Texas Instruments SN74HCS251DR

Overview

SN74HCS251DR from Texas Instruments is an 8-to-1 CMOS multiplexer with Schmitt-trigger inputs and complementary 3-state outputs, operating across 2 V to 6 V. It selects one of eight data inputs (D0–D7) using binary address lines A/B/C, drives bus systems via high-drive ±7.8-mA outputs at 6 V, and supports industrial temperature range (–40°C to +125°C). Used in digital signal routing where noise immunity and bus isolation are critical.

For engineers reviewing the SN74HCS251DR datasheet, SN74HCS251DR pinout, SN74HCS251DR application, or SN74HCS251DR equivalent, key selection criteria include Schmitt-trigger hysteresis (ΔVT = 0.6–1.6 V), 3-state enable timing (ten = 6 ns max at 6 V), low ICC (0.1 µA typical), and SOIC-16 package compatibility with legacy board layouts.

Technical Context

The SN74HCS251DR implements full binary decoding for 1-of-8 selection with strobe-controlled complementary outputs Y (non-inverted) and W (inverted), both placed in high-impedance state when OE is high. Its Schmitt-trigger inputs provide hysteresis (VT+ − VT−) of 0.6 V min at 6 V, enabling reliable operation with slow-rising or noisy control signals.

It features balanced CMOS 3-state outputs capable of sourcing/sinking ±7.8 mA at 6 V while maintaining VOH ≥ 5.4 V and VOL ≤ 0.33 V under load. Propagation delay is 6 ns typical (A/B/C → Y/W) at 6 V with 50-pF load, supporting up to 36 MHz switching frequency.

Key Specifications

Pinout & Package

SN74HCS251DR is housed in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm, optimized for automated assembly and thermal dissipation in dense PCB layouts.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-to-1 multiplexer applications.

Compared with SN74HCS251DR, SN74HC251N trades manufacturability for prototyping flexibility, while MC74HC251AD offers supply-chain redundancy at minor speed cost-both require verification of thermal performance and timing margins in final layout.

Availability

SN74HCS251DR is available at Aetrix Electronics and suitable for industrial data acquisition, automotive sensor interfaces, programmable logic configuration, and legacy bus expansion requiring stable component supply across extended temperature ranges.

Supply support for SN74HCS251DR 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 for industrial, automotive, and communications markets.

The SN74HCS251DR belongs to TI's HCS logic family, designed specifically for low-power, noise-immune digital signal routing in harsh environments where reliability and wide supply tolerance are mandatory.

FAQ

What is the maximum clock frequency supported by the SN74HCS251DR?

The SN74HCS251DR supports a maximum switching frequency of 36 MHz at 6 V supply, as specified in its switching characteristics table. This value assumes CL = 50 pF and operation within recommended conditions (–40°C to +125°C). At 2 V, fmax drops to 5 MHz due to reduced drive strength and slower internal propagation. Designers must verify timing margins against actual load capacitance and temperature extremes in their application.

Does the SN74HCS251DR require external pull-up resistors on its address inputs?

No, the SN74HCS251DR does not require external pull-up resistors on address inputs A, B, or C because its Schmitt-trigger inputs tolerate slow or floating transitions without unintended state changes. However, TI recommends tying unused inputs to VCC or GND to prevent undefined logic states. For actively driven address lines, no pull-ups are needed-only proper termination if traces exceed 10 cm or run near noise sources.

Can the SN74HCS251DR drive a 100-pF capacitive load reliably?

The SN74HCS251DR is characterized for 50-pF loads per datasheet; driving 100-pF will increase propagation delay and transition times beyond guaranteed limits. At 6 V, tt rises from 4 ns (typical) to ~12 ns, and tpd increases by ~50%. While functional, this degrades timing margins and may cause setup/hold violations in high-speed systems. For >50-pF loads, add a buffer stage or reduce trace length and parallel capacitance.

How does the Schmitt-trigger input hysteresis improve noise immunity in the SN74HCS251DR?

The SN74HCS251DR provides input hysteresis (ΔVT) of 0.6 V minimum at 6 V, meaning the rising threshold (VT+) is 2.1 V and falling threshold (VT−) is 1.5 V. This 600-mV window rejects noise spikes smaller than ±300 mV peak-to-peak, preventing false triggering on address or enable lines exposed to EMI from motors, relays, or RF sources-critical in industrial and automotive environments.

What is the purpose of the complementary W output in the SN74HCS251DR?

The W output in the SN74HCS251DR is the logical inverse of Y and shares the same 3-state control via OE. This enables single-chip implementation of true/complement data paths-e.g., driving differential receivers, generating active-low enable signals, or simplifying bus arbitration logic without adding external inverters. Both outputs switch simultaneously with matched propagation delays, ensuring timing coherence in dual-rail applications.