Texas Instruments SN74AHCT138PW

Overview

SN74AHCT138PW from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin TSSOP package, featuring TTL-compatible inputs, three enable inputs (G1 active-high, G2A/G2B active-low), and eight active-low decoded outputs (Y0–Y7). It operates from 1.5 V to 5.5 V, delivers ±8 mA output drive, and achieves 1 ns to 12 ns propagation delays at 5 V with 15 pF load - optimized for high-speed memory decoding and address demultiplexing in industrial control and embedded systems.

For engineers reviewing the SN74AHCT138PW datasheet, SN74AHCT138PW pinout, SN74AHCT138PW application, or SN74AHCT138PW equivalent, key selection criteria include its triple-enable architecture for cascaded decoding, TTL-voltage compatibility without level shifters, latch-up immunity (>250 mA), and ESD robustness (±2000 V HBM), making it suitable for noise-prone industrial PCBs requiring reliable address routing.

Technical Context

The SN74AHCT138PW implements positive-logic decoding with strict input threshold compatibility: VIH = 2.0 V min and VIL = 0.8 V max at VCC = 4.5–5.5 V, ensuring interoperability with legacy TTL logic families. Its functional modes are fully defined by the three enable inputs - only when G1 = HIGH and G2A = G2B = LOW does decoding activate, selecting one of eight outputs based on A/B/C inputs per the truth table.

Propagation delay is asymmetrically specified across input groups: tPLH/tPHL for select inputs (A/B/C) is 7.6–12 ns, while enable inputs (G1, G2A/G2B) exhibit faster response (6.6–11 ns), enabling precise timing control in hierarchical decode trees. All outputs are actively driven low during selection and high-impedance otherwise - no internal pull-ups or open-drain configuration.

Key Specifications

Pinout & Package

Packaged in a 16-pin Thin Shrink Small Outline Package (TSSOP-PW), body size 5.00 mm × 4.40 mm, 0.65 mm pitch, with exposed thermal pad not electrically connected (no thermal pad terminal listed for PW variant).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AHCT138PW, SN74HC138PWR trades speed and TTL compatibility for ultra-low static current, while 74LVC138APW,118 offers superior speed and footprint at the cost of 5 V operation - making SN74AHCT138PW the sole choice for mixed-voltage or legacy-TTL-coupled industrial systems.

Availability

SN74AHCT138PW is available at Aetrix Electronics and suitable for industrial control systems, embedded memory subsystems, and programmable logic peripheral selection requiring stable component supply and long-term manufacturability.

Supply support for SN74AHCT138PW 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 heritage in industry-standard logic families.

The SN74AHCT series was designed specifically for high-speed memory decoding and data-routing applications where short propagation delay and TTL compatibility are essential - targeting industrial, automotive, and communications infrastructure.

FAQ

What is the recommended operating voltage range for SN74AHCT138PW?

The SN74AHCT138PW operates reliably from 1.5 V to 5.5 V DC. At VCC = 5.0 V, it guarantees full TTL-compatible input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and ±8 mA output drive. Operation below 1.5 V is not characterized and may result in undefined logic states or increased propagation delay. Always reference the Recommended Operating Conditions table in the official SN74AHCT138PW datasheet for temperature-dependent limits.

Can SN74AHCT138PW be used as a demultiplexer?

Yes, SN74AHCT138PW functions as a 1-to-8 demultiplexer when the enable inputs (G1, G2A, G2B) are configured to activate decoding, and one input (e.g., G1) is used as the data line. With A/B/C held constant, asserting the data input on G1 causes the selected Yx output to mirror the data state (active-low), enabling time-multiplexed signal distribution. This mode is explicitly supported in TI's application notes for SN74AHCT138PW.

Does SN74AHCT138PW require external pull-up resistors on its outputs?

No, SN74AHCT138PW features push-pull (totem-pole) outputs that actively drive both HIGH and LOW states. Each output can source up to −8 mA and sink up to +8 mA at VCC = 5 V, eliminating the need for external pull-ups. Adding pull-up resistors would interfere with guaranteed VOH/VOL specifications and is not recommended unless interfacing with open-drain systems - which is not a standard use case for SN74AHCT138PW.

How many SN74AHCT138PW devices are needed to decode a 5-bit address bus?

Two SN74AHCT138PW devices are required to decode a 5-bit address bus into 32 unique outputs. The most significant bit (A4) selects between the two decoders: when A4 = LOW, the first SN74AHCT138PW activates outputs Y0–Y7; when A4 = HIGH, the second SN74AHCT138PW activates its Y0–Y7. TI's SN74AHCT138PW datasheet illustrates this exact 32-line decoding scheme in Figure 7-2.

Is SN74AHCT138PW pin-compatible with other 16-pin TSSOP logic devices?

SN74AHCT138PW uses the standard 16-pin TSSOP (PW) footprint with 0.65 mm pitch and 5.00 mm × 4.40 mm body size, matching JEDEC MO-153. It is mechanically compatible with other PW-packaged TI logic (e.g., SN74AHCT00PW, SN74AHCT04PW), but electrical pinout is unique to the '138 function. Substitution requires verifying signal mapping - no functional or pin-for-pin compatibility exists with non-decoder devices.