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Texas Instruments SN74HCT138DG4
- Part No.:
- SN74HCT138DG4
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74HCT138DG4.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
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Product details
Overview
SN74HCT138DG4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer IC operating at 4.5–5.5 V, delivering 17 ns typical propagation delay (tpd), ±4-mA output drive at 5 V, and low 80-µA max supply current (ICC). It features three enable inputs (two active-low, one active-high) for cascading in memory decoding and data-routing systems.
For engineers reviewing the SN74HCT138DG4 datasheet, SN74HCT138DG4 pinout, SN74HCT138DG4 application, or SN74HCT138DG4 equivalent, this device is selected for high-speed address decoding in SRAM/ROM subsystems, bus demultiplexing in microcontroller peripherals, and logic-level signal routing where TTL-compatible input thresholds and LSTTL-load drive capability are required.
Technical Context
The SN74HCT138DG4 implements a combinational logic decoder with binary-select inputs (A, B, C) and three independent enables (G1, G2A, G2B) that collectively determine which of eight active-low outputs (Y0–Y7) is asserted. Its architecture supports direct demultiplexing when an enable input serves as data input.
Designed for memory-decoding applications, it minimizes system-level decoding delay-its tpd (17 ns typ at 5.5 V) and memory enable time typically fall below memory access time, rendering its contribution to total latency negligible. Input compatibility with TTL voltage levels (VIH = 2 V, VIL = 0.8 V) ensures seamless integration with legacy 5-V logic families.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Supply Voltage | 4.5 V to 5.5 V - Ensures stable operation across standard 5-V rail tolerances without level-shifting. |
| Propagation Delay (tpd) | 17 ns typical at 5.5 V - Enables use in sub-20-ns timing-critical address decode paths. |
| Output Drive | ±4 mA at 5 V - Directly drives up to 10 LSTTL loads without external buffers. |
| Input Thresholds | VIH = 2.0 V, VIL = 0.8 V - Fully compatible with TTL-output devices and legacy 5-V logic interfaces. |
| Supply Current (ICC) | 80 µA max - Supports low-static-power system design in always-on control logic. |
| Input Leakage | ±1 µA max - Prevents unintended logic state shifts on unterminated or high-impedance select lines. |
| Operating Temperature | -40 °C to +85 °C - Qualified for industrial-grade embedded and computing applications. |
Pinout & Package
SN74HCT138DG4 is housed in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm with standard 1.27-mm lead pitch and gull-wing leads. The package is RoHS-compliant, moisture-sensitive level 1, and rated for reflow up to 260 °C.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be low to activate decoding; used with G2B and G1 to gate all outputs. |
| 2 (G2B) | Active-low enable input | Second active-low enable; both G2A and G2B must be low for function enable. |
| 3 (G1) | Active-high enable input | Must be high; combined with G2A/G2B, forms 3-input AND-enable logic. |
| 4 (C) | MSB binary select input | Most significant bit of 3-bit address; determines Y4–Y7 vs. Y0–Y3 group selection. |
| 5 (B) | Mid-bit binary select input | Second bit; works with A and C to uniquely select one of eight outputs. |
| 6 (A) | LSB binary select input | Least significant bit; completes 3-bit decode address mapping to Y0–Y7. |
| 7 (GND) | Ground reference | Return path for all internal logic and output currents; requires low-impedance PCB connection. |
| 8 (Y0) | Active-low decoded output | Asserted low when G1=H, G2A=L, G2B=L, and CBA=000; drives downstream enable or chip-select lines. |
| 9 (Y1) | Active-low decoded output | Asserted low for CBA=001; used for discrete peripheral selection in multi-device buses. |
| 10 (Y2) | Active-low decoded output | Asserted low for CBA=010; supports modular I/O expansion where each output enables a functional block. |
| 11 (Y3) | Active-low decoded output | Asserted low for CBA=011; commonly tied to memory bank enable or ADC channel select. |
| 12 (Y4) | Active-low decoded output | Asserted low for CBA=100; routes control signals to higher-address peripherals in hierarchical decode trees. |
