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Texas Instruments SN74ALS138AN3
- Part No.:
- SN74ALS138AN3
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- -
- Datasheet:
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SN74ALS138AN3.pdf
- Description:
- DECODER/DRIVER, ALS SERIES, INVE
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Product details
Overview
SN74ALS138AN3 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin PDIP (N) 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 5 ns (tPLH) at VCC = 5 V, and operates across 0°C to 70°C. It is used in address decoding for static RAM systems where minimal system delay is critical.
For engineers reviewing the SN74ALS138AN3 datasheet, SN74ALS138AN3 pinout, SN74ALS138AN3 application, or SN74ALS138AN3 equivalent, key selection criteria include its ALS logic family speed-power trade-off, dual-enable architecture for cascading, TTL-compatible input thresholds, and verified operation with fast-enable memories requiring sub-20 ns decode latency.
Technical Context
The SN74ALS138AN3 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 Schottky-clamped bipolar design ensures stable propagation delay across temperature and supply voltage variations.
It delivers true 3-to-8 line decoding with all eight outputs active-low and mutually exclusive under valid enable conditions. The device supports demultiplexing by using an enable input as a data channel, and its output drive capability (IOL = 4 mA min, VOL ≤ 0.4 V) meets standard TTL fan-out requirements.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - Ensures compatibility with standard 5 V TTL/ALS power rails and noise margin stability. |
| Propagation Delay (tPLH) | 5 ns max (A/B/C → Y) - Enables use in high-speed memory systems where decode latency must be <10% of typical 60 ns SRAM access time. |
| Output Drive (IOL) | 4 mA min at VOL ≤ 0.4 V - Supports direct interfacing to multiple standard TTL inputs (fan-out ≥ 20). |
| Input Thresholds (VIH/VIL) | VIH ≥ 2.0 V, VIL ≤ 0.8 V - Guarantees robust noise immunity and interoperability with legacy TTL and ALS logic families. |
| Operating Temperature | 0°C to 70°C - Qualified for commercial-grade embedded control, instrumentation, and industrial logic subsystems. |
| Supply Current (ICC) | 10 mA max at VCC = 5.5 V - Enables predictable power budgeting in multi-device decoding trees without thermal derating. |
Pinout & Package
SN74ALS138AN3 uses a 16-pin plastic dual in-line package (PDIP-N), 300-mil width, with standard through-hole mounting and 0.1-inch pin pitch. Pin 1 is marked by a notch or dot on the package body.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Enable input | Active-low enable; must be LOW with G2B LOW and G1 HIGH for decoding to occur. |
| 2 (G2B) | Enable input | Second active-low enable; both G2A and G2B must be LOW simultaneously for device activation. |
| 3 (G1) | Enable input | Active-high enable; complements G2A/G2B to support 3-input AND logic for cascade control. |
| 4 (C) | Select input | MSB of 3-bit binary address; determines output line Y0–Y7 in conjunction with A and B. |
| 5 (B) | Select input | Middle bit of address; combined with A and C to uniquely select one of eight decoded outputs. |
| 6 (A) | Select input | LSB of address; completes 3-bit selection vector for full 8-output decoding. |
| 7 (Y0) | Output | Active-low decoded output corresponding to A=B=C=0; sinks current when selected. |
| 8 (Y1) | Output | Active-low output for A=1, B=C=0; electrically identical to Y0–Y7 with same drive specs. |
| 9 (Y2) | Output | Active-low output for A=0, B=1, C=0; shares timing and loading characteristics across all Y outputs. |
| 10 (Y3) | Output | Active-low output for A=B=1, C=0; no internal buffering differences between Y outputs. |
| 11 (Y4) | Output | Active-low output for A=C=0, B=1; maintains consistent VOL and tPHL/tPLH across all outputs. |
| 12 (Y5) | Output | Active-low output for A=1, B=0, C=1; fully interchangeable in layout and routing with other Y pins. |
