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Texas Instruments SN74ALS138ADBR
- Part No.:
- SN74ALS138ADBR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- -
- Datasheet:
-
SN74ALS138ADBR.pdf
- Description:
- DECODER/DEMULTIPLEXER SINGLE 3-T
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Product details
Overview
SN74ALS138ADBR from Texas Instruments is a 3-line-to-8-line decoder/demultiplexer IC with active-low outputs, three binary select inputs (A, B, C), two active-low enable inputs (G2A, G2B), and one active-high enable input (G1). It operates at 5 V supply, delivers 8 mA sink current per output, supports propagation delays as low as 12 ns (tPHL, G2A→Y), and is used in address decoding for microprocessor systems and memory mapping.
For engineers reviewing the SN74ALS138ADBR datasheet, SN74ALS138ADBR pinout, SN74ALS138ADBR application, or SN74ALS138ADBR equivalent, this device is selected for logic-level decoding where low-power TTL-compatible operation, precise enable control, and deterministic 3-level propagation delay paths are required in industrial control and legacy computing interfaces.
Technical Context
The SN74ALS138ADBR implements a fully decoded 1-of-8 output architecture using standard TTL logic gates. Its enable structure-G1 (active-high) and G2A/G2B (active-low ORed)-allows hierarchical expansion to 24-line or 32-line decoders without external inverters in many configurations.
Each output is a NAND-based open-collector-equivalent stage with guaranteed VOL ≤ 0.5 V at IOL = 8 mA and VOH ≥ 2.7 V at IOH = –400 µA. Propagation delay varies by input path: select inputs A/B/C induce 2-level delays to even/odd outputs, while G1 and G2A/G2B each trigger 3-level delays to all eight outputs.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Supply Voltage | 4.75 V to 5.25 V - ensures compatibility with standard 5 V TTL bus systems and stable noise margins. |
| Output Sink Current | 8 mA per Yx output - drives standard TTL loads directly without external pull-ups in most decoding applications. |
| Propagation Delay (tPHL) | 12 ns min (G2A→Y), 32 ns max - enables reliable timing in 20–25 MHz address decode cycles with margin. |
| Input Thresholds | VIH ≥ 2.0 V, VIL ≤ 0.8 V - provides robust noise immunity against TTL-level signal transitions. |
| Operating Temperature | –20 °C to +75 °C - qualified for commercial and industrial ambient environments without derating. |
| Power Dissipation | 10 mA ICC max at VCC = 5.25 V - supports dense board layouts with minimal thermal impact. |
Pinout & Package
SN74ALS138ADBR is housed in a 16-pin SOIC package (JEDEC MS-012, body width 3.91 mm, pitch 1.27 mm), designated DB package by Texas Instruments and compatible with SOP-16 footprint standards.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | ORed with G2B; both must be low for decode activation - enables cascading with upstream enable logic. |
| 2 (G2B) | Active-low enable input | ORed with G2A; allows flexible gating of decoder group via shared low-active bus signals. |
| 3 (G1) | Active-high enable input | Must be high for function; used as primary chip-select or demux data path when tied to logic high. |
| 4 (C) | Binary select input | Highest-order address bit; determines Y4–Y7 vs. Y0–Y3 output group selection. |
| 5 (B) | Binary select input | Middle-order address bit; partitions outputs into four pairs (e.g., Y0/Y1, Y2/Y3). |
| 6 (A) | Binary select input | Lowest-order address bit; selects individual output within each pair (e.g., Y0 vs. Y1). |
| 7 (Y0) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=B=A=L - direct drive for chip-select lines. |
| 8 (Y1) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=B=L, A=H - used for peripheral enable sequencing. |
| 9 (Y2) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=L, B=H, A=L - supports memory bank selection. |
| 10 (Y3) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=L, B=H, A=H - matches LSB-aligned address ranges. |
| 11 (Y4) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=H, B=L, A=L - enables upper-half memory decoding. |
| 12 (Y5) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=H, B=L, A=H - used in I/O port address decoding. |
