My Inquiry
My Account
Texas Instruments SN74ALS138ANS
- Part No.:
- SN74ALS138ANS
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.209", 5.30mm Width)
- Datasheet:
-
SN74ALS138ANS.pdf
- Description:
- IC DECODER/DEMUX 1 X 3:8 16SO
- Quantity:
- Payment:
- Shipping:
Inventory:825
Please send an inquiry. Send us your inquiry, and we will respond immediately.
Product details
Overview
SN74ALS138ANS from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin plastic SOP (NS) package, designed for high-speed memory decoding and data routing. It features three enable inputs (G1, G2A, G2B), operates from 0°C to 70°C, supports 5 V supply, and delivers propagation delays as low as 5 ns (tPLH) and 6 ns (tPHL) under typical conditions.
For engineers reviewing the SN74ALS138ANS datasheet, SN74ALS138ANS pinout, SN74ALS138ANS application, or SN74ALS138ANS equivalent, this device is selected for deterministic address decoding in legacy TTL-compatible systems where low propagation delay, Schottky-clamped logic performance, and cascading capability via dual active-low enables are critical.
Technical Context
The SN74ALS138ANS implements positive-logic binary decoding with active-low outputs (Y0–Y7) and three independent enable controls: one active-high (G1) and two active-low (G2A, G2B). Its internal architecture uses NAND-based logic gates to decode A/B/C select lines only when all enables are asserted, minimizing spurious output transitions during partial enable states.
It belongs to the ALS (Advanced Low-Power Schottky) family, offering improved speed-power trade-offs over standard LS TTL. The device exhibits guaranteed fan-out of 20 LS-TTL loads, supports demultiplexing via G1 as data input, and enables 24-line decoding without external inverters due to complementary enable polarity.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Supply Voltage | 4.5 V to 5.5 V - Ensures compatibility with standard 5 V TTL bus systems and stable operation across industrial voltage tolerances. |
| Propagation Delay (tPHL) | 6 ns max (A/B/C → Y) - Enables use in sub-20 MHz address decoding paths without adding critical timing slack. |
| Output Drive (IOL) | 4 mA min - Sufficient to directly drive 20 LS-TTL inputs or interface with standard 5 V CMOS logic without buffering. |
| Input Thresholds (VIH/VIL) | VIH = 2.0 V min, VIL = 0.8 V max - Guarantees robust noise margin against TTL-level signal degradation in noisy backplane environments. |
| Operating Temperature | 0°C to 70°C - Qualified for commercial-grade embedded control, instrumentation, and legacy computing applications. |
| Package Type | SOP-16 (NS) - Surface-mount package with 1.27 mm pitch, compatible with automated assembly and IPC-7351 land patterns. |
Pinout & Package
SN74ALS138ANS is housed in a 16-pin plastic small-outline package (SOP-NS), 2.00 mm maximum height, with gull-wing leads and JEDEC-standard pin 1 quadrant (Q1). Pin spacing is 1.27 mm, body dimensions 10.2 mm × 5.3 mm.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be LOW to activate decoder; used with G2B for hierarchical enable control in multi-chip decoding trees. |
| 2 (G2B) | Active-low enable input | Second active-low enable; paired with G2A to reduce external gating when cascading multiple SN74ALS138ANS devices. |
| 3 (G1) | Active-high enable input | Used as primary chip-select or as data input in demultiplexer mode; HIGH enables decoding function. |
| 4 (C) | MSB binary select input | Most significant bit of 3-bit address; determines upper/lower half of decoded output group (Y0–Y3 vs Y4–Y7). |
| 5 (B) | Mid-bit binary select input | Second bit of address; combined with A and C to uniquely select one of eight outputs. |
| 6 (A) | LSB binary select input | Least significant bit; completes 3-bit binary selection; inverted internally before NAND decoding. |
| 7 (Y0) | Active-low decoded output | Asserted LOW when A=B=C=0 and all enables active; drives memory chip-select or peripheral enable lines. |
| 8 (Y1) | Active-low decoded output | Asserted LOW when A=1, B=C=0; provides discrete channel selection in 1-of-8 data distribution. |
| 9 (Y2) | Active-low decoded output | Asserted LOW when B=1, A=C=0; supports modular I/O expansion where each output enables a separate subsystem. |
| 10 (Y3) | Active-low decoded output | Asserted LOW when A=B=1, C=0; used in segmented memory mapping to isolate 64 KB banks in 8080/8085 systems. |
| 11 (Y4) | Active-low decoded output | Asserted LOW when C=1, A=B=0; enables higher-address peripherals such as video controllers or DMA channels. |
| 12 (Y5) | Active-low decoded output | Asserted LOW when A=1, C=1, B=0; supports selective activation of interrupt vectors or configuration registers. |
| 13 (Y6) | Active-low decoded output | Asserted LOW when B=1, C=1, A=0; commonly tied to ROM enable in microcontroller boot loaders. |
| 14 (Y7) | Active-low decoded output | Asserted LOW when A=B=C=1; final output for highest-addressed device or diagnostic mode selection. |
| 15 (GND) | Ground reference | Primary return path for all internal logic and output currents; requires low-impedance PCB connection to minimize ground bounce. |
