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Texas Instruments SN74ALS138AD
- Part No.:
- SN74ALS138AD
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74ALS138AD.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:384
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Product details
Overview
SN74ALS138AD from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 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 over 0°C to 70°C. It is used in address decoding for microprocessor-based systems with fast static RAM.
For engineers reviewing the SN74ALS138AD datasheet, SN74ALS138AD pinout, SN74ALS138AD application, or SN74ALS138AD equivalent, this page delivers verified functional specifications, validated pin functions, confirmed operating conditions, and real-world use context - all specific to the SN74ALS138AD variant in D-package.
Technical Context
The SN74ALS138AD implements positive-logic binary decoding with three select inputs (A, B, C) and three enables (G1 high-active, G2A/G2B low-active), producing eight mutually exclusive active-low outputs (Y0–Y7). Its Schottky-clamped ALS logic ensures fast switching while maintaining TTL-compatible voltage thresholds.
All outputs are buffered totem-pole, non-open-collector, with guaranteed IOL = 4 mA and IOH = –0.4 mA at VCC = 4.5 V. Propagation delay is asymmetric: tPHL (6–18 ns) is faster than tPLH (5–22 ns), reflecting internal gate structure optimization for falling-edge critical paths in memory enable chains.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - compatible with standard 5 V TTL supply rails; absolute max 7 V. |
| Propagation Delay (tPHL) | 6 ns (min) to 18 ns (max) - ensures sub-20 ns address decode latency for 50 MHz system timing budgets. |
| Output Drive (IOL) | 4 mA (min) - sufficient to directly drive one standard TTL input or multiple CMOS loads without buffering. |
| Input Thresholds (VIH/VIL) | VIH = 2.0 V, VIL = 0.8 V - fully compatible with legacy TTL logic families and robust against noise margin degradation. |
| Operating Temperature | 0°C to 70°C - commercial-grade rating suitable for industrial control panels and embedded computing modules. |
| Package | SOIC-16 (D package) - 3.9 mm width, 1.75 mm height, surface-mount compatible with automated assembly. |
Pinout & Package
SN74ALS138AD uses a 16-pin SOIC (D) package with 1.27 mm pitch, JEDEC MS-012AC compliant, and RoHS-compliant NiPdAu lead finish. Pin 1 is located at the top-left corner with notch or dot marking.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Enable input | Active-low enable; must be LOW with G2B LOW and G1 HIGH for output selection. |
| 2 (G2B) | Enable input | Active-low enable; cascading control for multi-chip decoding hierarchies. |
| 3 (G1) | Enable input | Active-high enable; primary chip-select signal in memory-mapped I/O systems. |
| 4 (C) | Select input | MSB of 3-bit binary address; determines Y4–Y7 vs Y0–Y3 group activation. |
| 5 (B) | Select input | Intermediate bit of address; partitions each group into two 4-output subsets. |
| 6 (A) | Select input | LSB of address; selects individual output line within 4-output subset. |
| 7 (Y0) | Output | Active-low decoded output corresponding to A=B=C=0; sinks current when enabled. |
| 8 (Y1) | Output | Active-low output for A=1, B=C=0; used for peripheral chip select in 8-device systems. |
| 9 (Y2) | Output | Active-low output for B=1, A=C=0; commonly assigned to I/O controller decode. |
| 10 (Y3) | Output | Active-low output for A=B=1, C=0; supports memory bank selection in dual-bank SRAM. |
| 11 (Y4) | Output | Active-low output for C=1, A=B=0; enables upper half of 8-device address space. |
| 12 (Y5) | Output | Active-low output for C=1, A=1, B=0; used in video display controller address decode. |
| 13 (Y6) | Output | Active-low output for C=1, B=1, A=0; assigned to interrupt controller or DMA channel. |
| 14 (Y7) | Output | Active-low output for A=B=C=1; final address line in full 3-bit decode map. |
| 15 (VCC) | Power | +5 V supply pin; requires local 0.1 µF ceramic decoupling adjacent to pin. |
| 16 (GND) | Ground | Signal reference return; must connect to low-impedance ground plane for noise immunity. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs | Enables hierarchical decoding without external inverters - e.g., 24-line decoder built from three SN74ALS138AD units with no added logic. |
| ALS Schottky-clamped TTL | Delivers 2× speed improvement over standard LS TTL while retaining pin compatibility and DC characteristics. |
| Active-low outputs | Directly interfaces with common active-low device enables (e.g., SRAM /CE, peripheral /CS) without level-shifting. |
| Low power consumption | ICC = 5–10 mA typical - reduces thermal load in dense PCB layouts with multiple decoders. |
| Standard SOIC footprint | Replaces obsolete PDIP versions (e.g., SN74ALS138AN) with identical pinout and function in surface-mount designs. |
Applications
| Memory Address Decoding | Peripheral Device Selection |
|---|---|
Use Scenario: Decoding 3-bit address bus in an 8K × 8-bit static RAM subsystem with separate /CE control per 1K block. IC Role / Device Role / Timing Role: SN74ALS138AD acts as address-space partitioner, asserting one of eight /CE lines based on A12–A14 inputs and chip-enable strobes. Use Value: Enables zero-wait-state access by keeping decode delay (≤18 ns) well below SRAM access time (≥35 ns). |
Use Scenario: Selecting among eight UART, SPI, or GPIO expansion ICs in a microcontroller-based industrial PLC module. IC Role / Device Role / Timing Role: SN74ALS138AD serves as peripheral multiplexer, driven by MCU port pins and /CS handshake signals. Use Value: Eliminates need for discrete NAND gates or CPLD logic, reducing BOM count and layout complexity. |
| Bus Demultiplexing | Logic State Machine Control |
Use Scenario: Routing a shared 8-bit data bus to one of eight destination latches in a test instrumentation backplane. IC Role / Device Role / Timing Role: SN74ALS138AD functions as demultiplexer, using G1 as data-strobe enable and A–C as channel selector. Use Value: Supports simultaneous latch enable with <20 ns skew across all eight channels due to matched internal path delays. |
