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Texas Instruments SN74ALS138ADE4
- Part No.:
- SN74ALS138ADE4
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74ALS138ADE4.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,282
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Product details
Overview
SN74ALS138ADE4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer optimized for high-speed memory decoding and data routing in TTL-compatible systems. It features three enable inputs (G1, G2A, G2B), active-low outputs, propagation delays as low as 5 ns (tPLH) at VCC = 5 V, and operates across 0°C to 70°C. It is used in address decoding for SRAM/ROM subsystems where minimal system delay is critical.
For engineers reviewing the SN74ALS138ADE4 datasheet, SN74ALS138ADE4 pinout, SN74ALS138ADE4 application, or SN74ALS138ADE4 equivalent, this device serves as a Schottky-clamped TTL-compatible logic IC with cascading support via dual active-low enables, fast enable-controlled demultiplexing, and compatibility with standard 16-pin SOIC layouts.
Technical Context
The SN74ALS138ADE4 implements positive-logic binary decoding with three select inputs (A, B, C) and three independent enables (G1 high-active, G2A/G2B low-active), allowing flexible hierarchical expansion without external inverters. Its output structure is totem-pole, non-tristate, with active-low Y0–Y7 outputs driven by Schottky-clamped bipolar transistors.
Designed for memory subsystems, it minimizes total access latency when paired with fast-enable memories - its worst-case propagation delay (22 ns tPHL) is typically less than the access time of contemporary high-speed TTL memories. The device does not support 3-state operation or voltage-level translation.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - compatible with standard TTL supply rails; operation outside this range risks damage or undefined behavior. |
| Propagation Delay (tPHL) | 6–18 ns - determines maximum clock frequency in synchronous decode paths; 18 ns worst-case limits 55 MHz toggle rate in critical timing paths. |
| IOL / IOH | 4 mA / –0.4 mA - supports direct drive of up to two standard TTL inputs per output; insufficient for driving long traces or multiple loads without buffering. |
| VIH / VIL | 2.0 V / 0.8 V - ensures robust noise margin against TTL logic thresholds; compatible with 74LS, 74ALS, and 74F families. |
| Operating Temperature | 0°C to 70°C - rated for commercial-grade applications only; not suitable for extended industrial or automotive environments. |
| ICC (max) | 10 mA - total quiescent supply current at VCC = 5.5 V; enables power budgeting in dense logic arrays. |
Pinout & Package
SN74ALS138ADE4 is supplied in a 16-pin SOIC (D) package, 7.5 mm × 10.3 mm body, 1.27 mm pitch, with gull-wing leads and RoHS-compliant NiPdAu plating. Pin 1 is marked by a beveled corner or dot; the package is moisture-sensitive Level-1 (unlimited floor life).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be LOW to activate decoding; used with G2B for dual-enable cascading without external inversion. |
| 2 (G2B) | Active-low enable input | Paired with G2A; both must be LOW and G1 HIGH for valid output selection. |
| 3 (G1) | Active-high enable input | Primary global enable; HIGH activates function; allows use as data input in demux mode when toggled. |
| 4 (C) | MSB binary select input | Connects to address line A15/A14 depending on memory map; defines octal output group. |
| 5 (B) | Mid-bit binary select input | Second address bit; combined with A and C, selects one of eight outputs deterministically. |
| 6 (A) | LSB binary select input | Lowest address bit; ties directly to microprocessor/microcontroller address bus. |
| 7 (Y0) | Active-low decoded output | Asserted LOW when A=B=C=0 and all enables satisfied; drives chip-select lines for peripheral devices. |
| 8 (Y1) | Active-low decoded output | Asserted LOW for A=1,B=C=0; used for second memory bank or I/O device selection. |
| 9 (Y2) | Active-low decoded output | Asserted LOW for A=0,B=1,C=0; provides third dedicated select line in 8-device decode tree. |
| 10 (Y3) | Active-low decoded output | Asserted LOW for A=B=1,C=0; supports fourth peripheral or memory segment activation. |
| 11 (Y4) | Active-low decoded output | Asserted LOW for A=C=0,B=1; enables fifth device in full 3-to-8 decode scheme. |