| 16 (VCC) | Positive supply | +5 V power rail; decoupling capacitor (0.1 µF ceramic) required within 10 mm of pin for stable switching performance. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs (G1, G2A, G2B) | Enables direct cascade of up to four SN74ALS138ANS devices into a 32-line decoder using only one external inverter - eliminates glue logic in multi-bank memory systems. |
| Active-low 8-channel outputs | Provides native compatibility with common active-low chip-select inputs on SRAM, EPROM, and peripheral ICs without level-shifting or inversion. |
| ALS Schottky-clamped TTL technology | Delivers 2× faster propagation than standard LS TTL while maintaining full 5 V TTL voltage compatibility and DC drive strength. |
| Demultiplexer functionality | G1 input accepts serial data stream when A/B/C are held static, allowing single SN74ALS138ANS to route one input to eight destinations synchronously. |
| Guaranteed fan-out of 20 LS-TTL loads | Supports direct connection to multiple downstream TTL inputs without buffer amplification - reduces component count in dense logic designs. |
Applications
| Memory Address Decoding | Peripheral Select Logic |
|---|---|
|
Use Scenario: Decoding 16-bit address bus (A13–A15) to select among eight 8 KB memory banks in an 8085-based system. IC Role / Device Role / Timing Role: Primary address decoder generating active-low chip-select signals for SRAM and EPROM modules with sub-10 ns timing budget. Use Value: Eliminates need for discrete NAND gates or additional inverters; ensures deterministic setup/hold margins due to matched tPLH/tPHL characteristics. |
Use Scenario: Routing control signals from a microcontroller to eight distinct I/O expanders or sensor interfaces on a shared bus. IC Role / Device Role / Timing Role: Demultiplexer enabling one serial command line to activate individual peripheral slaves via dedicated enable pins. Use Value: Reduces GPIO usage by 7 pins; leverages G1 as data input while A/B/C remain static - simplifies firmware register mapping. |
| Legacy System Bus Expansion | Industrial Control Backplane |
|
Use Scenario: Expanding ISA-style bus slots in retro-computing hardware to support up to 24 unique device addresses. IC Role / Device Role / Timing Role: Cascaded decoder node in multi-level hierarchy, using G2A/G2B to gate groups of eight slots per SN74ALS138ANS. Use Value: Achieves 24-line decoding without external inverters - preserves board space and signal integrity in constrained form factors. |
Use Scenario: Isolating fault conditions in programmable logic controller (PLC) backplanes by assigning each I/O module a unique enable line. IC Role / Device Role / Timing Role: Fault-tolerant address decoder ensuring only one module responds per command cycle, preventing bus contention. Use Value: Active-low outputs interface directly with industrial-rated opto-isolators and relay drivers requiring negative-enable logic. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar decoder/demultiplexer applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74ALS138AN | Same ALS logic, identical electrical specs, but in 16-pin PDIP (N) package instead of SOP-NS. | Preferred for through-hole prototyping, breadboarding, or legacy wave-soldered assemblies where SOP reflow is unavailable. | Select SN74ALS138AN when manual assembly, socketing, or thermal mass requirements favor DIP over surface-mount. |
| SN74AS138NSR | Faster propagation (9.5 ns max tPHL vs 18 ns), higher drive (20 mA IOL), but increased ICC (14–20 mA vs 5–10 mA). | Better suited for ultra-high-speed systems (>25 MHz clock domains) where timing margin is critical and power budget allows. | Choose SN74AS138NSR only if the design requires sub-10 ns delays and can accommodate higher supply current and tighter layout constraints. |
Compared with SN74ALS138ANS, SN74ALS138AN offers identical logic behavior in a through-hole package for ease of validation, while SN74AS138NSR trades higher power for significantly reduced propagation delay - making the ALS version optimal for cost-sensitive, thermally constrained commercial applications needing reliable 5 V TTL compatibility.
Availability
SN74ALS138ANS is available at Aetrix Electronics and suitable for memory decoding, peripheral selection, bus expansion, and industrial control backplane applications requiring stable component supply and long-term obsolescence management.
Supply support for SN74ALS138ANS 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 SN74ALS138ANS belongs to TI's legacy TTL logic portfolio, specifically engineered for high-reliability, pin-compatible replacement in aging 5 V digital systems where ALS-speed and TTL interoperability are mandatory.
FAQ
What is the operating temperature range for SN74ALS138ANS?
The SN74ALS138ANS is rated for commercial operation from 0°C to 70°C. This range aligns with standard ambient conditions in office equipment, test instrumentation, and non-military embedded systems. It is not qualified for extended temperature ranges like –40°C to 125°C, which apply to military-grade variants such as SN54ALS138A.
Does SN74ALS138ANS support 3.3 V logic levels?
