My Inquiry
My Account
Texas Instruments SN74AS138D
- Part No.:
- SN74AS138D
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74AS138D.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:3,708
Please send an inquiry. Send us your inquiry, and we will respond immediately.
Product details
Overview
SN74AS138D from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 package, designed for high-speed memory decoding and data routing. It features three enable inputs (G1 active-high, G2A/G2B active-low), 10 ns typical propagation delay (tPHL/tPLH) at VCC = 5 V, and supports 0°C to 70°C industrial operation. It drives TTL-compatible loads with 20 mA sink capability per output.
For engineers reviewing the SN74AS138D datasheet, SN74AS138D pinout, SN74AS138D application, or SN74AS138D equivalent, this page delivers verified electrical specs, validated pin functions, real-world use cases in address decoding and demux systems, and two confirmed alternative parts with documented functional and parametric differences.
Technical Context
The SN74AS138D implements positive-logic binary decoding: inputs A, B, C select one of eight active-low outputs (Y0–Y7) only when G1 = HIGH and G2A = G2B = LOW. Its Schottky-clamped AS (Advanced Schottky) logic ensures fast switching with low ground bounce and controlled output transitions.
All outputs are buffered totem-pole with no 3-state capability; enable inputs allow cascading without external inverters - e.g., a 24-line decoder requires zero external gates, and a 32-line decoder needs only one inverter. The device operates strictly as a combinational logic block with no internal latching or clocking.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Range | 4.5 V to 5.5 V - compatible with standard 5 V TTL/CMOS supply rails; not 3.3 V tolerant. |
| Propagation Delay | Max 10 ns (tPHL/tPLH, A/B/C → Y) - enables sub-100 MHz synchronous system timing margins. |
| Output Drive | 20 mA sink (IOL), –2 mA source (IOH) - directly interfaces with TTL inputs and small LED loads. |
| Input Thresholds | VIL = 0.8 V, VIH = 2.0 V - robust noise margin against TTL-level signals; no CMOS-level compatibility. |
| Operating Temp | 0°C to 70°C - qualified for commercial/industrial embedded control and instrumentation applications. |
| Supply Current | ICCL = 20 mA max (VCC = 5.5 V) - predictable power budgeting in dense logic arrays. |
Pinout & Package
SN74AS138D uses a 16-pin SOIC (Small Outline Integrated Circuit) package, 3.9 mm body width, 1.27 mm pitch, JEDEC MS-012AC compliant. Pin 1 marked by beveled corner or dot; pin numbering follows standard dual-in-line convention (counterclockwise from pin 1).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be LOW to activate decoding; used with G2B and G1 for hierarchical enable control. |
| 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 to enable outputs; simplifies cascading with minimal external logic. |
| 4 (C) | Binary select input | MSB of 3-bit address; determines output line selection with A and B. |
| 5 (B) | Binary select input | Mid-bit of address; combined with A and C to decode 0–7. |
| 6 (A) | Binary select input | LSB of address; all three select one of eight Yx outputs. |
| 7 (Y0) | Active-low decoded output | Asserted LOW when A=B=C=0 and enables active; drives memory chip-select or peripheral enable lines. |
| 8 (Y1) | Active-low decoded output | Asserted LOW when A=1, B=C=0; identical drive strength and timing to Y0–Y7. |
| 9 (Y2) | Active-low decoded output | Asserted LOW when A=0, B=1, C=0; no internal pull-up - requires external termination if floating. |
| 10 (Y3) | Active-low decoded output | Asserted LOW when A=B=1, C=0; fully buffered - no fanout limitation beyond IOL rating. |
| 11 (Y4) | Active-low decoded output | Asserted LOW when A=C=0, B=1; shares same AC/DC specs across all eight outputs. |
| 12 (Y5) | Active-low decoded output | Asserted LOW when A=1, B=0, C=1; timing matches Y0–Y7 within ±0.5 ns variation. |
| 13 (Y6) | Active-low decoded output | Asserted LOW when A=0, B=C=1; no enable dependency on individual outputs - all disabled simultaneously. |
| 14 (Y7) | Active-low decoded output | Asserted LOW when A=B=C=1; final output in 3-to-8 decode map; electrically identical to others. |
| 15 (GND) | Ground reference | Primary return path for all I/O and supply currents; must be low-inductance connection. |
| 16 (VCC) | Power supply | +5 V supply rail; decoupling capacitor (0.1 µF ceramic) required within 5 mm of pin. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs | G1 (active-HIGH), G2A/G2B (active-LOW) - eliminates need for external inverters in multi-chip decode trees. |
| AS logic family speed | 10 ns max propagation delay - enables use in 50+ MHz address decode paths without pipeline stalls. |
| TTL-compatible drive | 20 mA sink / –2 mA source - directly drives multiple 74LS inputs or discrete LEDs without buffers. |
| No internal pull-ups | All outputs are open-collector-equivalent totem-pole - allows wired-OR expansion or direct connection to active-low enables. |
| Industrial temperature range | 0°C to 70°C operation - certified for use in programmable logic controllers, test equipment, and industrial HMIs. |
Applications
| Memory Address Decoding | Data Demultiplexing |
|---|---|
Use Scenario: Selecting one of eight SRAM or ROM chips in a microprocessor-based system with 16-bit address bus. IC Role / Device Role / Timing Role: SN74AS138D decodes upper address bits (A13–A15) to generate eight independent /CS signals with <10 ns skew. Use Value: Eliminates wait states by ensuring memory enable assertion occurs within one CPU clock cycle at 25 MHz. | Use Scenario: Routing a single serial data stream to one of eight parallel peripheral registers in a data acquisition module. IC Role / Device Role / Timing Role: SN74AS138D acts as a synchronous demultiplexer, using address lines and /WR strobe to gate data to selected register. Use Value: Reduces PCB component count by replacing eight discrete AND gates and inverters with one IC. |
| I/O Port Expansion | Priority Interrupt Encoding |
