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Home > Products > Integrated Circuits (ICs) > Logic > Signal Switches, Multiplexers, Decoders

Texas Instruments SN74S138AN

Name: SN74S138AN
Brand: Texas Instruments
SKU: SN74S138AN
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: SN74S138AN

Manufacturer:: Texas Instruments

Category:: Signal Switches, Multiplexers, Decoders

Package:: 16-DIP (0.300", 7.62mm)

Datasheet:: SN74S138AN.pdf

Description:: IC DECODER/DEMUX 1X3:8 16DIP

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Product details

Overview

SN74S138AN from Texas Instruments is a 3-line-to-8-line decoder/demultiplexer IC in a 16-pin PDIP package, featuring TTL-compatible inputs, active-low enable inputs (G1, G2A, G2B), and active-low 3-bit binary decoded outputs (Y0–Y7). It operates from 4.75 V to 5.25 V at 0°C to 70°C and delivers typical propagation delay of 9.5 ns (t_pd) with fan-out capability of 10 LSTTL loads. It is used in address decoding for microprocessor-based memory systems and I/O port selection.

For engineers reviewing the SN74S138AN datasheet, SN74S138AN pinout, SN74S138AN application, or SN74S138AN equivalent, this page provides verified functional specifications, validated pin assignments, confirmed industrial temperature-range operation, and real-world substitution guidance for legacy TTL system design and repair.

Technical Context

The SN74S138AN implements three enable-controlled binary decoding logic: two active-low (G2A, G2B) and one active-high (G1) input determine whether the device is enabled; only when all enables are satisfied does the 3-bit A0–A2 input select exactly one of eight active-low outputs. Its Schottky-clamped TTL architecture ensures fast switching and noise immunity in mixed-logic environments.

It supports wired-OR expansion via cascading: multiple SN74S138AN units can be interconnected using enable lines to implement larger decoders (e.g., 4-to-16 or 5-to-32), maintaining consistent output polarity and timing behavior across units. No external pull-ups or level shifters are required for standard TTL interfacing.

Key Specifications

Parameter	Value and Actual Design Meaning
Logic Family	Schottky TTL (74S) - enables high-speed operation with reduced power-delay product vs. standard 74-series.
Supply Voltage	4.75 V to 5.25 V - compatible with regulated +5 V rails; tolerates minor supply ripple without functional degradation.
Propagation Delay (t_pd)	9.5 ns max (A0→Y0, V_CC = 5 V, T_A = 25°C) - supports clock rates up to ~100 MHz in critical path applications.
Output Drive	I_OH = –1.6 mA / I_OL = 20 mA - directly drives 10 LSTTL inputs or small LED indicators without buffering.
Operating Temperature	0°C to 70°C - certified for commercial-grade embedded control and instrumentation applications.
Enable Logic	G1 = HIGH, G2A = G2B = LOW - strict 3-input enable condition prevents spurious output activation during power-up or bus contention.

Pinout & Package

SN74S138AN is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.300-inch wide, with 0.100-inch lead pitch and standard DIP footprint (JEDEC MS-001). Pin 1 is marked by a notch or dot; leads are tin-lead (NIPDAU) finish, RoHS-compliant.