| 13 (Y5) | Active-low decoded output | Asserted low for CBA=101; enables secondary logic arrays or configurable interface modules. |
| 14 (Y6) | Active-low decoded output | Asserted low for CBA=110; used in FPGA configuration sequencers or CPLD address decoding. |
| 15 (Y7) | Active-low decoded output | Asserted low for CBA=111; often assigned to highest-priority interrupt controller or debug interface enable. |
| 16 (VCC) | Positive supply rail | 5-V power input; requires local 0.1-µF ceramic bypass capacitor placed adjacent to pin. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs | Two active-low (G2A, G2B) plus one active-high (G1) simplify hierarchical decoding without external inverters - e.g., 24-line decode possible with no added gates. |
| TTL-compatible inputs | VIH = 2.0 V and VIL = 0.8 V allow direct interfacing with 74LS, 74ALS, and other TTL-family outputs without level translation. |
| Low propagation delay | 17 ns typical at 5.5 V ensures decoder delay does not bottleneck memory access in high-speed SRAM/ROM systems. |
| High noise immunity | Input hysteresis and 0.4-V noise margin (VIL to VIH) suppress false triggering from board-level switching noise. |
| Demultiplexing capability | Any enable input (e.g., G1) can serve as data input while A/B/C act as select lines - enabling 1-to-8 signal distribution without redesign. |
Applications
| Memory Address Decoding | Microcontroller Peripheral Selection |
|---|---|
Use Scenario: Selecting among eight SRAM or ROM chips in a 64 KB memory map using A15–A13 as C/B/A inputs. IC Role / Device Role / Timing Role: Address decoder translating upper address bits into individual chip-enable signals with sub-20-ns latency. Use Value: Eliminates need for discrete NAND/NOT gates; enables full memory bandwidth utilization by keeping decode delay <10% of typical 100-ns access time. | Use Scenario: Routing UART, SPI, I²C, and GPIO interrupts from a single MCU port to dedicated handler circuits. IC Role / Device Role / Timing Role: Demultiplexer converting a shared interrupt request line into eight prioritized, isolated enable signals. Use Value: Reduces MCU pin count and firmware complexity; allows hardware-level priority arbitration without software polling. |
| Industrial Bus Expansion | FPGA Configuration Control |
Use Scenario: Expanding a 4-wire industrial bus (e.g., Modbus RTU) to drive eight remote I/O modules via enable-controlled transceivers. IC Role / Device Role / Timing Role: Enable distributor synchronizing transceiver activation with master command cycles. Use Value: Prevents bus contention by ensuring only one module responds per transaction; supports deterministic timing under 1-ms cycle budgets. | Use Scenario: Sequencing configuration bitstreams to multiple FPGA banks or selecting between dual-boot images during power-up. IC Role / Device Role / Timing Role: Power-on reset–gated decoder asserting bank-specific INIT or PROGRAM_B lines in defined order. Use Value: Guarantees safe, glitch-free configuration handoff; replaces discrete RC-delay networks with repeatable, temperature-stable logic. |
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 |
|---|---|---|---|
| SN74HC138N | CMOS version with wider 2–6 V supply range; 26 ns tpd at 6 V; lower ICC (4 µA), but TTL-incompatible inputs (VIH = 3.5 V min). | Suitable for mixed-voltage or battery-powered systems; not drop-in for TTL-driven designs. | Select SN74HC138N only if supply flexibility or ultra-low static power outweighs need for TTL input compatibility. |
| 74ACT138SCX | Advanced CMOS with 3.3 V/5 V tolerance; 8.5 ns tpd at 5 V; ±24 mA drive; requires 3.3-V logic-level inputs unless using 5-V tolerant variant. | Better for high-speed, low-skew systems requiring faster settling and stronger drive; incompatible with pure 5-V TTL sources. | Choose 74ACT138SCX when system timing budget is ≤10 ns and output loading exceeds 10 LSTTL units. |
Compared with SN74HC138N and 74ACT138SCX, the SN74HCT138DG4 uniquely balances TTL input compatibility, 17-ns speed, and industrial temperature range - making it optimal for retrofitting legacy 5-V controllers and memory subsystems where signal integrity across logic families is critical.