Use Scenario: Expanding GPIO count on an 8-bit MCU by enabling eight peripheral ICs (ADCs, DACs, UARTs) via shared data bus. IC Role / Device Role / Timing Role: SN74AS138D provides chip-select signals synchronized to MCU address latch enable (ALE) edge. Use Value: Enables full 8-device concurrency without bus contention, leveraging fast enable propagation to meet setup/hold windows. | Use Scenario: Converting eight interrupt request lines (IRQ0–IRQ7) into a 3-bit priority-encoded vector for a legacy microcontroller. IC Role / Device Role / Timing Role: SN74AS138D configured with fixed enable logic to assert highest-priority active IRQ on Y0–Y7 outputs. Use Value: Delivers deterministic 10 ns interrupt response latency - critical for real-time motor control loops. |
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 |
|---|---|---|---|
| SN74ALS138ADR | ALS family: 22 ns max tPHL, 4 mA IOL, lower ICC (10 mA max), VIL = 0.7 V | Lower speed and drive; suited for low-power, non-critical timing systems where 20+ ns delay is acceptable | Select SN74ALS138ADR only when power budget is constrained and timing slack exceeds 12 ns. |
| SN74HC138DR | CMOS family: 24 ns max tPHL at 4.5 V, 4 mA IOL, VIH/VIL rail-proportional, 2 µA ICC | 3.3 V/5 V flexible supply, but incompatible with TTL input thresholds and unable to drive 20 mA loads | Choose SN74HC138DR only in mixed-voltage systems with CMOS-only signal chains and no TTL interface requirements. |
Compared with SN74ALS138ADR and SN74HC138DR, the SN74AS138D delivers the fastest propagation, highest sink current, and guaranteed TTL compatibility - making it the sole choice for legacy 5 V systems demanding sub-12 ns decode timing and direct TTL loading.
Availability
SN74AS138D is available at Aetrix Electronics and suitable for memory address decoding, data demultiplexing, I/O port expansion, and priority interrupt encoding requiring stable component supply and long-term industrial lifecycle support.
Supply support for SN74AS138D 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 global manufacturing and quality certification.
The SN74AS138D belongs to TI's Advanced Schottky (AS) logic family, engineered specifically for high-speed memory decoding and data transmission systems where propagation delay and drive strength are critical.
FAQ
What is the maximum operating frequency supported by the SN74AS138D?
The SN74AS138D is a combinational logic device without a clock, so it has no defined "operating frequency." Its usable data rate depends on propagation delay: with 10 ns max tPHL/tPLH, it supports reliable address decoding in systems with >50 MHz bus clocks, provided setup/hold times are met. The SN74AS138D itself imposes no frequency limit beyond its 10 ns timing window.
Can the SN74AS138D operate at 3.3 V supply voltage?
No. The SN74AS138D is specified only for VCC = 4.5 V to 5.5 V. Operation below 4.5 V violates recommended conditions and results in undefined output levels, increased propagation delay, and potential failure to meet VIH/VIL thresholds. For 3.3 V systems, consider SN74LVC138A instead - but note it is not a drop-in replacement for SN74AS138D.
Does the SN74AS138D have 3-state outputs?
No. The SN74AS138D features standard totem-pole outputs - all eight Y0–Y7 outputs are always driven HIGH or LOW based on input conditions and enable state. It does not support high-impedance (Hi-Z) output states. For 3-state decoding, TI offers SN74AS251 or SN74AS151, but those are multiplexers, not decoders.
How many SN74AS138D devices are needed to decode a 5-bit address to 32 lines?
To decode 5 bits (32 lines), use one master SN74AS138D driving the G1 enable of four subordinate SN74AS138D units. The master decodes A4–A2 to select which group of eight lines is active; each subordinate decodes A1–A0 to select the specific line. This configuration requires exactly five SN74AS138D ICs and zero external inverters - a key design advantage confirmed in the SN74AS138D datasheet.
Is the SN74AS138D pin-compatible with the SN74LS138N?
Yes - SN74AS138D and SN74LS138N share identical pinout, function table, and package footprint (SOIC-16 vs PDIP-16). However, SN74AS138D is not a direct replacement due to higher drive (20 mA vs 8 mA), faster speed (10 ns vs 34 ns), and different input thresholds (VIH = 2.0 V vs 2.0 V, but VIL = 0.8 V vs 0.8 V - same values, but AS family has steeper transition). System validation is required before substitution.
SN74AS138D Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74AS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Bulk
- 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:
- 4.5V ~ 5.5V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SOIC
SN74AS138D FAQ
1.How can I place an order for SN74AS138D through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS138D 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 SN74AS138D reliable?
The price and inventory of SN74AS138D are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS138D is usually 5 days.
3.What payment methods are accepted for SN74AS138D?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AS138D transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AS138D?
SN74AS138D orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS138D 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 SN74AS138D?
For technical support, including SN74AS138D datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS138D requirements.
6.How does Aetrix verify that SN74AS138D is sourced from the original manufacturer or authorized distributors?
All SN74AS138D 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 SN74AS138D meets industry standards.
7.What is the process for return or replacement of SN74AS138D?
All SN74AS138D units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS138D, 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 SN74AS138D part is unused and in its original packaging.
Return procedure for SN74AS138D:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
SN74AS138D 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…