| 13 (Y6) | Output | Active-low output for A=0, B=C=1; supports wired-OR expansion when multiple decoders share bus lines. |
| 14 (Y7) | Output | Active-low output for A=B=C=1; final decoded line; identical electrical behavior to Y0–Y6. |
| 15 (VCC) | Power supply | +5 V supply pin; requires local 0.1 µF ceramic bypass capacitor adjacent to pin for noise suppression. |
| 16 (GND) | Ground | Signal and power return; must be connected to low-impedance system ground plane to maintain timing integrity. |
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 board area and signal path skew. |
| ALS Schottky-clamped TTL technology | Delivers 5 ns typical propagation delay with 10 mA max ICC - balances speed and power better than standard TTL or LS families. |
| Active-low outputs with TTL-compatible drive | Sinks 4 mA at VOL ≤ 0.4 V - directly drives multiple 74-series inputs without pull-up resistors or level-shifting. |
| Binary-select inputs (A, B, C) | Supports standard address bus connection (e.g., A0–A2) for memory chip select generation in microprocessor systems. |
| Demultiplexing capability | One enable input (e.g., G1) can serve as data input while A/B/C act as channel selectors - eliminates need for separate DMUX ICs. |
Applications
| Memory Address Decoding | Bus Interface Logic |
|---|---|
Use Scenario: Selecting one of eight SRAM or ROM chips in a 64 KB memory map using A13–A15 as select lines. IC Role / Device Role / Timing Role: Primary address decoder generating individual /CS signals with sub-10 ns latency to avoid memory access bottlenecks. Use Value: Eliminates wait states in 8085/8086-based systems by ensuring decode delay is less than 15% of typical 60 ns memory access time. | Use Scenario: Routing control signals from a microcontroller's parallel port to eight peripheral devices (ADCs, DACs, GPIO expanders). IC Role / Device Role / Timing Role: Demultiplexer converting single data/control line into eight dedicated enable paths synchronized to address strobes. Use Value: Reduces I/O pin count by 7× while maintaining deterministic timing - each output activates within 6 ns of address latch edge. |
| Logic State Expansion | Industrial Control Sequencing |
Use Scenario: Expanding limited PLC output points to drive 8 solenoid valves or relay coils using discrete address bits. IC Role / Device Role / Timing Role: Level-shifting and fan-out buffer that converts 3-bit BCD commands into isolated active-low control pulses. Use Value: Provides 4 mA sink per output - sufficient to drive ULN2003 inputs or small-signal relays without added transistors. | Use Scenario: Generating timed enable windows for sequential motor start-up in HVAC or packaging machinery controllers. IC Role / Device Role / Timing Role: Synchronized decoder triggered by clock-divided counter outputs to activate stages in fixed order. Use Value: Guarantees monotonic, glitch-free transitions between states due to simultaneous enable gating - prevents race conditions in safety-critical sequencing. |
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 |
|---|---|---|---|
| SN74AS138N | Faster propagation delay (2 ns min, 10 ns max), higher IOL (20 mA), but higher ICC (14–20 mA) and stricter VIH (2.0 V min). | Better suited for ultra-fast systems (e.g., 100+ MHz bus arbitration) where SN74ALS138AN3's 5 ns delay is marginal. | Choose SN74AS138N only if system timing budget demands <10 ns worst-case decode latency and power overhead is acceptable. |
| SN74HC138N | CMOS technology, lower ICC (<8 µA), wider VCC range (2–6 V), but slower (21 ns max at 6 V) and lower drive (4 mA @ 6 V). | Ideal for battery-powered or mixed-voltage designs where power efficiency and voltage flexibility outweigh speed needs. | Select SN74HC138N when migrating from TTL to CMOS logic families or integrating with 3.3 V microcontrollers via level translation. |
Compared with SN74ALS138AN3, SN74AS138N offers 2× faster switching at higher power cost, while SN74HC138N trades speed for 1000× lower quiescent current and broader supply tolerance - making SN74ALS138AN3 the optimal balance for legacy 5 V TTL systems needing reliable, moderate-speed decoding.