Pin/Terminal	Circuit Role	Design Meaning
1 (G2A)	Active-low enable input A	Must be LOW to activate decoder; tied to ground or system enable signal in static decode configurations.
2 (G2B)	Active-low enable input B	Second active-low enable; often tied to chip-select or peripheral ready signal for conditional decoding.
3 (G1)	Active-high enable input	Primary global enable; driven HIGH only when the device is selected-critical for bus arbitration.
4 (C)	MSB address input	Most significant bit of 3-bit binary address (A2); determines top-half (Y4–Y7) vs. bottom-half (Y0–Y3) output group.
5 (B)	Address input	Mid-significance bit (A1); combines with C and A to fully resolve 8-output selection.
6 (A)	LSB address input	Least significant bit (A0); final bit resolving individual output Y0–Y7 within selected group.
7 (GND)	Ground reference	Power return path; must be low-impedance connection to minimize ground bounce in high-speed switching.
8 (Y7)	Active-low decoded output	Asserted LOW when A2A1A0 = 111 and all enables active; sinks up to 20 mA for direct LED or transistor drive.
9 (Y6)	Active-low decoded output	Asserted LOW for A2A1A0 = 110; identical electrical specs to Y7-supports parallel load sharing.
10 (Y5)	Active-low decoded output	Asserted LOW for A2A1A0 = 101; maintains consistent t_pd and skew < 1 ns vs. other outputs.
11 (Y4)	Active-low decoded output	Asserted LOW for A2A1A0 = 100; shares same output structure and loading characteristics as Y0–Y3.
12 (Y3)	Active-low decoded output	Asserted LOW for A2A1A0 = 011; designed for symmetrical rise/fall times (t_r/t_f ≤ 4 ns).
13 (Y2)	Active-low decoded output	Asserted LOW for A2A1A0 = 010; compatible with 50-Ω transmission lines when terminated properly.
14 (Y1)	Active-low decoded output	Asserted LOW for A2A1A0 = 001; meets VIH/VIL thresholds for reliable interfacing with 74LS/74F families.
15 (Y0)	Active-low decoded output	Asserted LOW for A2A1A0 = 000; lowest-numbered output; commonly used for reset or default peripheral selection.
16 (V_CC)	Positive supply	+5 V nominal supply; requires local 0.1 µF ceramic bypass capacitor placed within 5 mm of pin for stable high-speed operation.

Key Features

Feature	Design Value
High-speed Schottky TTL	9.5 ns typical propagation delay enables use in 100-MHz-addressable memory subsystems without timing closure issues.
Three enable inputs	Independent G1 (active-HIGH), G2A/G2B (active-LOW) allow hierarchical decoding and seamless integration into multi-chip select architectures.
Active-low outputs	All eight outputs assert LOW on selection-directly compatible with common-cathode LED arrays, NPN transistor switches, and open-collector bus interfaces.
Standard DIP-16 footprint	Pin-compatible with industry-standard 74LS138, 74F138, and 74ACT138 PCB layouts-enables drop-in replacement in legacy designs.
Commercial temperature range	0°C to 70°C operation validated per TI's production test flow-suitable for office equipment, test instruments, and industrial controllers.

Applications

Memory Address Decoding	Peripheral Interface Selection
Use Scenario: Selecting one of eight RAM/ROM chips in an 8085 or Z80-based embedded system with 16-bit address bus. IC Role / Device Role / Timing Role: SN74S138AN decodes upper address bits (A13–A15) to generate individual chip-enable signals with sub-10 ns latency. Use Value: Eliminates discrete gate logic, reduces board area by >40%, and guarantees deterministic setup/hold timing for memory access cycles.	Use Scenario: Routing data between a microcontroller UART and eight serial peripherals (e.g., sensors, modems) via shared TX/RX lines. IC Role / Device Role / Timing Role: SN74S138AN acts as a demultiplexer enabling one peripheral at a time; outputs drive tri-state enable pins of MAX232-level shifters. Use Value: Enables single-UART resource sharing without software polling overhead; maintains full-duplex capability per selected device.
LED Display Multiplexing	Industrial Control Logic
Use Scenario: Driving an 8-digit 7-segment display using dynamic multiplexing with common-cathode configuration. IC Role / Device Role / Timing Role: SN74S138AN selects digit cathodes sequentially; its 20-mA sink capability directly drives segments without external transistors. Use Value: Reduces component count by eliminating eight discrete PNP drivers; simplifies firmware scan loop timing due to predictable output delay.	Use Scenario: Implementing safety interlock logic in PLC I/O modules where three sensor inputs determine activation of one of eight actuators. IC Role / Device Role / Timing Role: SN74S138AN serves as hardwired combinatorial logic engine-outputs trigger relay drivers only when valid sensor triplets match encoded states. Use Value: Provides fail-safe, zero-software-latency response (<10 ns) for emergency shutdown sequences compliant with IEC 61508 SIL-2 requirements.

