Texas Instruments SN74AHCT138NSR

Overview

SN74AHCT138NSR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOP-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 voltage.

For engineers reviewing the SN74AHCT138NSR datasheet, SN74AHCT138NSR pinout, SN74AHCT138NSR application, or SN74AHCT138NSR equivalent, this page provides verified functional modes, real-world timing behavior under CL = 15 pF and CL = 50 pF loads, confirmed ESD robustness (±2000 V HBM), latch-up immunity (>250 mA), and precise SOP-16 mechanical dimensions (10.2 mm × 7.8 mm body).

Technical Context

The SN74AHCT138NSR implements positive-logic 3-to-8 decoding with three independent enable controls-G1 (active-high) and G2A/G2B (both active-low)-enabling flexible cascading without external inverters. Its function table defines eight mutually exclusive active-low outputs (Y0–Y7), where only one output goes low per valid input combination when all enables are asserted.

It uses advanced AHCT logic architecture optimized for speed-power trade-offs in memory systems: propagation delay tPLH/tPHL is specified down to 1 ns (min) at VCC = 5 V and CL = 15 pF, while maintaining CMOS-level quiescent current (ICC ≤ 40 µA typical) and TTL-compatible input thresholds for seamless interface with legacy controllers.

Key Specifications

Pinout & Package

SOP-16 package (NS), body size 10.20 mm × 5.30 mm, overall outline 10.2 mm × 7.8 mm, 1.27 mm pitch, 2.00 mm max height. RoHS-compliant, NIPDAU lead finish, MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 74HC138PW,118 and SN74LV138APWR, the SN74AHCT138NSR uniquely bridges TTL and CMOS domains with guaranteed 2 V VIH at 5 V supply, delivers faster worst-case timing (12 ns vs. 21 ns), and maintains full 1.5–5.5 V operation-making it the only choice for mixed-signal industrial controllers requiring both legacy compatibility and sub-15 ns decode latency.

Availability

SN74AHCT138NSR is available at Aetrix Electronics and suitable for industrial control panels, memory subsystems, and automated test equipment requiring stable component supply, long-term lifecycle support, and RoHS-compliant packaging.

Supply support for SN74AHCT138NSR 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 delivering analog, embedded processing, and connectivity solutions for industrial, automotive, and communications markets.

The SN74AHCT138NSR belongs to TI's industry-standard 74AHCT logic family, engineered specifically for high-speed memory decoding and data routing where TTL compatibility, low propagation delay, and robust ESD/latch-up performance are critical.

FAQ

What is the maximum clock frequency supported by SN74AHCT138NSR in demultiplexing mode?

The SN74AHCT138NSR is not a clocked device and has no internal clock-it operates asynchronously. Its usable switching frequency depends on propagation delay and load capacitance. With tPD ≤ 12 ns (max) at CL = 15 pF, the SN74AHCT138NSR supports clean signal routing up to ~40 MHz in edge-triggered applications, assuming proper PCB layout and termination. For higher-frequency use, verify setup/hold margins against your controller's timing budget using the SN74AHCT138NSR's published switching characteristics.

Can SN74AHCT138NSR drive LEDs directly without current-limiting resistors?

No. While the SN74AHCT138NSR can sink up to 8 mA per output (IOL = 8 mA), driving LEDs directly risks exceeding absolute maximum ratings and damaging the output stage. Each Yx output must be paired with a series current-limiting resistor calculated for the target LED forward voltage and desired current. For example, with a 5 V supply and red LED (VF ≈ 1.8 V), a 390 Ω resistor limits current to ~8.2 mA - within SN74AHCT138NSR's safe operating range.

Does SN74AHCT138NSR require external pull-up resistors on its Y0–Y7 outputs?

Yes, when used in active-low wired-OR configurations or driving high-impedance inputs. The SN74AHCT138NSR outputs are push-pull, not open-drain, but its Y0–Y7 outputs are active-low. To interface with logic that expects active-high enables (e.g., most memory CE# pins), external pull-up resistors (typically 4.7 kΩ to VCC) are required to generate a HIGH default state. Without pull-ups, outputs float HIGH when inactive - potentially causing unintended device activation.

Is SN74AHCT138NSR pin-compatible with older 74LS138 devices?

Yes, the SN74AHCT138NSR shares identical SOP-16 pinout and function table with 74LS138, including A/B/C inputs, G1/G2A/G2B enables, and Y0–Y7 active-low outputs. However, SN74AHCT138NSR offers superior specs: lower ICC (<40 µA vs. ~20 mA), faster tPD (12 ns vs. 25 ns), and higher noise immunity. No PCB changes are needed for drop-in replacement, but verify VCC decoupling and input drive strength match LS-family requirements.

What is the thermal resistance (RθJA) of SN74AHCT138NSR in its SOP-16 package?

The junction-to-ambient thermal resistance RθJA for SN74AHCT138NSR in SOP-16 (NS) package is 64°C/W, measured under JEDEC-standard conditions (1-layer board, 1 in² copper pad). This value assumes standard PCB layout with adequate copper pour on VCC/GND planes. At 8 mA output loading and 5 V supply, typical power dissipation remains below 25 mW - well within safe thermal limits even in enclosed industrial enclosures up to +85°C ambient.