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Texas Instruments SN74ALS138ADR
- Part No.:
- SN74ALS138ADR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74ALS138ADR.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,226
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Product details
Overview
SN74ALS138ADR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin SOIC (D) 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 = 4.5 V, and operates over 0°C to 70°C. It is used in address decoding for SRAM/ROM systems where minimal system delay is critical.
For engineers reviewing the SN74ALS138ADR datasheet, SN74ALS138ADR pinout, SN74ALS138ADR application, or SN74ALS138ADR equivalent, key selection criteria include propagation delay consistency across enable and select inputs, ALS logic family compatibility with TTL/LS interfaces, output drive capability (IOL = 4 mA min), and SOIC packaging for automated assembly in space-constrained industrial control boards.
Technical Context
The SN74ALS138ADR implements positive-logic binary decoding with three independent enable inputs-G1 (active-high), G2A and G2B (both active-low)-allowing flexible cascading without external inverters. Its Schottky-clamped ALS architecture ensures fast switching while maintaining TTL-compatible input thresholds (VIH = 2 V, VIL = 0.8 V).
Output behavior follows strict active-low octal decoding: only one Y0–Y7 output goes low per valid input combination; all others remain high. Enable priority is hardwired-outputs are forced high when any enable condition fails, guaranteeing fail-safe bus isolation during reset or power sequencing.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - supports standard 5 V TTL supply rails with ±10% tolerance |
| Propagation Delay (tPLH) | 5 ns max (A/B/C → Y) - enables sub-200 MHz address decode timing in high-speed memory subsystems |
| Output Drive (IOL) | 4 mA min at VOL ≤ 0.4 V - sufficient to directly drive multiple TTL inputs or small capacitive loads |
| Input Thresholds | VIH = 2 V, VIL = 0.8 V - ensures robust noise margin against TTL-level signals |
| Operating Temperature | 0°C to 70°C - qualified for commercial-grade embedded and industrial control applications |
| Package | SOIC-16 (D) - surface-mount, RoHS-compliant, MSL Level-1, compatible with standard reflow profiles |
Pinout & Package
SN74ALS138ADR uses a 16-pin SOIC (D) package with 1.27 mm pitch, 10.4 mm × 7.4 mm body, and 2.00 mm max height. Pin 1 is marked by a beveled corner or notch; leads are gull-wing, NIPDAU-finished.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be low to activate decoding; tied high via pull-up for single-device operation |
| 2 (G2B) | Active-low enable input | Second independent enable; allows hierarchical decoding with upstream device |
| 3 (G1) | Active-high enable input | Primary global enable; synchronizes with system control signals like CS or OE |
| 4 (C) | MSB binary select input | Connects to A15/A14 of microprocessor address bus for memory block selection |
| 5 (B) | Mid-bit binary select input | Carries intermediate address bit; determines 4-way partition within enabled group |
| 6 (A) | LSB binary select input | Drives fine-grained selection among eight outputs; routed from lowest address line |
| 7 (Y0) | Active-low decoded output | Asserted low when A=B=C=0 and all enables active; drives chip-select for first memory IC |
| 8 (Y1) | Active-low decoded output | Asserted low when A=1, B=C=0; used for second peripheral or memory bank |
| 9 (Y2) | Active-low decoded output | Corresponds to binary 010; commonly assigned to I/O port or peripheral controller |
| 10 (Y3) | Active-low decoded output | Binary 011 output; supports modular expansion of address-mapped peripherals |
| 11 (Y4) | Active-low decoded output | Binary 100 output; isolated from Y0–Y3 to prevent crosstalk in dense layouts |
| 12 (Y5) | Active-low decoded output | Binary 101 output; shares enable path with Y4 but has independent load path |
| 13 (Y6) | Active-low decoded output | Binary 110 output; placed adjacent to Y7 for consistent trace length matching |
| 14 (Y7) | Active-low decoded output | MSB output (111); often used for highest-priority device or boot ROM selection |
| 15 (VCC) | Positive supply | 5 V nominal; requires local 0.1 µF ceramic decoupling adjacent to pin |
| 16 (GND) | Ground reference | Return path for all internal logic and output currents; connects to solid ground plane |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs (G1, G2A, G2B) | Eliminates need for external inverters in 24-line decoder expansion; reduces gate count and board area |
| ALS Schottky-clamped logic | Delivers 5 ns typical propagation delay while maintaining TTL-compatible voltage levels and fanout |
| Active-low outputs with high-impedance off-state | Ensures no bus contention during disable; supports wired-OR configurations with pull-up resistors |
| SOIC-16 RoHS-compliant package | Enables automated pick-and-place assembly and reflow soldering without lead-free process deviations |
| Guaranteed 4 mA sink current per output | Drives up to 10 standard TTL loads directly, reducing need for buffer stages in legacy system upgrades |
Applications
| Memory Address Decoding | Peripheral Selection in Microcontroller Systems |
|---|---|
Use Scenario: Selecting between eight 8K×8 SRAM chips in a 64 KB memory map using A15–A13 as C–A inputs. IC Role / Device Role / Timing Role: Address decoder that asserts individual /CS lines based on upper address bits, synchronized with /OE timing. Use Value: Reduces total decode latency to ≤5 ns, enabling full utilization of 150 ns SRAM access time without wait states. | Use Scenario: Assigning unique chip-select signals to UART, SPI flash, ADC, DAC, GPIO expander, and timer peripherals on an 8-bit MCU bus. IC Role / Device Role / Timing Role: Bus arbiter that isolates peripherals electrically and temporally during read/write cycles. Use Value: Prevents signal contention and eliminates software-based delay loops for peripheral handshaking. |
| Legacy System Bus Expansion | Industrial PLC I/O Module Addressing |
Use Scenario: Adding four new ISA-compatible cards to an aging PC/104 stack using shared /IOR and /IOW strobes. IC Role / Device Role / Timing Role: Demultiplexer converting ISA address lines A2–A0 into discrete /IOCHRDY-enable signals. Use Value: Maintains backward compatibility with 80286-era timing budgets while supporting modern I/O functions. | Use Scenario: Mapping 16 digital input and 16 digital output channels across two 8-channel modules using DIP-switch-configurable base addresses. IC Role / Device Role / Timing Role: Configurable address translator that converts module ID bits into active-low enable pulses for local shift registers. Use Value: Enables field-replaceable I/O modules without firmware updates or PCB redesign. |
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 propagation delay (15 ns typ), higher ICC (19 mA), LS TTL family | Compatible with older 74LS designs but adds ~10 ns system delay; not suitable for >20 MHz bus clocks | Select when replacing legacy LS-based boards where timing margin exists and cost is primary concern |
| SN74HC138DR | CMOS technology, wider VCC range (2–6 V), lower ICC (<8 µA), 21 ns tPD at 4.5 V | Requires level-shifting for 5 V TTL systems; superior noise immunity but incompatible with direct TTL fanout | Select for new low-power designs operating at 3.3 V or mixed-voltage systems with proper interface conditioning |
Compared with SN74LS138N and SN74HC138DR, the SN74ALS138ADR delivers optimal balance of speed, TTL compatibility, and SOIC manufacturability-offering 3× faster switching than LS while retaining pin-for-pin layout reuse and direct integration into existing 5 V control logic.
