Texas Instruments SN74AS158D

Overview

SN74AS158D from Texas Instruments is a high-speed 2-line-to-1-line data selector/multiplexer with inverted output logic, designed for TTL-compatible digital systems. It features four independent 2:1 multiplexed channels, buffered inputs/outputs, a common strobe (G) enable, and operates across 0°C to 70°C with 4.5–5.5 V supply. It delivers sub-11 ns propagation delay (tPHL/tPLH) on data paths and supports bus-oriented routing in industrial control logic.

For engineers reviewing the SN74AS158D datasheet, SN74AS158D pinout, SN74AS158D application, or SN74AS158D equivalent, key selection criteria include its inverted-output architecture, 20 mA sink capability, SOIC-16 package compatibility, and guaranteed performance under 50 pF load at 5 V.

Technical Context

The SN74AS158D implements four independent 2:1 multiplexers sharing a single select line (A/B) and global strobe (G), with all outputs inverted to minimize propagation delay versus non-inverting variants like SN74AS157D. Its internal structure includes Schottky-clamped bipolar transistor logic for improved speed and noise immunity.

It uses standard TTL input thresholds (VIH = 2 V, VIL = 0.8 V), drives loads up to 20 mA low and 2 mA high, and maintains full functionality with CL = 50 pF and RL = 500 Ω - confirming suitability for medium-speed bus gating and address/data path selection in legacy 5 V systems.

Key Specifications

Pinout & Package

SN74AS158D is housed in a 16-pin SOIC (D) package per JEDEC MS-012, 7.5 mm body width, 1.75 mm max height, with 1.27 mm pitch and gull-wing leads. Pin 1 identifier located at top-left corner with beveled edge marking.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar quad 2:1 multiplexer applications.

Compared with SN74ALS158D, SN74AS158D provides 2.5× faster propagation and 2.5× higher sink current but consumes ~50% more quiescent power; versus SN74AS157D, it offers identical speed and drive but requires logic-level inversion in downstream signal paths.

Availability

SN74AS158D is available at Aetrix Electronics and suitable for industrial PLC I/O expansion, legacy microprocessor address decoding, and test equipment signal routing requiring stable component supply and long-term obsolescence management.

Supply support for SN74AS158D 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 and automotive qualification.

The SN74AS series was developed for high-speed TTL-compatible logic applications demanding sub-10 ns propagation in 5 V systems, targeting industrial control, instrumentation, and legacy computing infrastructure.

FAQ

What is the function of the G (strobe) pin on the SN74AS158D?

The G pin on the SN74AS158D is an active-low strobe enable that gates all four multiplexer outputs. When G = high, all Y outputs are forced high (inactive); when G = low, outputs reflect the selected A/B data with inversion. This allows synchronous bus arbitration and prevents glitches during select transitions - a critical feature in the SN74AS158D's role as a controlled data router.

Does the SN74AS158D support mixed-voltage interfacing, such as 3.3 V logic inputs?

No, the SN74AS158D does not support reliable 3.3 V logic inputs. Its VIH minimum is 2.0 V and VIL maximum is 0.8 V under 4.5–5.5 V operation, optimized for 5 V TTL levels. Driving it with 3.3 V signals risks marginal high-level recognition and increased static power; level-shifting circuitry is required for interoperability with 3.3 V systems - a constraint inherent to the SN74AS158D's bipolar TTL architecture.

How does the inverted output behavior of the SN74AS158D affect timing-critical designs?

Because the SN74AS158D inherently inverts all outputs, timing-critical designs must account for this polarity in setup/hold calculations and downstream latch triggering. For example, if feeding an active-high clocked register, the inverted output may require complementary clock edge sampling or an external inverter - unlike the SN74AS157D, which preserves polarity. This inversion is fixed in the SN74AS158D silicon and cannot be disabled.

Can the SN74AS158D replace the SN74LS158 in existing designs?

The SN74AS158D can replace SN74LS158 in most 5 V designs but requires validation of timing margins and power delivery. While pin-compatible and functionally identical, the SN74AS158D offers ~3× faster propagation (5.5 ns vs 15 ns typical) and higher drive (20 mA vs 8 mA), potentially causing overshoot or increased ground bounce. Layout review and decoupling reinforcement are recommended before drop-in substitution of SN74AS158D.

What is the maximum capacitive load the SN74AS158D can drive while maintaining specified timing?

The SN74AS158D's switching characteristics are explicitly characterized at CL = 50 pF, and timing parameters (e.g., tPHL = 1–5.5 ns) are guaranteed only under those conditions. Driving >50 pF increases propagation delay nonlinearly and may violate setup/hold windows. For loads exceeding 50 pF, buffer staging or transmission-line layout techniques are required - a hard limit defined in the SN74AS158D datasheet and not adjustable via bias or supply tuning.