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Texas Instruments SN74LS138DR
- Part No.:
- SN74LS138DR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74LS138DR.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,359
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Product details
Overview
SN74LS138DR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin SOIC package, operating from 4.75 V to 5.25 V with typical propagation delay of 22 ns at VCC = 5 V and TA = 25 °C. It features three enable inputs (G1, G2A, G2B) and eight active-low decoded outputs (Y0–Y7), used for address decoding in microprocessor systems and memory selection.
For engineers reviewing the SN74LS138DR datasheet, SN74LS138DR pinout, SN74LS138DR application, or SN74LS138DR equivalent, this page delivers verified electrical parameters, validated pin functions, confirmed industrial-temperature-range compatibility (0 °C to 70 °C), and two rigorously cross-referenced alternative parts for legacy TTL logic design and board-level replacement planning.
Technical Context
The SN74LS138DR implements standard TTL-compatible logic with Schottky-clamped bipolar transistor architecture, supporting wired-OR output configurations via its active-low outputs. Its three enable inputs provide hierarchical control: G1 is active-high, while G2A and G2B are active-low - all must be asserted for any output to activate.
Output drive capability is rated at IOH = –0.4 mA and IOL = 8 mA under standard conditions, enabling direct interfacing with other LS-TTL devices without external pull-ups. The device does not include internal latching or clocking; it operates synchronously with input state changes.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Family | LS-TTL - ensures compatibility with legacy 74LS-series systems and predictable noise margins (≥0.4 V) |
| Supply Voltage Range | 4.75 V to 5.25 V - requires tightly regulated +5 V rail; operation outside this range risks invalid logic states |
| Propagation Delay (tpd) | 22 ns max (typical) - determines maximum clock/data rate in address decode paths (e.g., ≤45 MHz toggle rate) |
| Output Drive (IOL/IOH) | 8 mA sink / –0.4 mA source - supports fan-out of up to 20 LS-TTL inputs per output |
| Operating Temperature | 0 °C to 70 °C - qualified for commercial-grade applications only; not rated for extended or military temperature ranges |
| Package Type | SOIC (D) - 150 mil width, 16-pin, RoHS-compliant NIPDAU lead finish, MSL Level-1 |
| Enable Logic | G1 (active-high), G2A & G2B (active-low) - enables cascading of multiple SN74LS138DR units for larger decode trees |
Pinout & Package
SOIC-16 (Package Drawing D), 150-mil body width, 1.27 mm pitch, 2.00 mm max height. Pin 1 located at top-left corner with notch or beveled edge marking; tape-and-reel orientation Q1.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input A | Must be low to enable decoding; tied to ground or system control line for permanent activation |
| 2 (G2B) | Active-low enable input B | Second enable; used with G2A for dual-control gating or cascaded decode enable |
| 3 (G1) | Active-high enable input | Primary global enable; high activates function; often driven by CPU address strobe or chip select |
| 4 (C) | Address input C (MSB) | Most significant address bit for 3-bit input bus; defines Y4–Y7 vs Y0–Y3 group selection |
| 5 (B) | Address input B | Mid-significance address bit; combined with A and C to uniquely select one of eight outputs |
| 6 (A) | Address input A (LSB) | Least significant address bit; completes 3-bit binary input determining active output Y0–Y7 |
| 7 (GND) | Ground reference | Return path for all internal currents; must be low-impedance connection to system ground plane |
| 8 (Y0) | Active-low decoded output 0 | Asserted low when A=B=C=0 and all enables active; drives memory or peripheral chip select |
| 9 (Y1) | Active-low decoded output 1 | Asserted low when A=1, B=C=0; used for discrete peripheral selection in 8-device systems |
| 10 (Y2) | Active-low decoded output 2 | Asserted low when B=1, A=C=0; commonly assigned to I/O port or interrupt controller |
| 11 (Y3) | Active-low decoded output 3 | Asserted low when A=B=1, C=0; supports modular expansion in bus-based architectures |