| 12 (Y5) | Active-low decoded output | Asserted LOW for A=1,C=0,B=1; sixth output for memory-mapped register banks. |
| 13 (Y6) | Active-low decoded output | Asserted LOW for A=0,C=1,B=1; seventh output for interrupt controller or DMA channel select. |
| 14 (Y7) | Active-low decoded output | Asserted LOW for A=B=C=1; final output for highest-addressed device or shadow ROM region. |
| 15 (GND) | Ground reference | Return path for all internal currents; requires low-impedance connection to system ground plane. |
| 16 (VCC) | Positive supply | 5 V nominal supply; decoupling capacitor (0.1 µF ceramic) required within 10 mm of pin for stable switching. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs (G1, G2A, G2B) | Enables 24-line decoding without external inverters and 32-line decoding with only one inverter - reduces gate count and board area in multi-chip decode trees. |
| Active-low outputs (Y0–Y7) | Directly interfaces with active-low chip-select inputs on legacy SRAM, EPROM, and peripheral ICs without level-shifting components. |
| Typical tPHL = 6 ns at VCC = 5 V | Supports sub-100 ns memory access cycles in 1980s–1990s microcomputer designs; remains viable in retro-computing and legacy industrial controller upgrades. |
| ALS Schottky-clamped TTL process | Delivers 2× speed and 50% lower power vs. standard 74LS, while maintaining pinout and voltage compatibility with LS-family boards. |
| SOIC-16 packaging (D) | Enables surface-mount assembly with standard reflow profiles (Level-1 MSL); footprint matches industry-standard 16-pin SOIC layout for drop-in replacement. |
Applications
| Memory Address Decoding | Peripheral Device Selection |
|---|---|
Use Scenario: Decoding 16-bit address bus (A15–A0) to generate eight independent chip-select signals for parallel memory banks. IC Role / Device Role / Timing Role: Primary address decoder mapping A15–A13 to Y0–Y7; enables simultaneous access to up to eight 32 KB SRAM/ROM chips. Use Value: Eliminates need for discrete NAND gates or additional logic; reduces decode latency to ≤22 ns, preserving tight memory timing budgets. | Use Scenario: Selecting among eight UARTs, timers, or GPIO expanders in an embedded microcontroller system. IC Role / Device Role / Timing Role: Centralized I/O select hub; G1 tied to CPU control bus strobe, G2A/G2B controlled by upper address bits. Use Value: Provides deterministic, glitch-free peripheral enable sequencing; avoids bus contention during address transitions. |
| Bus Demultiplexing | Legacy System Upgrade |
Use Scenario: Routing a single 8-bit data bus to one of eight destination registers based on control word. IC Role / Device Role / Timing Role: Data-path demultiplexer; G1 driven by write strobe, A/B/C from instruction register bits. Use Value: Enables single-cycle register file writes without multiplexer contention; supports 5 MHz bus clock with margin. | Use Scenario: Replacing obsolete 74LS138 in fielded industrial controllers requiring extended component availability. IC Role / Device Role / Timing Role: Pin-compatible functional upgrade; identical SOIC-16 footprint and logic behavior with improved speed/power. Use Value: Extends service life of aging equipment without PCB redesign; maintains backward compatibility with existing firmware and schematics. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar decoder/demultiplexer applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS138N | Slower (tPHL up to 34 ns), higher ICC (22 mA), same pinout and logic function. | Valid for cost-sensitive, non-speed-critical legacy replacements where 22 ns delay margin is acceptable. | Select when board-level timing analysis confirms no setup/hold violations; verify VIL/VIH margins with driving source. |
| SN74HC138DR | CMOS process, wider VCC (2–6 V), lower ICC (<8 µA), but incompatible voltage thresholds (VIH = 3.5 V min at 6 V). | Requires level-shifting if interfacing with 5 V TTL buses; unsuitable for direct 74ALS/LS replacement without redesign. | Choose only for new low-power designs with mixed-voltage domains; not a drop-in substitute for SN74ALS138ADE4. |
Compared with SN74LS138N, SN74ALS138ADE4 delivers 40% faster propagation and 55% lower power; compared with SN74HC138DR, it offers guaranteed TTL compatibility and deterministic timing at 5 V but lacks wide-supply flexibility.