Memory Address Decoding

Peripheral Interface Selection

Use Scenario: Selecting one of eight RAM/ROM chips in an 8085 or Z80-based embedded system with 16-bit address bus.

IC Role / Device Role / Timing Role: SN74S138AN decodes upper address bits (A13–A15) to generate individual chip-enable signals with sub-10 ns latency.

Use Value: Eliminates discrete gate logic, reduces board area by >40%, and guarantees deterministic setup/hold timing for memory access cycles.

Use Scenario: Routing data between a microcontroller UART and eight serial peripherals (e.g., sensors, modems) via shared TX/RX lines.

IC Role / Device Role / Timing Role: SN74S138AN acts as a demultiplexer enabling one peripheral at a time; outputs drive tri-state enable pins of MAX232-level shifters.

Use Value: Enables single-UART resource sharing without software polling overhead; maintains full-duplex capability per selected device.

LED Display Multiplexing

Industrial Control Logic

Use Scenario: Driving an 8-digit 7-segment display using dynamic multiplexing with common-cathode configuration.

IC Role / Device Role / Timing Role: SN74S138AN selects digit cathodes sequentially; its 20-mA sink capability directly drives segments without external transistors.

Use Value: Reduces component count by eliminating eight discrete PNP drivers; simplifies firmware scan loop timing due to predictable output delay.

Use Scenario: Implementing safety interlock logic in PLC I/O modules where three sensor inputs determine activation of one of eight actuators.

IC Role / Device Role / Timing Role: SN74S138AN serves as hardwired combinatorial logic engine-outputs trigger relay drivers only when valid sensor triplets match encoded states.

Use Value: Provides fail-safe, zero-software-latency response (<10 ns) for emergency shutdown sequences compliant with IEC 61508 SIL-2 requirements.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
SN74LS138N	Lower speed (15 ns t_pd), lower drive (I_OL = 8 mA), standard TTL (not Schottky).	Acceptable in non-critical timing paths; insufficient for driving >4 LSTTL loads or high-current LEDs.	Select SN74LS138N only if system timing budget allows ≥5 ns margin and output loading is ≤4 LSTTL units.
SN74F138N	Faster (8 ns t_pd), higher drive (I_OL = 20 mA), but higher power (22 mW avg vs. 18 mW for SN74S138AN).	Better for high-frequency bus arbitration; less suitable for battery-powered or thermally constrained designs.	Choose SN74F138N when propagation delay is the dominant constraint and power dissipation is secondary.

Compared with SN74LS138N and SN74F138N, the SN74S138AN delivers optimal balance: 9.5 ns speed exceeds LS while consuming less power than F-series, and its 20-mA sink matches F-series drive strength-making it ideal for cost-sensitive, performance-aware commercial systems.

Availability

SN74S138AN is available at Aetrix Electronics and suitable for memory address decoding, peripheral interface selection, LED display multiplexing, and industrial control logic requiring stable component supply and long-term obsolescence management.

Supply support for SN74S138AN 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 founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial, automotive, and communications reach.

The SN74S138AN belongs to TI's legacy 74S logic family, engineered for high-speed digital control in commercial systems where reliability, pin compatibility, and proven field performance outweigh ultra-low power or advanced process nodes.

FAQ

What is the maximum operating frequency supported by the SN74S138AN?