| 13 (Y6) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=H, B=H, A=L - supports multi-function peripheral addressing. |
| 14 (Y7) | Active-low decoded output | Asserted only when G1=H, G2A=G2B=L, and C=B=A=H - final output for full 3-bit decode space. |
| 15 (GND) | Ground reference | Return path for all internal logic and output currents - requires low-impedance PCB plane connection. |
| 16 (VCC) | Positive supply | +5 V power rail - must be bypassed locally with 0.1 µF ceramic capacitor near pin 16. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs with mixed polarity | Enables hierarchical decoding: G1 (high-enable) and G2A/G2B (low-enable) allow stacking without external inverters. |
| Guaranteed 8 mA output sink capability | Drives multiple standard TTL inputs or small LEDs directly - eliminates need for buffer stages in simple control logic. |
| Matched propagation delay paths | Known 2- or 3-level delay groups (per input type) simplify timing analysis in synchronous address decode trees. |
| Standard SOIC-16 footprint | Compatible with automated assembly and rework workflows; pinout identical to industry-standard 74LS138 and 74HC138 variants. |
| Industrial temperature range support | Validated operation from –20 °C to +75 °C - suitable for non-automotive embedded systems with moderate thermal profiles. |
Applications
| Memory Address Decoding | Peripheral Chip Select Logic |
|---|---|
Use Scenario: Mapping 3-bit address segments to enable specific RAM/ROM chips or I/O ports in an 8-bit microcontroller system. IC Role / Device Role / Timing Role: SN74ALS138ADBR acts as a synchronous address decoder, asserting one of eight active-low chip-select lines based on A0–A2 and global enable signals. Use Value: Reduces glue logic count by replacing discrete NAND/NOR gates; maintains <32 ns worst-case setup-to-output timing for 4 MHz bus systems. |
Use Scenario: Assigning dedicated select lines to UART, SPI, or parallel LCD controllers in a resource-constrained MCU design. IC Role / Device Role / Timing Role: SN74ALS138ADBR serves as a static demultiplexer, routing a single enable strobe to one of eight peripherals under firmware control. Use Value: Enables deterministic peripheral arbitration without software polling; supports hot-swap detection via Yx line monitoring. |
| Bus Expansion Interface | Legacy System Logic Replacement |
Use Scenario: Expanding a 16-bit data bus to drive 24 separate control signals across multiple PCBs in test equipment. IC Role / Device Role / Timing Role: SN74ALS138ADBR functions as a level-shifting demux stage, translating compact bus commands into distributed enable pulses. Use Value: Achieves 24-line decode using three SN74ALS138ADBR units with no external inverters - cuts BOM cost by 30% vs. discrete solutions. |
Use Scenario: Replacing obsolete 74LS138 devices in field-repair scenarios for telecom line cards and industrial PLC backplanes. IC Role / Device Role / Timing Role: SN74ALS138ADBR operates as a drop-in functional replacement with improved speed and lower ICC than original LS parts. Use Value: Maintains pinout and logic behavior while reducing power consumption by ~40% and improving tPHL by 3–5 ns over 74LS138. |
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 |
|---|---|---|---|
| SN74HC138DR | CMOS technology; VCC = 2–6 V; IOL = 4 mA; tPD ≈ 20 ns typical; higher noise immunity but lower drive strength. | Suitable for mixed-voltage systems and battery-powered designs; not recommended for driving >2 standard TTL loads. | Select SN74HC138DR when supply flexibility or static power reduction is prioritized over raw drive capability. |
| SN74LS138N | Bipolar LS process; identical pinout; VCC = 4.75–5.25 V; IOL = 8 mA; tPD ≈ 35 ns max - slower and higher ICC than ALS variant. | Direct legacy replacement where timing margin exists; higher power draw limits density in thermally constrained layouts. | Choose SN74LS138N only for exact form-fit-function replacement in documented LS-only designs with verified timing slack. |
Compared with SN74HC138DR and SN74LS138N, the SN74ALS138ADBR delivers superior speed-power trade-off for 5 V TTL systems: 25% faster than LS and 2× stronger output drive than HC, making it optimal for high-reliability industrial decode where both timing precision and load drive matter.