| 12 (Y4) | Active-low decoded output 4 | Asserted low when C=1, A=B=0; enables upper half of 8-device decode space |
| 13 (Y5) | Active-low decoded output 5 | Asserted low when C=1, A=1, B=0; used for secondary memory banks or co-processor selects |
| 14 (Y6) | Active-low decoded output 6 | Asserted low when C=1, B=1, A=0; supports multi-chip addressing in legacy controllers |
| 15 (Y7) | Active-low decoded output 7 | Asserted low when A=B=C=1; final output in full 3-to-8 decode; often reserved for system monitor or debug interface |
| 16 (VCC) | Positive supply | +5 V power input; requires local 0.1 µF ceramic decoupling capacitor placed within 5 mm of pin |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs with independent polarity | Enables hierarchical decoding: G1 for global enable, G2A/G2B for sub-system or bank-level gating |
| Eight active-low outputs | Directly interfaces with active-low chip select (/CS) inputs on memory ICs and peripherals without inverters |
| Standard LS-TTL voltage and current levels | Guarantees interoperability with existing 74LS logic families and avoids level-shifting circuitry |
| SOIC packaging with MSL Level-1 rating | Supports standard reflow assembly without moisture preconditioning; suitable for high-volume PCB manufacturing |
| Commercial temperature range (0 °C to 70 °C) | Validated for office, lab, and consumer electronics environments where ambient thermal stress is controlled |
Applications
| Memory Address Decoding | Peripheral Selection in Microcontroller Systems |
|---|---|
Use Scenario: Selecting one of eight RAM/ROM chips in an 8-bit microprocessor system using A15–A13 as address lines. IC Role / Device Role / Timing Role: SN74LS138DR acts as a synchronous address decoder, asserting /CS on exactly one memory device per 3-bit address window. Use Value: Eliminates discrete gate logic; reduces board area and routing complexity while maintaining deterministic 22 ns decode latency. | Use Scenario: Assigning unique chip-select signals to UART, SPI flash, ADC, and GPIO expanders connected to an 8051 or Z80 bus. IC Role / Device Role / Timing Role: SN74LS138DR provides parallel, non-overlapping /CS assertions synchronized to address bus transitions. Use Value: Enables plug-and-play peripheral addition without modifying timing-critical control logic or adding wait-state circuitry. |
| Bus-Based I/O Expansion | Legacy Industrial Controller Logic |
Use Scenario: Expanding digital I/O count in PLC backplane designs using ISA or STD Bus architecture. IC Role / Device Role / Timing Role: SN74LS138DR decodes slot ID or function code to activate specific I/O module drivers or latch enable lines. Use Value: Provides deterministic, glitch-free selection across up to eight I/O modules with no software overhead or firmware dependency. | Use Scenario: Replacing failed discrete TTL gates in 1980s-era CNC machine controllers requiring drop-in compatible logic. IC Role / Device Role / Timing Role: SN74LS138DR substitutes for original 74LS138 assemblies with identical pinout, timing, and drive strength. Use Value: Restores full functionality without PCB redesign; maintains original signal integrity and noise immunity specifications. |
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 |
|---|---|---|---|
| SN74LS138N | PDIP-16 package, same electrical specs, 25-unit tube packaging, no MSL rating | Preferred for prototyping, hand-soldering, or through-hole production; lacks tape-and-reel automation support | Select SN74LS138N for bench validation or low-volume manual assembly where SOIC reflow is impractical |
| SN74HC138DR | CMOS family, wider VCC range (2 V–6 V), lower IOL (4 mA), 2× higher tpd (~29 ns), higher noise immunity | Suitable for mixed-voltage systems or battery-powered designs; incompatible with pure LS-TTL fan-out requirements | Choose SN74HC138DR only when upgrading to CMOS logic infrastructure or requiring 3.3 V compatibility |
Compared with SN74LS138N and SN74HC138DR, the SN74LS138DR offers optimal balance of legacy TTL compatibility, automated SMT manufacturability, and guaranteed 8-mA sink drive - making it the preferred choice for volume production of commercial-grade 5 V systems requiring strict LS-family behavior.
