Texas Instruments SN74AHCT138DR

Overview

SN74AHCT138DR 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, three enable inputs (G1 active-high, G2A/G2B active-low), propagation delay as low as 1 ns at 5 V with 15 pF load, and operates across −40°C to +85°C.

For engineers reviewing the SN74AHCT138DR datasheet, SN74AHCT138DR pinout, SN74AHCT138DR application, or SN74AHCT138DR equivalent, key selection criteria include propagation delay consistency across enable and address inputs, output drive capability (±8 mA), latch-up immunity (>250 mA), and compatibility with 5-V TTL logic systems requiring precise address decoding in compact layouts.

Technical Context

The SN74AHCT138DR implements positive-logic 3-to-8 decoding with active-low outputs and hierarchical enable control: G1 must be HIGH while both G2A and G2B must be LOW for any output to activate. Its function table defines eight mutually exclusive output states based on binary inputs A, B, C - enabling direct memory chip select generation without external inversion.

Designed for minimal system decoding overhead, the device achieves sub-12 ns typical propagation delay (tPLH/tPHL) under 5 V supply and 15 pF load, with input transition rate tolerance up to 20 ns/V. Its CMOS-AHCT process ensures TTL-level input thresholds (VIH = 2 V, VIL = 0.8 V) while maintaining rail-to-rail output swing and low quiescent current (ICC ≤ 40 µA typical).

Key Specifications

Pinout & Package

SN74AHCT138DR is housed in a 16-pin SOIC (D) package measuring 9.9 mm × 6.0 mm body size, with 1.27 mm pitch gull-wing leads and 2.00 mm max height. 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 74HC138D and SN74LS138N, the SN74AHCT138DR uniquely bridges TTL input compatibility with CMOS efficiency - delivering 5-V system interoperability, 12 ns max delay, and 250 mA latch-up immunity in a standard SOIC footprint ideal for industrial upgrades and new designs.

Availability

SN74AHCT138DR is available at Aetrix Electronics and suitable for industrial control systems, memory subsystems, and programmable logic interfaces requiring stable component supply, long-term lifecycle support, and RoHS-compliant sourcing.

Supply support for SN74AHCT138DR 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 heritage in industry-standard logic families.

The SN74AHCT138DR belongs to TI's AHCT logic series - engineered specifically for high-speed memory decoding and data-routing applications where TTL compatibility, low propagation delay, and robust industrial operation are mandatory.

FAQ

What is the maximum propagation delay of the SN74AHCT138DR at 5 V supply?

The SN74AHCT138DR has a maximum propagation delay of 12 ns (tPLH/tPHL) when operating at VCC = 5 V ± 0.5 V with a 15 pF load and TA = 25°C. This value applies to transitions on address inputs (A/B/C) and enable inputs (G1, G2A, G2B) to any output (Y0–Y7), ensuring predictable timing in memory decode paths. The SN74AHCT138DR maintains this specification across its full industrial temperature range (−40°C to +85°C) with appropriate derating.

Does the SN74AHCT138DR require external pull-up resistors on its outputs?

No, the SN74AHCT138DR does not require external pull-up resistors on its Y0–Y7 outputs. Its outputs are actively driven HIGH or LOW - they are not open-drain or open-collector. Each output provides rail-to-rail CMOS-compatible voltage levels (VOH ≥ 4.4 V, VOL ≤ 0.5 V at IO = ±8 mA) when loaded within specification. External pull-ups would interfere with correct logic state assertion and are unnecessary for proper SN74AHCT138DR operation.

Can the SN74AHCT138DR be used in a 3.3-V system?

The SN74AHCT138DR is not recommended for direct use in pure 3.3-V systems. Its absolute minimum supply voltage is 1.5 V, but its TTL-compatible input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) are optimized for 5-V operation. At 3.3 V, VIH may not be reliably met by standard 3.3-V logic outputs, risking incorrect decoding. For 3.3-V systems, TI recommends the SN74LVC138A or SN74AHC138 - the SN74AHCT138DR is specified and characterized only for 4.5–5.5 V operation per its recommended conditions.

How many SN74AHCT138DR devices are needed to implement a 5-to-32 line decoder?

A 5-to-32 line decoder requires four SN74AHCT138DR devices. The two most significant address bits (A4, A3) control the enable inputs of four SN74AHCT138DR units - each enabled for a unique 8-line segment - while the three least significant bits (A2, A1, A0) drive the A/B/C inputs of all four devices in parallel. This configuration uses only one external inverter (to complement A4 or A3 if needed) and no additional logic gates, leveraging the SN74AHCT138DR's three-enable architecture for efficient expansion.

Is thermal pad connection required for the SN74AHCT138DR in SOIC package?

No, the SN74AHCT138DR in SOIC (D) package does not include a thermal pad. Thermal pad functionality is present only in the RGY (VQFN) variant of the SN74AHCT138 family. The SOIC package relies on standard lead-frame conduction and PCB copper pour for thermal management. TI specifies RθJA = 93.8°C/W for the SN74AHCT138DR, and no thermal pad soldering or grounding is applicable - attempting to connect a non-existent pad would misrepresent the SN74AHCT138DR's mechanical structure.