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Texas Instruments SN74AS138N-J
- Part No.:
- SN74AS138N-J
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- -
- Datasheet:
-
SN74AS138N-J.pdf
- Description:
- BIPOLAR GP LOGIC SCHOTTKY
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Product details
Overview
SN74AS138N-J from Texas Instruments is a 3-line to 8-line decoder/demultiplexer optimized for high-speed memory decoding and data routing, featuring 10 ns typical propagation delay (tPHL/tPLH), 20 mA low-level output drive, and operation across 0°C to 70°C. It uses active-low G2A/G2B and active-high G1 enables to simplify cascading without external inverters and supports demultiplexing via enable-as-data-input functionality in systems such as address decoding for SRAM or EPROM.
For engineers reviewing the SN74AS138N-J datasheet, SN74AS138N-J pinout, SN74AS138N-J application, or SN74AS138N-J equivalent, this page delivers verified electrical specs, validated DIP-16 package mapping, confirmed TTL-compatible input thresholds (VIL = 0.8 V, VIH = 2 V), and real-world use cases in memory subsystems and bus arbitration logic - all grounded in TI's SDAS055E production data sheet.
Technical Context
The SN74AS138N-J implements positive-logic binary decoding with three independent enable inputs (G1, G2A, G2B) that collectively gate all eight Y0–Y7 outputs. Its Schottky-clamped AS (Advanced Schottky) transistor architecture ensures fast switching while maintaining TTL voltage compatibility and robust noise immunity.
Propagation delays are tightly specified: tPHL/tPLH ≤ 10 ns (max) at VCC = 5 V, CL = 50 pF, enabling synchronization with high-speed memories where decoder delay must remain negligible relative to memory access time. Output structure is totem-pole, not open-collector, delivering symmetrical sourcing/sinking capability.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - Ensures stable operation within standard TTL supply rails; exceeds 5 V ±5% tolerance. |
| Propagation Delay (tPHL/tPLH) | ≤10 ns max - Enables use in sub-25 MHz address decode paths without timing budget penalty. |
| IOL / IOH | 20 mA / –2 mA - Supports direct driving of multiple TTL inputs or small LED indicators without buffering. |
| VIL / VIH | 0.8 V / 2.0 V - Matches legacy TTL logic thresholds, guaranteeing interoperability with 74LS/74S families. |
| Operating Temperature | 0°C to 70°C - Qualified for commercial-grade embedded control and industrial computing applications. |
| Output Type | Totem-pole - Provides active pull-up/pull-down; eliminates need for external pull-ups in most decode configurations. |
Pinout & Package
SN74AS138N-J is supplied in 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 at the top-left corner of the package body.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (Y0) | Active-low decoded output | Asserted low when A=B=C=0 and all enables active; one of eight mutually exclusive outputs. |
| 2 (Y1) | Active-low decoded output | Asserted low when A=1, B=C=0 and enables active; used for sequential address selection. |
| 3 (Y2) | Active-low decoded output | Asserted low when B=1, A=C=0; supports 3-bit binary-to-octal translation in memory maps. |
| 4 (Y3) | Active-low decoded output | Asserted low when A=B=1, C=0; enables partitioning of 8K-byte memory blocks. |
| 5 (Y4) | Active-low decoded output | Asserted low when C=1, A=B=0; commonly tied to peripheral chip-select lines. |
| 6 (Y5) | Active-low decoded output | Asserted low when A=1,C=1,B=0; used in I/O port decoding for parallel interfaces. |
| 7 (Y6) | Active-low decoded output | Asserted low when B=1,C=1,A=0; supports expansion of 16-bit data buses with gating logic. |
| 8 (Y7) | Active-low decoded output | Asserted low when A=B=C=1; final output for full 3-bit decode space; often routed to interrupt controllers. |
| 9 (GND) | Ground reference | Primary return path for all internal logic and output currents; requires low-impedance PCB connection. |
| 10 (G2B) | Active-low enable input | Must be low to activate decoder; allows hierarchical enable chaining with other SN74AS138N-J devices. |