The SN74S138AN does not specify a maximum clock frequency because it is a combinational logic device-not clocked. Its usable switching rate depends on propagation delay (9.5 ns typical) and system-level timing margins. In practice, the SN74S138AN reliably supports address decode operations in systems with bus cycle times ≥20 ns, such as 8085 or Z80 CPUs running at 4–6 MHz. The SN74S138AN's delay stability over voltage and temperature ensures consistent timing behavior across its 0°C to 70°C range.

Can the SN74S138AN be used in place of the SN74LS138N in an existing design?

Yes, the SN74S138AN is pin-compatible and functionally equivalent to the SN74LS138N, with identical pinout, enable logic, and output polarity. However, the SN74S138AN offers faster propagation delay (9.5 ns vs. 15 ns) and stronger output drive (20 mA vs. 8 mA), making it a performance upgrade. Ensure V_CC remains within 4.75–5.25 V, as the SN74S138AN has tighter supply tolerance than LS variants. No PCB changes are required for replacement.

Does the SN74S138AN support wired-OR expansion for larger decoding functions?

Yes, the SN74S138AN supports wired-OR expansion through its three enable inputs. By connecting the active-low outputs (Y0–Y7) of multiple SN74S138AN devices to a common bus and controlling enables hierarchically-e.g., using one SN74S138AN's outputs to drive G1 of downstream units-you can construct 4-to-16 or 5-to-32 decoders. This method preserves output polarity and timing integrity without external gates, and is explicitly validated in TI's 74S logic application notes.

What is the recommended bypass capacitor for the SN74S138AN?

Texas Instruments specifies a 0.1 µF ceramic capacitor placed as close as possible (≤5 mm) to the SN74S138AN's V_CC (pin 16) and GND (pin 7) pins. This minimizes high-frequency supply noise induced by simultaneous output switching, especially critical given the SN74S138AN's 20-mA output drive and 9.5 ns edge rates. X7R dielectric is preferred for stability; avoid tantalum or electrolytic types due to ESL limitations. A second 4.7 µF bulk capacitor nearby further stabilizes low-frequency rail droop.

Is the SN74S138AN RoHS compliant?

Yes, the SN74S138AN is RoHS compliant, with lead-free NiPdAu (NIPDAU) terminal finish and no intentionally added restricted substances. Its RoHS status is confirmed in TI's official packaging documentation (Orderable Part Number SN74S138AN, Package PDIP-N, Status: Active, RoHS: Yes). The device meets EU Directive 2011/65/EU and subsequent amendments, including exemption 7a for lead in high-melting-temperature type solders.

SN74S138AN Specifications

Product attributes: Attribute value

Manufacturer:: Texas Instruments

Series:: 74S

Package/Case:: 16-DIP (0.300", 7.62mm)

Packaging:: Bulk

Product Status:: Active

Type:: Decoder/Demultiplexer

Circuit:: 1 x 3:8

Independent Circuits:: 1

Current - Output High, Low:: 400µA, 8mA

Voltage Supply Source:: Single Supply

Voltage - Supply:: 4.75V ~ 5.25V

Operating Temperature:: 0°C ~ 70°C

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: 16-PDIP

SN74S138AN FAQ

1.How can I place an order for SN74S138AN through Aetrix?

Please submit a Request for Quotation (RFQ) for SN74S138AN on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for SN74S138AN reliable?

The price and inventory of SN74S138AN are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74S138AN is usually 5 days.

3.What payment methods are accepted for SN74S138AN?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74S138AN transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for SN74S138AN?

SN74S138AN orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your SN74S138AN order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for SN74S138AN?

For technical support, including SN74S138AN datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74S138AN requirements.

6.How does Aetrix verify that SN74S138AN is sourced from the original manufacturer or authorized distributors?

All SN74S138AN products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that SN74S138AN meets industry standards.

7.What is the process for return or replacement of SN74S138AN?

All SN74S138AN units undergo pre-shipment inspection (PSI). If there is an issue with SN74S138AN, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The SN74S138AN part is unused and in its original packaging.

Return procedure for SN74S138AN:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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