Availability
SN74HCT138DG4 is available at Aetrix Electronics and suitable for memory address decoding, microcontroller peripheral selection, and industrial bus expansion requiring stable component supply and long-term industrial-grade availability.
Supply support for SN74HCT138DG4 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 50 years of innovation in industrial, automotive, and communications markets.
The SN74HCT138DG4 belongs to TI's HCT (High-Speed CMOS TTL-compatible) logic family, engineered specifically for high-performance memory decoding and data-routing applications where fast propagation delay and robust 5-V TTL interoperability are essential.
FAQ
What is the maximum clock or signal frequency supported by SN74HCT138DG4?
The SN74HCT138DG4 is a combinational logic device with no internal clock; its usable signal frequency depends on propagation delay and system timing margins. With 17 ns typical tpd at 5.5 V, it supports reliable operation in address decode paths up to ~30 MHz toggle rate (based on 2×tpd setup/hold windows), provided input transitions meet the 500-ns max rise/fall time spec.
Can SN74HCT138DG4 operate from a 3.3-V supply?
No. The SN74HCT138DG4 requires a minimum 4.5-V supply and is not rated for 3.3-V operation. Its input thresholds (VIH = 2.0 V, VIL = 0.8 V) and output drive characteristics are specified only across 4.5–5.5 V. For 3.3-V systems, consider SN74LVC138A or SN74AHC138 instead.
How many SN74HCT138DG4 devices are needed to decode a 5-bit address to 32 outputs?
Three SN74HCT138DG4 devices are required: two for primary 3-bit decode (Y0–Y7), and one master unit whose outputs enable the two slaves. The master uses two address bits (e.g., A4,A3) to select which slave is active; each slave decodes A2–A0. This achieves full 32-line decode with only one external inverter (to complement one enable line), per TI's documented cascade method.
Does SN74HCT138DG4 require external pull-up or pull-down resistors on unused inputs?
Yes. All unused inputs (including enables and select lines) must be externally biased to a valid logic level - either VCC or GND - to prevent floating states that cause excessive ICC, oscillation, or undefined outputs. TI explicitly mandates this in Section 5.2 of the datasheet to ensure predictable operation and avoid increased power consumption or EMI.
Is SN74HCT138DG4 pin-compatible with SN74HCT138DRG4?
Yes. SN74HCT138DG4 and SN74HCT138DRG4 share identical SOIC-16 (D) package dimensions, pinout, electrical specifications, and thermal ratings. The "G4" and "RG4" suffixes denote different tape-and-reel packaging and lead finish (NIPDAU), but the silicon and pin functions are identical - enabling direct substitution in existing layouts.
SN74HCT138DG4 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74HCT
- 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:
- 4mA, 4mA
- Voltage Supply Source:
- Single Supply
- Voltage - Supply:
- 4.5V ~ 5.5V
- Operating Temperature:
- -40°C ~ 85°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SOIC
SN74HCT138DG4 FAQ
1.How can I place an order for SN74HCT138DG4 through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74HCT138DG4 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 SN74HCT138DG4 reliable?
The price and inventory of SN74HCT138DG4 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74HCT138DG4 is usually 5 days.
3.What payment methods are accepted for SN74HCT138DG4?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74HCT138DG4 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74HCT138DG4?
SN74HCT138DG4 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74HCT138DG4 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 SN74HCT138DG4?
For technical support, including SN74HCT138DG4 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74HCT138DG4 requirements.
6.How does Aetrix verify that SN74HCT138DG4 is sourced from the original manufacturer or authorized distributors?
All SN74HCT138DG4 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 SN74HCT138DG4 meets industry standards.
7.What is the process for return or replacement of SN74HCT138DG4?
All SN74HCT138DG4 units undergo pre-shipment inspection (PSI). If there is an issue with SN74HCT138DG4, 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 SN74HCT138DG4 part is unused and in its original packaging.
Return procedure for SN74HCT138DG4:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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