Availability
SN74ALS138ADBR is available at Aetrix Electronics and suitable for memory address decoding, peripheral chip select logic, bus expansion interfaces, and legacy system logic replacement requiring stable component supply and long-term industrial availability.
Supply support for SN74ALS138ADBR 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 and automotive qualification.
The SN74ALS138ADBR belongs to TI's 74ALS advanced low-power Schottky logic family, designed specifically for high-speed, low-power 5 V TTL-compatible digital systems in industrial control and communications infrastructure.
FAQ
What is the operating voltage range for SN74ALS138ADBR?
The SN74ALS138ADBR operates over a supply voltage range of 4.75 V to 5.25 V. This tight tolerance ensures compatibility with regulated 5 V TTL buses and provides sufficient noise margin for reliable logic-level interpretation. Operation outside this range may cause undefined output states or accelerated parametric drift. The SN74ALS138ADBR is not rated for 3.3 V or dual-supply operation.
Does SN74ALS138ADBR support demultiplexing functionality?
Yes, SN74ALS138ADBR supports demultiplexing: its G1 input can serve as a data input while A, B, C act as select lines, routing the active-high G1 signal to one of eight active-low outputs. This mode requires G2A and G2B held low. The SN74ALS138ADBR retains full decode functionality in this configuration, with propagation delay matching standard enable-path timing.
What is the maximum output sink current per Yx pin on SN74ALS138ADBR?
The SN74ALS138ADBR guarantees 8 mA sink current per output (Y0–Y7) at VCC = 5.25 V and TA = 75 °C, with VOL ≤ 0.5 V. This rating allows direct interfacing with standard TTL inputs (requiring ≤ 1.6 mA per input) or driving small indicator LEDs (with series resistor). Exceeding 8 mA risks violating VOL specification or accelerating long-term parameter shift in the SN74ALS138ADBR.
Is SN74ALS138ADBR pin-compatible with SN74LS138N?
Yes, SN74ALS138ADBR is pin-compatible with SN74LS138N: both use identical 16-pin SOIC (DB) packaging and share identical pin assignments for G1, G2A, G2B, A, B, C, Y0–Y7, VCC, and GND. No PCB layout changes are needed when upgrading from LS to ALS, though timing and power characteristics differ meaningfully in the SN74ALS138ADBR.
What is the worst-case propagation delay for SN74ALS138ADBR?
The worst-case propagation delay for SN74ALS138ADBR is 39 ns (tPLH, G1→Y) and 41 ns (tPHL, G2A→Y) at VCC = 5.25 V, CL = 15 pF, and RL = 2 kΩ. These values apply to the longest internal logic path (3-level delay) and define the maximum time between enable assertion and valid output transition. The SN74ALS138ADBR achieves faster delays on select-input paths (e.g., 26 ns max for A→Y).
SN74ALS138ADBR 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:
- -
- Operating Temperature:
- -
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- -
- Supplier Device Package:
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SN74ALS138ADBR FAQ
1.How can I place an order for SN74ALS138ADBR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138ADBR 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 SN74ALS138ADBR reliable?
The price and inventory of SN74ALS138ADBR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138ADBR is usually 5 days.
3.What payment methods are accepted for SN74ALS138ADBR?
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4.How is shipping managed for SN74ALS138ADBR?
SN74ALS138ADBR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138ADBR 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 SN74ALS138ADBR?
For technical support, including SN74ALS138ADBR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138ADBR requirements.
6.How does Aetrix verify that SN74ALS138ADBR is sourced from the original manufacturer or authorized distributors?
All SN74ALS138ADBR 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 SN74ALS138ADBR meets industry standards.
7.What is the process for return or replacement of SN74ALS138ADBR?
All SN74ALS138ADBR units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138ADBR, 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 SN74ALS138ADBR part is unused and in its original packaging.
Return procedure for SN74ALS138ADBR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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