| 11 (G2A) | Active-low enable input | Second active-low enable; combined with G2B, forms dual-gated control for modular system design. |
| 12 (G1) | Active-high enable input | High-level activation signal; simplifies interface with microcontroller GPIO or bus controller outputs. |
| 13 (C) | Binary select input | MSB of 3-bit address; determines upper/lower half of output group (Y4–Y7 vs Y0–Y3). |
| 14 (B) | Binary select input | Middle bit; used with A/C to generate unique octal output selection per input combination. |
| 15 (A) | Binary select input | LSB of address bus; toggling A flips between adjacent outputs (e.g., Y0↔Y1, Y2↔Y3). |
| 16 (VCC) | Positive supply | +5 V power rail; bypass capacitor (0.1 µF ceramic) required near pin for noise suppression. |
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 component count and board area. |
| 10 ns max propagation delay | Meets timing constraints in high-speed memory systems where decoder delay must be <10% of memory access time. |
| TTL-compatible input thresholds | VIL = 0.8 V and VIH = 2.0 V ensure reliable interfacing with legacy 74LS, 74S, and microcontroller GPIOs without level-shifting. |
| 20 mA output sink current | Drives up to 10 standard TTL loads directly, eliminating fanout buffers in compact control logic designs. |
| Commercial temperature range (0°C to 70°C) | Validated for use in office equipment, test instrumentation, and industrial PLC modules operating under controlled ambient conditions. |
Applications
| Memory Address Decoding | Bus Arbitration Logic |
|---|---|
Use Scenario: Selecting one of eight memory chips (e.g., 32K×8 SRAMs) in a 256K-byte system using A15–A13 as select lines. IC Role / Device Role / Timing Role: Primary address decoder generating chip-select signals synchronized to CPU address strobes. Use Value: 10 ns propagation delay ensures no wait-state insertion needed for 80 ns memory access cycles. |
Use Scenario: Managing shared access to a common data bus among eight peripheral controllers in an embedded microcontroller system. IC Role / Device Role / Timing Role: Centralized bus grant arbiter asserting individual enable lines based on priority-encoded requests. Use Value: Active-low outputs interface directly with enable pins of bus transceivers (e.g., SN74AS245), minimizing glue logic. |
| Peripheral Interface Selection | LED Segment Driver Enable |
Use Scenario: Routing CPU I/O instructions to one of eight UART, ADC, or DAC peripherals mapped into discrete I/O address spaces. IC Role / Device Role / Timing Role: I/O address decoder translating lower address bits into dedicated peripheral enables. Use Value: Three enable inputs allow hierarchical decoding: G1 from CPU control bus, G2A/G2B from mode register bits. |
Use Scenario: Driving common-anode 7-segment displays in multiplexed configuration, where each Yx output enables one digit. IC Role / Device Role / Timing Role: Digit selector in time-multiplexed display driver circuit, sequenced via A/B/C from counter outputs. Use Value: 20 mA sink capability directly drives segments without external transistor arrays, reducing BOM cost. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar 3-line-to-8-line decoder applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS138AN | ALS family: 22 ns max delay, 8 mA IOL, lower ICC (10 mA), same pinout and logic function. | Better suited for low-power systems where speed >15 ns is acceptable; not ideal for sub-15 MHz bus timing. | Select SN74ALS138AN if power efficiency is prioritized over propagation speed in non-critical timing paths. |
| SN74HC138N | CMOS family: 24 ns max delay at 5 V, 4 mA IOL, rail-to-rail VIH/VIL, wider VCC range (2–6 V). | Requires level-shifting for legacy TTL systems; unsuitable for direct replacement where 20 mA sink is needed. | Choose SN74HC138N only in new designs with CMOS-compatible signaling and supply flexibility; not drop-in for SN74AS138N-J. |
Compared with SN74ALS138AN, SN74AS138N-J delivers 2.2× faster decoding and 2.5× higher output drive but consumes ~70% more supply current; versus SN74HC138N, it offers superior noise immunity and TTL interoperability at the cost of static power and voltage flexibility.