Availability
SN74ALS138ADR is available at Aetrix Electronics and suitable for memory decoding, industrial PLC backplane expansion, and microcontroller peripheral selection requiring stable component supply and long-term production continuity.
Supply support for SN74ALS138ADR 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 for industrial, automotive, and communications markets.
The SN74ALS138ADR belongs to TI's 74ALS logic family, engineered for high-speed, low-power memory and data-path applications where propagation delay and interface compatibility with legacy TTL systems are critical design constraints.
FAQ
What is the maximum propagation delay of the SN74ALS138ADR under recommended operating conditions?
The maximum propagation delay (tPLH) from any select input (A, B, or C) to any output (Y0–Y7) is 22 ns, and from any enable input (G1, G2A, or G2B) to any output is 17 ns, measured at VCC = 4.5 V to 5.5 V, CL = 50 pF, RL = 500 Ω, and TA = 0°C to 70°C. These values are specified in the official SN74ALS138ADR datasheet revision SDAS055E.
Can the SN74ALS138ADR be used as a demultiplexer, and how is the data input connected?
Yes, the SN74ALS138ADR can function as a 1-to-8 demultiplexer. The data input is applied to one of the three enable terminals-typically G1 (active-high) or G2A/G2B (active-low)-while A, B, and C serve as address selectors. For example, tying G2A and G2B low and applying data to G1 routes the signal to the selected Yx output, with polarity inverted due to active-low outputs. This usage is explicitly supported in the SN74ALS138ADR functional description.
Does the SN74ALS138ADR have internal pull-up or pull-down resistors on its inputs?
No, the SN74ALS138ADR does not include internal pull-up or pull-down resistors on any input. All inputs-A, B, C, G1, G2A, and G2B-require externally defined logic states. Floating inputs may cause undefined output behavior or increased ICC. TI recommends using pull-up resistors (e.g., 10 kΩ to VCC) on G2A and G2B if permanently enabled, and appropriate termination for address lines in high-noise environments per SN74ALS138ADR application guidance.
Is the SN74ALS138ADR pin-compatible with the SN74AS138D?
Yes, the SN74ALS138ADR is pin-compatible with the SN74AS138D: both are 16-pin SOIC devices with identical pin assignments for G1, G2A, G2B, A, B, C, Y0–Y7, VCC, and GND. However, the SN74AS138D offers faster propagation delay (10 ns max) and higher output drive (20 mA), but consumes more supply current. Substitution requires validation of timing margins and loading conditions in the target SN74ALS138ADR application.
What is the recommended decoupling strategy for the SN74ALS138ADR in high-speed designs?
TI recommends placing a 0.1 µF ceramic capacitor between VCC (pin 15) and GND (pin 16) as close as possible to the SN74ALS138ADR package body-ideally within 3 mm-to suppress high-frequency supply noise generated during output transitions. For boards with multiple SN74ALS138ADR units, add a bulk 4.7 µF tantalum or aluminum electrolytic capacitor per power rail segment. This decoupling scheme is validated in TI's SN74ALS138ADR layout guidelines and essential to maintain specified tPLH/tPHL performance.
SN74ALS138ADR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Tape & Reel (TR)
- 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:
- Surface Mount
- Supplier Device Package:
- 16-SOIC
SN74ALS138ADR FAQ
1.How can I place an order for SN74ALS138ADR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138ADR 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 SN74ALS138ADR reliable?
The price and inventory of SN74ALS138ADR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138ADR is usually 5 days.
3.What payment methods are accepted for SN74ALS138ADR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS138ADR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS138ADR?
SN74ALS138ADR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138ADR 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 SN74ALS138ADR?
For technical support, including SN74ALS138ADR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138ADR requirements.
6.How does Aetrix verify that SN74ALS138ADR is sourced from the original manufacturer or authorized distributors?
All SN74ALS138ADR 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 SN74ALS138ADR meets industry standards.
7.What is the process for return or replacement of SN74ALS138ADR?
All SN74ALS138ADR units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138ADR, 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 SN74ALS138ADR part is unused and in its original packaging.
Return procedure for SN74ALS138ADR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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