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Texas Instruments SN74AHCT138N
- Part No.:
- SN74AHCT138N
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74AHCT138N.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16DIP
- Quantity:
- Payment:
- Shipping:
Inventory:917
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Product details
Overview
SN74AHCT138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in 16-pin PDIP package, featuring TTL-compatible inputs, three enable inputs (G1 active-high, G2A/G2B active-low), propagation delay ≤12 ns at 5 V/15 pF, and operation across −40°C to +85°C. It enables high-speed memory decoding and data routing in microcontroller-based address demultiplexing systems.
For engineers reviewing the SN74AHCT138N datasheet, SN74AHCT138N pinout, SN74AHCT138N application, or SN74AHCT138N equivalent, this page delivers verified functional modes, real-world switching characteristics, thermal resistance (RθJA = 67°C/W), and validated alternative options for industrial control, embedded logic expansion, and memory subsystem design.
Technical Context
The SN74AHCT138N implements positive-logic 3-to-8 decoding with cascading support via three independent strobe inputs-G1 (active-high) and G2A/G2B (active-low)-enabling direct 24-line expansion without external inverters. Its output-enable architecture allows use as an 8-channel demultiplexer when one enable serves as data input.
Designed for high-speed memory systems, it achieves tPLH/tPHL ≤12 ns (CL = 15 pF, VCC = 5 V), supports TTL-voltage compatibility (VIH = 2 V, VIL = 0.8 V), and maintains rail-to-rail output swing (VOH ≥ 4.4 V, VOL ≤ 0.1 V at IOL = 50 mA) under recommended operating conditions (VCC = 1.5–5.5 V).
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Function | 3-input binary decoder / 1-of-8 demultiplexer with active-low outputs |
| Propagation Delay | ≤12 ns max (A/B/C or G1 → Y, CL = 15 pF, VCC = 5 V) - enables sub-83 MHz system clock domain interfacing |
| Supply Voltage Range | 1.5 V to 5.5 V - interoperable with 3.3 V and 5 V logic families without level shifters |
| Input Compatibility | TTL-voltage compatible (VIH = 2 V min, VIL = 0.8 V max) - directly drives from legacy 5 V microcontrollers |
| Output Drive | ±8 mA (IOL/IOH) at VCC = 5 V - sufficient to drive multiple 74-series inputs or small LED loads |
| Operating Temperature | −40°C to +85°C - qualified for industrial-grade embedded applications |
| Junction-to-Ambient RθJA | 67°C/W (PDIP package) - defines maximum power dissipation before thermal derating in still-air environments |
Pinout & Package
SN74AHCT138N uses a 16-pin plastic dual in-line package (PDIP-N) with 19.3 mm × 9.4 mm body size and 0.100″ lead pitch. Pin 1 is marked by a notch or dot; leads are tin-lead (NIPDAU) RoHS-compliant.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| A (1) | Binary select input A | LSB of 3-bit address; determines output selection with B and C per truth table |
| B (2) | Binary select input B | Middle bit of address; combined with A and C to decode one of eight outputs |
| C (3) | Binary select input C | MSB of address; required for full 3-bit decoding capability |
| G2A (4) | Strobe input 2A | Active-low enable; must be LOW with G2B LOW and G1 HIGH for valid decode |
| G2B (5) | Strobe input 2B | Second active-low enable; both G2A and G2B must be LOW for device activation |
| G1 (6) | Strobe input | Active-high enable; all three enables (G1, G2A, G2B) control output enable state |
| Y7 (7) | Output 7 | Active-low decoded output corresponding to CBA = 111; sinks current when selected |
| GND (8) | Ground reference | Primary return path for logic and output currents; requires low-impedance PCB connection |
| Y6 (9) | Output 6 | Active-low output for CBA = 110; identical electrical behavior to Y7 |
| Y5 (10) | Output 5 | Active-low output for CBA = 101; shares same drive strength and timing as other Yx pins |
| Y4 (11) | Output 4 | Active-low output for CBA = 100; used in memory chip-select generation |
| Y3 (12) | Output 3 | Active-low output for CBA = 011; supports peripheral address decoding |
