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Texas Instruments SN74AS138N
- Part No.:
- SN74AS138N
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74AS138N.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16DIP
- Quantity:
- Payment:
- Shipping:
Inventory:4,902
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Product details
Overview
SN74AS138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin PDIP package, designed for high-speed memory decoding and data routing. It features three enable inputs (G1, G2A, G2B), active-low outputs, propagation delays as low as 2 ns (tPLH/tPHL), and operates over 0°C to 70°C with 4.5–5.5 V supply. It is used in address decoding for SRAM/ROM systems where minimal system delay is critical.
For engineers reviewing the SN74AS138N datasheet, SN74AS138N pinout, SN74AS138N application, or SN74AS138N equivalent, key selection criteria include propagation delay consistency across enable and select inputs, output drive capability (20 mA sink), TTL-compatible input thresholds, and compatibility with legacy 74-series logic families in through-hole designs.
Technical Context
The SN74AS138N implements positive-logic binary decoding with three independent enable controls-G1 (active-high) and G2A/G2B (both active-low)-enabling flexible cascading without external inverters. Its Schottky-clamped bipolar architecture delivers guaranteed sub-10 ns propagation delays under loaded conditions (CL = 50 pF, RL = 500 Ω).
It supports demultiplexing by using an enable input as a data line, with all eight outputs asserted low when enabled. Output logic states follow strict truth-table behavior per input combinations, with no 3-state or open-collector functionality-outputs are standard totem-pole.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - Ensures stable operation with standard 5 V TTL power rails and tolerates typical supply ripple. |
| Propagation Delay (tPHL/tPLH) | 2 ns (min) to 10 ns (max) - Enables use in systems with >100 MHz clock domain timing margins. |
| Output Sink Current (IOL) | 20 mA - Drives multiple TTL inputs or small LEDs directly without buffering. |
| Input Thresholds (VIH/VIL) | VIH = 2.0 V, VIL = 0.8 V - Fully compatible with standard TTL logic levels and noise margins. |
| Operating Temperature | 0°C to 70°C - Qualified for commercial-grade embedded control and industrial board-level applications. |
| Supply Current (ICCL) | 20 mA (max) - Predictable dynamic power draw aids thermal design in dense logic arrays. |
Pinout & Package
SN74AS138N uses a 16-pin plastic dual in-line package (PDIP-N), 300-mil width, with 0.1-inch lead pitch and through-hole mounting. Pin 1 is marked by a notch or dot at the top-left corner of the package body.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be low to activate decoding; used with G2B and G1 for hierarchical enable control. |
| 2 (G2B) | Active-low enable input | Second active-low enable; both G2A and G2B must be low for function enable. |
| 3 (G1) | Active-high enable input | Primary enable; high enables device when G2A/G2B are low. |
| 4 (C) | MSB binary select input | Connects to address bus A2 or equivalent MSB for 3-bit decode mapping. |
| 5 (B) | Mid-bit binary select input | Connects to address bus A1; forms 3-bit select code with A and C. |
| 6 (A) | LSB binary select input | Connects to address bus A0; determines Y0–Y7 output activation per truth table. |
| 7 (Y0) | Active-low decoded output | Asserted low when A=B=C=0 and enables active; drives downstream enable or chip-select lines. |
| 8 (Y1) | Active-low decoded output | Asserted low when A=1, B=C=0; one of eight mutually exclusive outputs. |
| 9 (Y2) | Active-low decoded output | Asserted low when B=1, A=C=0; supports discrete peripheral selection in microcontroller systems. |
| 10 (Y3) | Active-low decoded output | Asserted low when A=B=1, C=0; used in memory bank switching or I/O port expansion. |
| 11 (Y4) | Active-low decoded output | Asserted low when C=1, A=B=0; enables dedicated functions like interrupt routing or status flag latching. |
| 12 (Y5) | Active-low decoded output | Asserted low when A=1,C=1,B=0; provides deterministic output assignment for fixed-address peripherals. |
| 13 (Y6) | Active-low decoded output | Asserted low when B=1,C=1,A=0; supports multi-chip select schemes with shared bus arbitration. |
| 14 (Y7) | Active-low decoded output | Asserted low only when A=B=C=1; final output in 3-to-8 decode sequence. |
| 15 (GND) | Ground reference | Power return path; must be connected to system ground plane with low-inductance trace. |
| 16 (VCC) | Positive supply | 5 V DC supply input; requires local 0.1 µF ceramic decoupling capacitor near pin. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs (G1, G2A, G2B) | Enables direct 24-line decoding without external inverters and reduces gate count in multi-level decode trees. |
| Sub-10 ns propagation delay | Minimizes added latency in high-speed memory access paths-delays are shorter than typical fast SRAM access times. |
| TTL-compatible input thresholds | Ensures interoperability with legacy 74LS/74F logic and microcontroller GPIO without level-shifting. |
| 20 mA output sink capability | Drives up to 10 standard TTL loads or interfaces directly with optocouplers and small-signal transistors. |
| Standard PDIP-16 footprint | Supports manual prototyping, socket-based testing, and legacy PCB rework without redesign. |
Applications
| Memory Address Decoding | Peripheral Selection in Microcontroller Systems |
|---|---|
Use Scenario: Selecting individual SRAM or ROM chips within a 8-KB address space using A0–A12 bus lines. IC Role / Device Role / Timing Role: SN74AS138N decodes upper address bits (e.g., A13–A15) to generate chip-select signals with deterministic timing. Use Value: Eliminates race conditions during memory access due to guaranteed <10 ns delay matching microprocessor bus timing. | Use Scenario: Assigning unique chip-select lines to UART, SPI flash, and ADC peripherals on an 8-bit MCU bus. IC Role / Device Role / Timing Role: SN74AS138N acts as a static peripheral selector, asserting one active-low CS line per decoded address window. Use Value: Reduces firmware overhead by enabling hardware-based peripheral isolation without software polling or GPIO bit-banging. |
| Legacy System Bus Expansion | Industrial Control Logic Sequencing |
