conditions. Its input transition rate tolerance (20 ns/V) and low power dissipation capacitance (14 pF) support stable operation in fast-switching digital interfaces.

Key Specifications

Pinout & Package

SN74AHCT138D is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package with 9.9 mm × 6.0 mm body size and 1.27 mm lead pitch. The package is RoHS-compliant, lead-finished with NiPdAu, and rated MSL Level-1 (unlimited floor life at ≤30°C/60% RH).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138N and 74LV138PW, the SN74AHCT138D uniquely bridges 5 V TTL and modern CMOS logic domains while delivering sub-12 ns timing-making it the optimal choice for retrofitting legacy systems or designing dual-voltage controllers without level shifters.

Availability

SN74AHCT138D is available at Aetrix Electronics and suitable for industrial control systems, memory subsystems, and peripheral interface modules requiring stable component supply and long-term manufacturability.

Supply support for SN74AHCT138D 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 decades of expertise in high-reliability industrial and automotive ICs.

The SN74AHCT138D belongs to TI's AHCT logic family-engineered for TTL-compatible operation across wide voltage ranges and optimized for high-speed, low-glitch decoding in memory and data-path applications.

FAQ

What is the maximum recommended supply voltage for SN74AHCT138D?

The absolute maximum supply voltage for SN74AHCT138D is 7 V, but the recommended operating range is 1.5 V to 5.5 V. Operating above 5.5 V risks exceeding electrical characteristics limits and may degrade reliability. TI specifies 5.5 V as the upper bound for guaranteed performance across temperature and process corners, and all published timing and drive specs assume VCC ≤ 5.5 V. For sustained operation, maintain VCC within this range.

Does SN74AHCT138D support cascading for larger decoding schemes?

Yes, SN74AHCT138D supports cascading via its three enable inputs: G1 (active-high) and G2A/G2B (both active-low). By connecting outputs of a master decoder to these enables, up to 24-line decoding is achievable without external inverters, and 32-line decoding requires only one inverter. This architecture eliminates additional logic gates and preserves timing integrity across stages-confirmed in TI's application schematics (Figures 7-1 and 7-2).

Can SN74AHCT138D be used as a demultiplexer?

Yes, SN74AHCT138D functions as a 1-to-8 demultiplexer when G1 is used as the data input and A/B/C serve as select lines. In this mode, the selected output mirrors the state of G1 (active-high), while all other outputs remain HIGH. This behavior is explicitly documented in TI's functional description and verified in the device's truth table-enabling simple data routing in serial-to-parallel conversion or signal distribution circuits.

What is the thermal resistance (RθJA) of SN74AHCT138D in SOIC package?

The junction-to-ambient thermal resistance (RθJA) for SN74AHCT138D in the SOIC (D) package is 93.8°C/W, as specified in TI's Thermal Information table (Section 4.4). This value assumes standard JEDEC PCB mounting conditions (2-layer board, 1 in² copper pad). Actual thermal performance depends on PCB layout, airflow, and copper area-TI recommends using a 0.1 µF bypass capacitor near VCC and avoiding floating inputs to minimize dynamic power dissipation.

Are unused inputs on SN74AHCT138D required to be terminated?

Yes, all unused inputs on SN74AHCT138D must be tied to a defined logic level-either VCC or GND-to prevent floating states that cause increased ICC, erratic outputs, or ESD susceptibility. TI mandates this in Section 7.3.1 and the Recommended Operating Conditions table. Leaving inputs unconnected violates the device's functional specification and may result in unpredictable enable behavior or excessive current draw, especially under temperature extremes.