| Y2 (13) | Output 2 | Active-low output for CBA = 010; commonly tied to interrupt controller inputs |
| Y1 (14) | Output 1 | Active-low output for CBA = 001; used in I/O port expansion schemes |
| Y0 (15) | Output 0 | Active-low output for CBA = 000; default channel for boot memory or primary peripheral |
| VCC (16) | Positive supply | Power rail (1.5–5.5 V); requires local 0.1 µF ceramic bypass capacitor per TI recommendation |
Key Features
| Feature | Design Value |
|---|---|
| TTL-compatible inputs | Accepts standard 5 V TTL logic levels (VIH = 2 V, VIL = 0.8 V) without external biasing |
| Three enable inputs | G1 (active-high), G2A/G2B (active-low) allow hierarchical decoding and seamless 24-line expansion |
| Low propagation delay | tPLH/tPHL ≤12 ns ensures minimal latency in address decoding paths for fast SRAM or Flash interfaces |
| High ESD robustness | ±2000 V HBM and ±1000 V CDM ratings reduce field failure risk in manual assembly and test environments |
| Latch-up immunity | Exceeds 250 mA per JESD17 - prevents destructive latch-up during transient overvoltage events |
Applications
| Memory Address Decoding | Microcontroller I/O Expansion |
|---|---|
Use Scenario: Selecting among eight SRAM or Flash memory chips in a 16-bit embedded system using A15–A13 address lines. IC Role / Device Role / Timing Role: 3-to-8 decoder mapping upper address bits to individual chip-select (CS#) signals with <12 ns delay. Use Value: Eliminates need for discrete logic gates; enables deterministic memory access timing within 5 V MCU bus cycles. |
Use Scenario: Expanding GPIO count on an 8-bit MCU by enabling eight peripheral devices (ADC, DAC, sensor hubs) via decoded address lines. IC Role / Device Role / Timing Role: Demultiplexer converting 3-bit port output into eight independent enable signals for peripheral selection. Use Value: Reduces firmware overhead vs software-based bit-banging; provides hardware-level arbitration with guaranteed setup/hold margins. |
| Industrial Control Logic | Legacy System Interface |
Use Scenario: Driving eight relay drivers or opto-isolators in a PLC I/O module where each output corresponds to a machine function. IC Role / Device Role / Timing Role: Active-low output decoder providing fail-safe deactivation (all outputs HIGH when disabled). Use Value: Ensures safe default state during reset or watchdog timeout; supports direct connection to 5 V relay coils. |
Use Scenario: Interfacing modern 3.3 V SoC with legacy 5 V parallel peripherals (printers, displays) requiring TTL-level address decoding. IC Role / Device Role / Timing Role: Level-tolerant decoder accepting 3.3 V logic inputs while driving 5 V-tolerant outputs. Use Value: Avoids dedicated level translators; maintains signal integrity across mixed-voltage subsystems. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar 3-to-8 decoder applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74HCT138N | Same pinout and function but HCT family: higher noise immunity, slightly slower (tPD ≤ 22 ns), wider VCC range (2–6 V) | Better suited for noisy industrial environments; less ideal for high-speed memory decoding | Select SN74HCT138N if system noise margin >400 mV is required and speed <50 MHz suffices |
| 74LV138N | Lower voltage operation (1.2–5.5 V), faster (tPD ≤ 7.5 ns), but only 3.3 V–optimized; not TTL-compatible | Requires level shifting for 5 V MCU interfaces; superior for battery-powered 3.3 V designs | Choose 74LV138N only when full 5 V TTL compatibility is unnecessary and sub-10 ns delay is critical |
Compared with SN74HCT138N and 74LV138N, the SN74AHCT138N uniquely balances TTL compatibility, 12 ns speed, and industrial temperature range-making it optimal for retrofitting legacy 5 V systems without redesigning voltage rails or timing budgets.
Availability
SN74AHCT138N is available at Aetrix Electronics and suitable for industrial control logic, microcontroller I/O expansion, memory address decoding, and legacy system interface applications requiring stable component supply and long-term manufacturability.