Use Scenario: Adding ISA-compatible I/O cards to retro-computing platforms requiring TTL-level address decoding. IC Role / Device Role / Timing Role: SN74AS138N serves as a drop-in replacement for obsolete 74LS138 in vintage bus architectures. Use Value: Maintains pinout and timing compatibility while improving speed and noise immunity over LS-family predecessors. | Use Scenario: Controlling sequencing of relay drivers, sensor multiplexers, and indicator LEDs in PLC backplane modules. IC Role / Device Role / Timing Role: SN74AS138N generates synchronized enable pulses for discrete output stages based on control register bits. Use Value: Provides deterministic, glitch-free activation of safety-critical outputs via hardwired logic instead of software state machines. |
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 |
|---|---|---|---|
| SN74ALS138AN | Slower propagation delay (max 22 ns), lower IOL (8 mA), ALS-family Schottky TTL. | Suitable for cost-sensitive, lower-speed systems where power efficiency is prioritized over speed. | Select SN74ALS138AN only if timing budget allows ≥20 ns delay and load current ≤8 mA. |
| SN74HC138N | CMOS technology, wider VCC range (2–6 V), higher fan-out, but slower (max 29 ns @ 6 V) and non-TTL input thresholds. | Used in mixed-voltage or battery-powered systems; requires level translation when interfacing with 5 V TTL buses. | Choose SN74HC138N for low-power or wide-supply applications-but verify input compatibility and add translation if driving from TTL sources. |
Compared with SN74ALS138AN and SN74HC138N, the SN74AS138N delivers the fastest propagation performance and strongest drive strength in a TTL-compatible 5 V PDIP package, making it optimal for legacy speed-critical decoding where signal integrity and timing predictability are paramount.
Availability
SN74AS138N is available at Aetrix Electronics and suitable for memory address decoding, microcontroller peripheral selection, legacy bus expansion, and industrial control logic sequencing requiring stable component supply and long-term obsolescence management.
Supply support for SN74AS138N 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 communications portfolio coverage.
The SN74AS138N belongs to TI's 74AS family of advanced Schottky TTL logic devices, engineered specifically for high-speed memory decoding and data transmission systems where propagation delay and output drive are critical.
FAQ
What is the maximum operating frequency supported by the SN74AS138N?
The SN74AS138N does not specify a maximum clock frequency because it is a combinational logic device-not clocked. Its usable speed is determined by propagation delay: with tPHL/tPLH ≤ 10 ns (max), it supports address transitions up to approximately 100 MHz in well-designed systems with matched trace lengths and proper termination. The SN74AS138N remains functional across its full 0°C to 70°C range under these conditions.
Can the SN74AS138N be used as a demultiplexer?
Yes-the SN74AS138N can function as a 1-to-8 demultiplexer by using one enable input (e.g., G1) as the data input while holding the other two enables (G2A, G2B) statically active. When G2A and G2B are low and G1 toggles, the selected Yx output mirrors the G1 logic state (inverted due to active-low output). This configuration is explicitly supported in the datasheet and requires no external components. The SN74AS138N maintains full timing specifications in this mode.
Is the SN74AS138N RoHS compliant?
Yes, the SN74AS138N is RoHS compliant, as confirmed in TI's official packaging addendum: "RoHS: Yes", with lead finish specified as NIPDAU (nickel/palladium/gold). It meets EU Directive 2011/65/EU requirements and is suitable for commercial and industrial products sold in regulated markets. The SN74AS138N carries no exemptions and uses fully lead-free terminations.
Does the SN74AS138N have 3-state outputs?
No, the SN74AS138N does not feature 3-state outputs. All eight Y0–Y7 outputs are standard totem-pole, actively driven high or low depending on decode state. There is no output-enable control separate from the three enable inputs (G1, G2A, G2B), and outputs go high-impedance only when the device is disabled-i.e., when any enable condition fails. The SN74AS138N differs from 3-state variants like SN74LS138N in this fundamental architectural aspect.
What is the recommended bypass capacitor for the SN74AS138N?
Texas Instruments recommends a 0.1 µF ceramic capacitor placed as close as possible between VCC (pin 16) and GND (pin 15) to suppress high-frequency supply noise. For boards with multiple SN74AS138N devices or high-current loads, a bulk 4.7–10 µF tantalum or aluminum electrolytic capacitor should supplement the local 0.1 µF unit at the power entry point. This ensures stable operation of the SN74AS138N under fast switching conditions and prevents ground bounce-induced glitches.
SN74AS138N Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74AS
- 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:
- 4.5V ~ 5.5V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 16-PDIP
SN74AS138N FAQ
1.How can I place an order for SN74AS138N through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS138N 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 SN74AS138N reliable?
The price and inventory of SN74AS138N are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS138N is usually 5 days.
3.What payment methods are accepted for SN74AS138N?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AS138N transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AS138N?
SN74AS138N orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS138N 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 SN74AS138N?
For technical support, including SN74AS138N datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS138N requirements.
6.How does Aetrix verify that SN74AS138N is sourced from the original manufacturer or authorized distributors?
All SN74AS138N 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 SN74AS138N meets industry standards.
7.What is the process for return or replacement of SN74AS138N?
All SN74AS138N units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS138N, 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 SN74AS138N part is unused and in its original packaging.
Return procedure for SN74AS138N:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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