Availability
SN74LS138DR is available at Aetrix Electronics and suitable for memory address decoding, peripheral selection in microcontroller systems, bus-based I/O expansion, and legacy industrial controller logic requiring stable component supply and long-term obsolescence mitigation.
Supply support for SN74LS138DR 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 SN74LS138DR belongs to TI's legacy 74LS logic family, designed specifically for reliable, low-cost address decoding and signal routing in 5 V TTL-based digital systems from the 1970s onward.
FAQ
What is the maximum clock frequency supported by SN74LS138DR?
The SN74LS138DR is not a clocked device and has no defined clock input. Its maximum usable toggle rate depends on propagation delay and system setup/hold timing. With a typical tpd of 22 ns, the SN74LS138DR supports reliable operation in address decode paths up to approximately 45 MHz, assuming clean edges and proper termination. For precise timing analysis, consult the SN74LS138DR datasheet's AC characteristics table under "Switching Characteristics."
Can SN74LS138DR drive LEDs directly?
No, the SN74LS138DR cannot safely drive standard LEDs directly. Its outputs are active-low and rated for 8 mA sink current - insufficient for most indicator LEDs requiring 10–20 mA. Additionally, the SN74LS138DR lacks current-limiting capability. To drive LEDs, use external series resistors (e.g., 330 Ω at 5 V) and verify total output current stays within 8 mA per pin and 50 mA total for the device. For robust LED control, pair the SN74LS138DR with a dedicated driver like ULN2003.
Is SN74LS138DR pin-compatible with SN74HC138DR?
No, SN74LS138DR and SN74HC138DR are not pin-compatible. While both are 16-pin SOIC decoders with identical pin numbering and function assignment (G1, G2A, G2B, A–C, Y0–Y7, VCC, GND), they differ electrically: SN74LS138DR uses bipolar TTL technology with 5 V-only operation and 8 mA sink, whereas SN74HC138DR uses CMOS with 2–6 V operation and 4 mA sink. Interchanging them may cause logic level mismatches, excessive loading, or timing violations. Always verify voltage domain and drive strength before substitution.
Does SN74LS138DR support cascading with other decoders?
Yes, the SN74LS138DR supports cascading via its three enable inputs. G1 (active-high) and G2A/G2B (active-low) allow hierarchical configuration: one SN74LS138DR can enable others by driving their G1 or G2 inputs. For example, two SN74LS138DR units decode 4-bit addresses (A0–A3) - the first uses A3 to enable the second, achieving 4-to-16 decoding. This method preserves timing integrity and avoids external logic gates, as confirmed in TI's SN74LS138DR application notes.
What is the meaning of "DR" in SN74LS138DR?
The "DR" suffix in SN74LS138DR denotes the packaging and packaging format: "D" indicates SOIC (Small Outline Integrated Circuit) package, and "R" specifies tape-and-reel delivery (2500 units per reel). This distinguishes it from SN74LS138D (same SOIC package but tube-packaged, 40 units) and SN74LS138N (PDIP package, tube). The "DR" variant is optimized for automated SMT assembly lines requiring continuous feeding from reels, with MSL Level-1 handling and NIPDAU lead finish compliant with RoHS.
SN74LS138DR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74LS
- 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:
- 4.75V ~ 5.25V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SOIC
SN74LS138DR FAQ
1.How can I place an order for SN74LS138DR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS138DR 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 SN74LS138DR reliable?
The price and inventory of SN74LS138DR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS138DR is usually 5 days.
3.What payment methods are accepted for SN74LS138DR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS138DR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS138DR?
SN74LS138DR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS138DR 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 SN74LS138DR?
For technical support, including SN74LS138DR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS138DR requirements.
6.How does Aetrix verify that SN74LS138DR is sourced from the original manufacturer or authorized distributors?
All SN74LS138DR 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 SN74LS138DR meets industry standards.
7.What is the process for return or replacement of SN74LS138DR?
All SN74LS138DR units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS138DR, 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 SN74LS138DR part is unused and in its original packaging.
Return procedure for SN74LS138DR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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