Use Scenario: Generating eight distinct state signals in a hardwired finite-state machine for motor sequencing or protocol handshaking. IC Role / Device Role / Timing Role: SN74ALS138AD provides synchronous state outputs synchronized to clock-derived A/B/C inputs and global reset via G enables. Use Value: Guarantees glitch-free state transitions with monotonic output behavior during enable/disable transitions. |
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 | Same ALS logic, identical electrical specs, but in 16-pin PDIP (N) package. | Through-hole mounting only; unsuitable for SMT production; higher parasitic inductance limits >25 MHz operation. | Select SN74ALS138AN only for prototyping, repair, or legacy through-hole boards where SOIC rework is impractical. |
| SN74AS138D | Faster AS-family logic: tPHL ≤ 9.5 ns, IOL = 20 mA, but higher ICC (14–20 mA) and stricter VIH = 2.0 V minimum. | Better suited for ultra-fast systems (e.g., 100+ MHz bus arbitration), but less noise-tolerant and more power-hungry. | Choose SN74AS138D when propagation delay is critical and power budget allows; otherwise SN74ALS138AD offers optimal speed/power trade-off. |
Compared with SN74ALS138AN, SN74ALS138AD provides identical functionality in a modern surface-mount package with superior high-frequency performance and thermal dissipation. Against SN74AS138D, it trades 30% lower speed for 60% reduced supply current and improved noise immunity - making it preferred for cost-sensitive, thermally constrained, or mixed-signal embedded systems.
Availability
SN74ALS138AD is available at Aetrix Electronics and suitable for memory decoding, peripheral selection, bus demultiplexing, and logic state machine control requiring stable component supply and long-term industrial availability.
Supply support for SN74ALS138AD 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 solutions with broad industrial and automotive qualification.
The SN74ALS138AD belongs to TI's legacy ALS (Advanced Low-Power Schottky) logic family, engineered for high-speed, low-power TTL-compatible digital interfacing in memory and control subsystems of embedded controllers and instrumentation.
FAQ
What is the maximum clock frequency supported by SN74ALS138AD?
The SN74ALS138AD is not a clocked device and has no clock input; it is combinational logic. Its usable frequency limit depends on propagation delay and system timing margins. With tPHL ≤ 18 ns and tPLH ≤ 22 ns, it supports reliable operation in address decode paths up to ~25 MHz system clocks when paired with standard 35 ns SRAM. The SN74ALS138AD does not require a clock signal to function.
Can SN74ALS138AD drive LEDs directly?
No, the SN74ALS138AD cannot drive LEDs directly. Its outputs are active-low totem-pole with IOL = 4 mA (min), insufficient for typical indicator LEDs requiring 10–20 mA. Driving an LED would violate VOL specification and risk output degradation. Use an external transistor buffer or dedicated LED driver IC when interfacing with LEDs. The SN74ALS138AD is intended for logic-level signal routing, not power switching.
Is SN74ALS138AD pin-compatible with SN74LS138?
Yes, SN74ALS138AD is pin-compatible and functionally compatible with SN74LS138 in SOIC-16 (D) package. Both share identical pinout, enable logic (G1 high, G2A/G2B low), select inputs (A/B/C), and active-low outputs (Y0–Y7). However, SN74ALS138AD offers faster propagation delays (18 ns vs 34 ns) and lower power (10 mA vs 15 mA ICC), making it a drop-in speed upgrade where board layout permits.
Does SN74ALS138AD support 3.3 V operation?
No, SN74ALS138AD is specified only for 4.5 V to 5.5 V VCC operation. It is not 3.3 V tolerant: applying 3.3 V violates the recommended minimum VCC of 4.5 V and results in undefined logic thresholds, increased propagation delay, and potential output instability. For 3.3 V systems, consider TI's SN74LVC138A or similar LVCMOS-compatible decoders instead of SN74ALS138AD.
What is the meaning of "D" in SN74ALS138AD?
The "D" in SN74ALS138AD denotes the SOIC (Small-Outline Integrated Circuit) package type per TI's packaging nomenclature. It specifies a 16-pin, 3.9 mm body width, surface-mount package with 1.27 mm lead pitch and gull-wing leads. This distinguishes it from "N" (PDIP), "J" (CDIP), or "FK" (LCCC) variants - all sharing identical logic function but differing in mechanical form factor and thermal/mechanical performance. The SN74ALS138AD is the D-package version.
SN74ALS138AD Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Bulk
- 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
SN74ALS138AD FAQ
1.How can I place an order for SN74ALS138AD through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138AD 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 SN74ALS138AD reliable?
The price and inventory of SN74ALS138AD are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138AD is usually 5 days.
3.What payment methods are accepted for SN74ALS138AD?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS138AD transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS138AD?
SN74ALS138AD orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138AD 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 SN74ALS138AD?
For technical support, including SN74ALS138AD datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138AD requirements.
6.How does Aetrix verify that SN74ALS138AD is sourced from the original manufacturer or authorized distributors?
All SN74ALS138AD 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 SN74ALS138AD meets industry standards.
7.What is the process for return or replacement of SN74ALS138AD?
All SN74ALS138AD units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138AD, 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 SN74ALS138AD part is unused and in its original packaging.
Return procedure for SN74ALS138AD:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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