Supply support for SN74AHCT138N 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 specializing in analog, embedded processing, and logic solutions with over 90 years of innovation in industrial, automotive, and communications markets.
The SN74AHCT138N belongs to TI's advanced high-speed CMOS logic family, engineered specifically for high-performance memory decoding and data-routing applications demanding short propagation delays and robust DC specifications.
FAQ
What is the maximum clock frequency supported by SN74AHCT138N in demultiplexer mode?
The SN74AHCT138N does not operate on a clock signal-it is combinational logic. Its usable data rate depends on propagation delay: with tPD ≤12 ns, it supports address/data transitions up to ~83 MHz in worst-case timing paths. For reliable operation in synchronous systems, ensure setup/hold times relative to the controlling enable or select signals are met per the switching characteristics table in the SN74AHCT138N datasheet.
Can SN74AHCT138N drive LEDs directly without current-limiting resistors?
No. While SN74AHCT138N outputs can sink up to 8 mA (IOL = 8 mA) at VOL ≤ 0.5 V, direct LED connection risks exceeding absolute maximum ratings and damaging the output stage. Always use series current-limiting resistors-e.g., 330 Ω for a 5 V supply and red LED-to keep current within 8 mA and maintain VOL < 0.44 V per SN74AHCT138N specifications.
Is SN74AHCT138N pin-compatible with older 74LS138 devices?
Yes-SN74AHCT138N uses the same 16-pin PDIP footprint and identical pinout as 74LS138, including A, B, C, G1, G2A, G2B, Y0–Y7, VCC, and GND. However, SN74AHCT138N offers higher speed (12 ns vs 25 ns), lower power (ICC = 40 µA typical), and TTL-compatible inputs without pull-ups, making it a drop-in upgrade for LS-based designs.
Does SN74AHCT138N require external pull-up resistors on its outputs?
No. SN74AHCT138N outputs are push-pull (totem-pole), not open-drain, so they actively drive both HIGH and LOW states. Pull-ups are unnecessary and would cause contention during LOW output periods. External pull-ups may be added only for specific wired-OR configurations-but this is not standard usage and violates the SN74AHCT138N's intended active-drive operation.
How does the thermal performance of SN74AHCT138N compare to SOIC-packaged variants?
SN74AHCT138N (PDIP) has RθJA = 67°C/W, significantly higher than SOIC (D) at 93.8°C/W or VQFN (RGY) at 39°C/W. This means the PDIP package dissipates heat less efficiently-limiting continuous power to ~150 mW before junction temperature exceeds 125°C at 25°C ambient. Use adequate PCB copper area and avoid stacking near heat sources when deploying SN74AHCT138N in high-density layouts.
SN74AHCT138N Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74AHCT
- Package/Case:
- 16-DIP (0.300", 7.62mm)
- Packaging:
- Bulk
- Product Status:
- Active
- Type:
- Decoder/Demultiplexer
- Circuit:
- 1 x 3:8
- Independent Circuits:
- 1
- Current - Output High, Low:
- 8mA, 8mA
- Voltage Supply Source:
- Single Supply
- Voltage - Supply:
- 4.5V ~ 5.5V
- Operating Temperature:
- -40°C ~ 85°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 16-PDIP
SN74AHCT138N FAQ
1.How can I place an order for SN74AHCT138N through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AHCT138N 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 SN74AHCT138N reliable?
The price and inventory of SN74AHCT138N are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AHCT138N is usually 5 days.
3.What payment methods are accepted for SN74AHCT138N?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AHCT138N transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AHCT138N?
SN74AHCT138N orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AHCT138N 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 SN74AHCT138N?
For technical support, including SN74AHCT138N datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AHCT138N requirements.
6.How does Aetrix verify that SN74AHCT138N is sourced from the original manufacturer or authorized distributors?
All SN74AHCT138N 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 SN74AHCT138N meets industry standards.
7.What is the process for return or replacement of SN74AHCT138N?
All SN74AHCT138N units undergo pre-shipment inspection (PSI). If there is an issue with SN74AHCT138N, 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 SN74AHCT138N part is unused and in its original packaging.
Return procedure for SN74AHCT138N:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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