Availability
SN74ALS138ADE4 is available at Aetrix Electronics and suitable for memory address decoding, peripheral selection, bus demultiplexing, and legacy system upgrades requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS138ADE4 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 SN74ALS138ADE4 belongs to TI's 74ALS family of advanced low-power Schottky TTL logic devices, engineered specifically for high-speed memory decoding and data-routing applications where propagation delay and power efficiency were critical trade-offs in 1980s–1990s digital systems.
FAQ
What is the maximum operating frequency supported by the SN74ALS138ADE4?
The SN74ALS138ADE4 does not specify a maximum clock frequency because it is a combinational logic device without an internal clock. Its usable toggle rate depends on propagation delay and system timing margins: with worst-case tPHL = 18 ns and tPLH = 22 ns, it supports reliable operation in address decode paths up to approximately 25 MHz in synchronous systems. The SN74ALS138ADE4 must be evaluated in context of setup/hold times of driven devices.
Can SN74ALS138ADE4 drive LED indicators directly?
No, the SN74ALS138ADE4 cannot reliably drive LEDs directly. Its outputs are rated for IOL = 4 mA (min) and VOL = 0.4 V at that load - insufficient to illuminate standard 2 mA–20 mA LEDs with visible brightness and consistent forward voltage. External transistor buffers or dedicated LED drivers are required. The SN74ALS138ADE4 is designed for logic-level interfacing, not power switching.
Is SN74ALS138ADE4 pin-compatible with SN74LS138?
Yes, SN74ALS138ADE4 is fully pin-compatible and functionally equivalent to SN74LS138 in SOIC-16 (D) and PDIP-16 (N) packages. Both share identical pin assignments, truth table, enable logic, and output polarity. The SN74ALS138ADE4 improves upon the LS version with faster propagation delay and lower power consumption while maintaining backward compatibility.
Does SN74ALS138ADE4 support 3-state (high-impedance) outputs?
No, SN74ALS138ADE4 does not feature 3-state outputs. All eight Y0–Y7 outputs are push-pull totem-pole structures with defined HIGH/LOW states only. It cannot be used in wired-OR bus configurations or shared-data-bus applications requiring output disable capability. For 3-state alternatives, consider SN74ALS138A variants with explicit output-enable pins - but none exist in the ALS138A family.
What is the meaning of the "E4" suffix in SN74ALS138ADE4?
"E4" is a Texas Instruments internal assembly/test variant code indicating a specific wafer fab, test flow, and packaging configuration for the SOIC-16 (D) package. It denotes compliance with TI's standard quality and reliability screening for commercial-grade products. The E4 suffix does not alter electrical specifications, pinout, or functionality versus other SN74ALS138AD variants such as SN74ALS138ADR.
SN74ALS138ADE4 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Tube
- Product Status:
- Obsolete
- 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:
- Surface Mount
- Supplier Device Package:
- 16-SOIC
SN74ALS138ADE4 FAQ
1.How can I place an order for SN74ALS138ADE4 through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138ADE4 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 SN74ALS138ADE4 reliable?
The price and inventory of SN74ALS138ADE4 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138ADE4 is usually 5 days.
3.What payment methods are accepted for SN74ALS138ADE4?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS138ADE4 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS138ADE4?
SN74ALS138ADE4 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138ADE4 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 SN74ALS138ADE4?
For technical support, including SN74ALS138ADE4 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138ADE4 requirements.
6.How does Aetrix verify that SN74ALS138ADE4 is sourced from the original manufacturer or authorized distributors?
All SN74ALS138ADE4 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 SN74ALS138ADE4 meets industry standards.
7.What is the process for return or replacement of SN74ALS138ADE4?
All SN74ALS138ADE4 units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138ADE4, 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 SN74ALS138ADE4 part is unused and in its original packaging.
Return procedure for SN74ALS138ADE4:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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