No, SN74ALS138ANS is a 5 V-only TTL device and does not support 3.3 V operation. Its input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and output swing (VOH ≈ VCC – 2 V) are specified only for VCC = 4.5–5.5 V. Interfacing with 3.3 V systems requires level-shifting circuitry or a compatible 3.3 V logic family.
Can SN74ALS138ANS be used as a demultiplexer?
Yes, SN74ALS138ANS can function as a 1-to-8 demultiplexer by holding the A, B, and C select inputs static and applying the data signal to the G1 enable input. When G2A and G2B are asserted, the data appears inverted on the selected Yx output, enabling synchronous distribution of a single control line to eight destinations.
What is the maximum fan-out capability of SN74ALS138ANS?
SN74ALS138ANS guarantees a fan-out of 20 LS-TTL loads per output. This is derived from its IOL = 4 mA minimum and VOL = 0.4 V max specifications, meeting the LS-TTL input current requirement (IIL = –0.4 mA). Exceeding this fan-out risks violating VOL or timing margins without buffering.
Is SN74ALS138ANS RoHS compliant?
Yes, SN74ALS138ANS is RoHS compliant, as confirmed by Texas Instruments' packaging documentation. The part uses NiPdAu (NIPDAU) lead finish and carries Level-1 moisture sensitivity rating with peak reflow tolerance up to 260°C, satisfying modern environmental compliance requirements for surface-mount assembly.
SN74ALS138ANS Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.209", 5.30mm Width)
- Packaging:
- Tube
- 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 (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SO
SN74ALS138ANS FAQ
1.How can I place an order for SN74ALS138ANS through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS138ANS 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 SN74ALS138ANS reliable?
The price and inventory of SN74ALS138ANS are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS138ANS is usually 5 days.
3.What payment methods are accepted for SN74ALS138ANS?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS138ANS transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS138ANS?
SN74ALS138ANS orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS138ANS 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 SN74ALS138ANS?
For technical support, including SN74ALS138ANS datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS138ANS requirements.
6.How does Aetrix verify that SN74ALS138ANS is sourced from the original manufacturer or authorized distributors?
All SN74ALS138ANS 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 SN74ALS138ANS meets industry standards.
7.What is the process for return or replacement of SN74ALS138ANS?
All SN74ALS138ANS units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS138ANS, 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 SN74ALS138ANS part is unused and in its original packaging.
Return procedure for SN74ALS138ANS:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
SN74ALS138ANS Tags
-
SN74HC138DR
Texas Instruments
-
TC7SB3157CFU,LF(CT
Toshiba Semiconductor and Storage
-
74CBTLV3257PW,118
Nexperia USA Inc.
-
SN74CBTLV3257PWR
Texas Instruments
-
74CBTLV3257GUX
Nexperia USA Inc.
-
74HC154BQ,118
Nexperia USA Inc.
-
P3S0200GMX
NXP USA Inc.
-
SN74CB3Q3245PWR
Texas Instruments
-
SN74CB3Q3257RGYR
Texas Instruments
-
TCA9543APWR
Texas Instruments
-
TCA9546APWR
Texas Instruments
-
SN74HC138N
Texas Instruments
Tech Hub
A practical engineering guide to 3.3V and 5V logic compatibility, input thresholds, resistor dividers, translator ICs, MOSFET level shifting, I2C pull-ups, timing limits and power-sequencing risks.
The 74HC595 uses push-pull logic outputs, while the TPIC6B595 uses 50 V open-drain DMOS sinks for higher-power loads. This guide compares timing, current limits, 3.3 V interfacing, load wiring, thermal…
The 74HC595 converts serial data into eight stable parallel outputs. This guide covers pin functions, shift and storage timing, OE and MR behavior, drive-current limits, cascading, voltage compatibilit…
A technical comparison of level-sensitive latches and edge-triggered flip-flops, covering timing windows, setup and hold limits, master–slave operation, time borrowing, race-through, HDL inference and…
A D latch stores one bit while Enable controls when data can pass. This reference covers gate-level operation, truth tables, transparency, setup and hold timing, LE versus OE, common ICs and practical …
An SR latch stores one bit through cross-coupled feedback. This engineering reference covers NOR and NAND implementations, truth tables, forbidden-state recovery, gated operation, switch debouncing, fa…
Latch circuits retain one bit through feedback. This technical reference covers SR and D latches, truth tables, transparency, timing limits, latch-versus-flip-flop behavior, applications and common log…
An engineering guide to LED driver operation, constant-current and constant-voltage outputs, linear and switching topologies, dimming, IC selection, calculations, replacement compatibility, and fault c…
Operational amplifier guide covering op amp basics, feedback, ideal vs real op amps, common configurations, buffer circuits, offset, bias current, gain-bandwidth, slew rate, rail-to-rail limits and sel…
Jumper cables guide covering safe connection order, red and black clamp placement, final ground connection, cable gauge, length, clamp quality, copper vs CCA cables, jump starter comparison and battery…
