Texas Instruments SN74AS138NSR

Overview

SN74AS138NSR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer optimized for high-speed memory decoding and data routing, featuring 2–10 ns propagation delay (tPHL/tPLH), 20 mA output sink current, and operation across 0°C to 70°C. It uses active-low G2A/G2B and active-high G1 enables to simplify cascading without external inverters and supports demultiplexing via enable-as-data-input functionality in systems such as address decoding for SRAM or FPGA configuration buses.

For engineers reviewing the SN74AS138NSR datasheet, SN74AS138NSR pinout, SN74AS138NSR application, or SN74AS138NSR equivalent, this page delivers verified electrical specs, SOP-16 package details, functional truth table behavior, real-world timing constraints for high-speed bus interfacing, and validated drop-in alternatives for legacy TTL-compatible logic design.

Technical Context

The SN74AS138NSR implements positive-logic binary decoding with three independent enable inputs (G1, G2A, G2B) that collectively gate all eight Y0–Y7 outputs - only one output goes low per valid A/B/C input combination when enables are satisfied. Its Schottky-clamped bipolar architecture ensures fast switching while maintaining TTL-compatible voltage thresholds (VIH = 2 V, VIL = 0.8 V).

Propagation delays are tightly specified: tPHL/tPLH from select inputs (A/B/C) to any Y output is ≤10 ns at VCC = 5 V, CL = 50 pF; enable-to-output delays (G1/G2 → Y) are ≤10 ns. Output drive capability (IOL = 20 mA, IOH = –2 mA) supports direct interfacing with multiple TTL loads or pull-up resistors in wired-OR configurations.

Key Specifications

Pinout & Package

SOP-16 (NS) package: 16-pin small-outline plastic package with gull-wing leads, 1.27 mm pitch, 10.4 mm × 5.3 mm body size, 2.00 mm max height, and pin 1 identifier in quadrant Q1 per tape-and-reel specification.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AS138NSR, SN74ALS138ANSR trades speed for lower power and noise immunity, while SN74HC138DR offers voltage flexibility at the cost of drive strength and TTL compatibility - making SN74AS138NSR optimal for high-speed, 5 V-only, noise-tolerant decoding where 20 mA drive is required.

Availability

SN74AS138NSR is available at Aetrix Electronics and suitable for industrial control panels, test equipment front-ends, and legacy computer system upgrades requiring stable component supply, long-term obsolescence mitigation, and guaranteed RoHS-compliant sourcing.

Supply support for SN74AS138NSR 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and aerospace qualification.

The SN74AS138NSR belongs to TI's 74AS logic family - engineered for high-speed, low-noise TTL-compatible operation in memory subsystems and real-time control applications where sub-10 ns timing and 20 mA drive are essential.

FAQ

What is the maximum operating frequency supported by the SN74AS138NSR?

The SN74AS138NSR does not specify a maximum clock frequency because it is a combinational logic device without internal clocking. Its usable speed is determined by propagation delay: with tPHL/tPLH ≤10 ns, it supports reliable operation in systems where input changes occur no faster than ~50 MHz (20 ns period), assuming proper setup/hold margins and load capacitance ≤50 pF. The SN74AS138NSR is typically deployed in address decode and static control paths rather than synchronous clock domains.

Can the SN74AS138NSR be used as a demultiplexer, and how?

Yes, the SN74AS138NSR can operate as a 1-to-8 demultiplexer by using one of its enable inputs - typically G1 - as the data input, while A, B, and C serve as the 3-bit channel select lines. When G1 is driven with the incoming serial data stream and G2A/G2B are held low, the data appears inverted on the selected Yx output. This configuration is documented in the function table and widely applied in legacy bus architectures for routing control signals. The SN74AS138NSR supports this mode without external components.

Is the SN74AS138NSR pin-compatible with the SN74LS138N?

No, the SN74AS138NSR is not pin-compatible with the SN74LS138N. While both are 16-pin 3-to-8 decoders with identical logic functions and pin assignments for A, B, C, G1, G2A, G2B, Y0–Y7, VCC, and GND, the SN74LS138N uses a PDIP-16 (N) package with 0.3" width and through-hole leads, whereas the SN74AS138NSR uses a SOP-16 (NS) surface-mount package with 0.3" width but gull-wing leads and 1.27 mm pitch. Physical mounting and PCB layout differ; adapter boards or redesign are required for substitution.

What is the meaning of "NS" in SN74AS138NSR?

The "NS" in SN74AS138NSR denotes the SOP (Small Outline Package) variant with a 16-pin, 1.27 mm lead pitch, 10.4 mm × 5.3 mm body, and gull-wing terminations - standardized under TI's NS0016A package drawing. The "R" suffix indicates tape-and-reel packaging (2000 units per reel), RoHS compliance, and NIPDAU lead finish. This distinguishes it from PDIP (N), SOIC (D), or ceramic DIP (J) variants of the same logic function.

Does the SN74AS138NSR require external pull-up resistors on its outputs?

No, the SN74AS138NSR features totem-pole (push-pull) outputs capable of actively driving both high and low logic states. Its VOH is specified at VCC – 2 V (≥3 V at VCC = 5 V) and VOL ≤0.5 V at IOL = 20 mA, eliminating the need for external pull-ups in standard TTL interfacing. Pull-ups are only required if open-collector behavior is desired - which the SN74AS138NSR does not support natively. Using pull-ups would degrade speed and increase power consumption unnecessarily.