Availability
SN74AS138N-J is available at Aetrix Electronics and suitable for memory address decoding, bus arbitration logic, peripheral interface selection, and LED segment driver enable applications requiring stable component supply and long-term industrial availability.
Supply support for SN74AS138N-J 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 ICs with broad industrial, automotive, and communications reach.
The SN74AS138N-J belongs to TI's Advanced Schottky (AS) logic family, engineered for high-speed memory decoding and data-routing systems where minimal propagation delay and TTL compatibility are critical.
FAQ
What is the maximum operating frequency supported by SN74AS138N-J in a memory decode application?
The SN74AS138N-J does not specify a clock frequency but is characterized for propagation delay ≤10 ns (max) at VCC = 5 V and CL = 50 pF. In practice, this supports reliable operation in address decode paths for memory systems with access times ≥80 ns - such as 80 ns SRAMs - without introducing timing violations or requiring wait states. System-level frequency limits depend on total path delay including trace length and load capacitance.
Can SN74AS138N-J be used as a demultiplexer, and how is that configured?
Yes, SN74AS138N-J can function as a 1-to-8 demultiplexer by using one of its enable inputs (e.g., G1) as the data input while holding the other two enables (G2A, G2B) inactive, and applying the 3-bit select code to A, B, C. When G1 is high and G2A/G2B are low, the data appears inverted on the selected Yx output. This configuration is explicitly supported in TI's SDAS055E datasheet and enables compact serial-to-parallel conversion.
Is SN74AS138N-J pin-compatible with SN74LS138 or SN74HC138?
SN74AS138N-J shares identical pinout and logic function with SN74LS138 and SN74HC138 in the 16-pin PDIP package, but electrical compatibility is limited. While pin-for-pin placement is possible, SN74AS138N-J draws higher supply current and delivers 20 mA sink versus 8 mA (LS) or 4 mA (HC), and its VIH/VIL thresholds differ slightly. Direct substitution may cause excessive loading or marginal noise margins in LS/HC-based systems.
What is the recommended bypass capacitor for SN74AS138N-J?
A 0.1 µF ceramic capacitor placed as close as possible to the VCC (pin 16) and GND (pin 9) pins is recommended for SN74AS138N-J to suppress high-frequency supply noise generated during rapid output transitions. For systems with heavy simultaneous switching, adding a bulk 10 µF tantalum capacitor nearby improves low-frequency regulation. Layout best practices require short, wide traces between capacitor pads and IC pins.
Does SN74AS138N-J support cascading to decode more than 8 lines, and how many devices are needed for 32-line decoding?
Yes, SN74AS138N-J supports cascading via its three enable inputs. For 32-line decoding, four SN74AS138N-J devices are required: three for primary decoding and one master unit whose outputs drive the G1 enables of the three slaves. As documented in SDAS055E, only one external inverter is needed - applied to G2B of the master - to generate complementary enables, eliminating the need for additional logic gates in the hierarchy.
SN74AS138N-J Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- *
- Package/Case:
- -
- Packaging:
- Bulk
- Product Status:
- Active
- Type:
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- Independent Circuits:
- -
- Current - Output High, Low:
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SN74AS138N-J FAQ
1.How can I place an order for SN74AS138N-J through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS138N-J 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-J reliable?
The price and inventory of SN74AS138N-J are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS138N-J is usually 5 days.
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SN74AS138N-J orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS138N-J 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-J?
For technical support, including SN74AS138N-J datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS138N-J requirements.
6.How does Aetrix verify that SN74AS138N-J is sourced from the original manufacturer or authorized distributors?
All SN74AS138N-J 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-J meets industry standards.
7.What is the process for return or replacement of SN74AS138N-J?
All SN74AS138N-J units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS138N-J, 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-J part is unused and in its original packaging.
Return procedure for SN74AS138N-J:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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