Availability
SN74ALS138AN3 is available at Aetrix Electronics and suitable for memory address decoding, bus interface logic, industrial control sequencing, and logic state expansion requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS138AN3 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 SN74ALS138AN3 belongs to TI's legacy 74ALS logic family, engineered specifically for high-speed memory decoding and data-routing applications where propagation delay and TTL compatibility are critical design constraints.
FAQ
What is the maximum operating temperature range for SN74ALS138AN3?
The SN74ALS138AN3 is rated for operation from 0°C to 70°C, conforming to commercial-grade specifications. This range is validated per TI's SDAS055E datasheet and applies to all SN74-series variants in PDIP (N) and SOIC (D) packages. Operation outside this range may result in timing violations or increased ICC drift. For extended temperature applications, consider the military-grade SN54ALS138AJ (–55°C to 125°C).
Does SN74ALS138AN3 support demultiplexing functionality?
Yes, SN74ALS138AN3 supports demultiplexing by using one of its enable inputs - typically G1 - as a data input while A, B, and C serve as channel select lines. When G1 is driven with a data signal and G2A/G2B are held active-low, the selected Y output mirrors the G1 state (inverted due to active-low output). This eliminates the need for a separate demultiplexer IC in space-constrained designs.
What are the absolute maximum ratings for SN74ALS138AN3?
Per the SDAS055E datasheet, SN74ALS138AN3 has an absolute maximum VCC of 7 V, input voltage (VI) limit of 7 V, and storage temperature range of –65°C to 150°C. Exceeding these values - even momentarily - risks permanent damage. Recommended operating conditions restrict VCC to 4.5–5.5 V and ambient temperature to 0–70°C to ensure specified timing and drive performance.
Can SN74ALS138AN3 be cascaded to decode more than 8 lines?
Yes, SN74ALS138AN3 supports hierarchical cascading using its three enable inputs. Two devices can implement a 24-line decoder without external inverters by connecting G1 of downstream units to Y outputs of upstream units. A 32-line decoder requires only one external inverter - leveraging G2A/G2B polarity asymmetry to minimize glue logic and propagation delay accumulation.
Is SN74ALS138AN3 pin-compatible with SN74AS138N?
Yes, SN74ALS138AN3 and SN74AS138N share identical 16-pin PDIP (N) packaging, pinout, and terminal functions - including G1, G2A, G2B, A–C, Y0–Y7, VCC, and GND assignments. However, they differ in electrical characteristics: SN74AS138N offers faster propagation delay (10 ns max vs. 22 ns max) and higher output drive (20 mA vs. 4 mA), but draws more supply current (14–20 mA vs. 5–10 mA).
SN74ALS138AN3 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- *
- Package/Case:
- -
- Packaging:
- Bulk
- Product Status:
- Active
- Type:
- -
- Circuit:
- -
- Independent Circuits:
- -
- Current - Output High, Low:
- -
- Voltage Supply Source:
- -
- Voltage - Supply:
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- Operating Temperature:
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SN74ALS138AN3 FAQ
1.How can I place an order for SN74ALS138AN3 through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138AN3 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 SN74ALS138AN3 reliable?
The price and inventory of SN74ALS138AN3 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138AN3 is usually 5 days.
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SN74ALS138AN3 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138AN3 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 SN74ALS138AN3?
For technical support, including SN74ALS138AN3 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138AN3 requirements.
6.How does Aetrix verify that SN74ALS138AN3 is sourced from the original manufacturer or authorized distributors?
All SN74ALS138AN3 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 SN74ALS138AN3 meets industry standards.
7.What is the process for return or replacement of SN74ALS138AN3?
All SN74ALS138AN3 units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138AN3, 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 SN74ALS138AN3 part is unused and in its original packaging.
Return procedure for SN74ALS138AN3:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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