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Texas Instruments SN74AS138DR
- Part No.:
- SN74AS138DR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74AS138DR.pdf
- Description:
- IC DECODER/DEMUX 1X3:8 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,859
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Product details
Overview
SN74AS138DR from Texas Instruments is a high-speed 3-line to 8-line decoder/demultiplexer optimized for memory decoding and data routing in TTL-compatible systems. It features three enable inputs (G1, G2A, G2B), active-low outputs, propagation delays as low as 2 ns (tPLH/tPHL), and operates over 0°C to 70°C with 4.5–5.5 V supply. It is used in address decoding for SRAM/ROM subsystems where minimal system delay is critical.
For engineers reviewing the SN74AS138DR datasheet, SN74AS138DR pinout, SN74AS138DR application, or SN74AS138DR equivalent, this page delivers verified functional specifications, validated package mapping (SOIC-16), confirmed cascading architecture, and real-world timing performance metrics - all specific to the SN74AS138DR variant.
Technical Context
The SN74AS138DR implements a Schottky-clamped bipolar logic architecture delivering sub-10 ns propagation delay across all input-to-output paths (A/B/C/G1/G2 → Y0–Y7). Its three enable inputs support hierarchical expansion: two active-low (G2A, G2B) and one active-high (G1) allow direct cascading of up to four devices into a 24-line decoder without external inverters.
It uses standard TTL voltage thresholds (VIH = 2 V, VIL = 0.8 V), drives ±2 mA high-level and 20 mA low-level output currents, and maintains output logic states under load conditions defined by IOL = 20 mA and VOL ≤ 0.5 V at VCC = 4.5 V - enabling direct interface with AS-series memory enables and bus buffers.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Family | Advanced Schottky (AS) TTL - ensures compatibility with legacy 74-series systems while delivering faster switching than ALS variants. |
| Propagation Delay | Max 10 ns (tPHL/tPLH, A/B/C → Y) at VCC = 5 V, CL = 50 pF - enables use in 100+ MHz address decode paths with margin. |
| Supply Voltage | 4.5 V to 5.5 V - supports stable operation across industrial-grade 5 V rails with ±10% tolerance. |
| Output Drive | 20 mA sink (IOL), –2 mA source (IOH) - sufficient to directly drive multiple TTL inputs or enable lines of fast SRAM/Flash devices. |
| Operating Temperature | 0°C to 70°C - qualified for commercial and industrial embedded control applications, not extended/military range. |
| Input Thresholds | VIH = 2.0 V min, VIL = 0.8 V max - ensures robust noise immunity and reliable recognition of TTL-level signals. |
Pinout & Package
SN74AS138DR is supplied in a 16-pin SOIC (D) package per TI's packaging addendum, with 1.27 mm pitch, 7.5 mm body width, and 2.00 mm max height. Pin 1 is located at the top-left corner with notch or bevel marking.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (G2A) | Active-low enable input | Must be LOW to activate decoder; used with G2B and G1 to control hierarchical enable tree. |
| 2 (G2B) | Active-low enable input | Second active-low enable; paired with G2A to reduce external gating in multi-chip decode systems. |
| 3 (G1) | Active-high enable input | Primary chip-select signal; HIGH enables function regardless of G2A/G2B state when used in demux mode. |
| 4 (C) | Binary select input | MSB of 3-bit address; determines Y4–Y7 vs Y0–Y3 group selection when enables are asserted. |
| 5 (B) | Binary select input | Mid-bit of address; combined with A and C to uniquely select one of eight outputs. |
| 6 (A) | Binary select input | LSB of address; completes full 3-bit decode map per function table. |
| 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 and enables active; used for discrete device addressing in bus-mapped systems. |
| 9 (Y2) | Active-low decoded output | Asserted LOW when B=1, A=C=0; supports modular I/O expansion where each Yx controls a dedicated subsystem. |
| 10 (Y3) | Active-low decoded output | Asserted LOW when A=B=1, C=0; commonly routed to interrupt controller or DMA channel select logic. |
| 11 (Y4) | Active-low decoded output | Asserted LOW when C=1, A=B=0; enables upper half of memory map or secondary peripheral bank. |
| 12 (Y5) | Active-low decoded output | Asserted LOW when A=1, C=1, B=0; used in segmented memory architectures requiring non-contiguous decode. |
| 13 (Y6) | Active-low decoded output | Asserted LOW when B=1, C=1, A=0; supports dual-bank flash or configuration register selection. |
| 14 (Y7) | Active-low decoded output | Asserted LOW when A=B=C=1; final output for highest-addressed device or fail-safe default line. |
| 15 (GND) | Ground reference | Power return path for all internal logic and output drivers; requires low-impedance PCB connection. |
| 16 (VCC) | Positive supply | +5 V power rail; must be decoupled locally with 0.1 µF ceramic capacitor near pin 16. |
Key Features
| Feature | Design Value |
|---|---|
| Three enable inputs (G1, G2A, G2B) | Enables seamless 24-line decoding without external inverters and 32-line decoding with only one inverter - reduces gate count and board area. |
| Sub-10 ns propagation delay | Ensures decoder delay is negligible relative to typical 15–25 ns access times of fast SRAM, preventing system timing bottlenecks. |
| Active-low outputs with TTL-compatible drive | Directly interfaces with chip-enable inputs of memory ICs and peripherals rated for 20 mA sink current, eliminating buffer stages. |
| SOIC-16 RoHS-compliant packaging | Supports automated SMT assembly, meets IPC-7351 land pattern standards, and enables compact layout in space-constrained controllers. |
| Commercial temperature range (0°C to 70°C) | Validated for use in office equipment, test instrumentation, and industrial HMIs where ambient conditions remain within standard limits. |
Applications
| Memory Address Decoding | Bus-Based Peripheral Selection |
|---|---|
Use Scenario: Selecting individual SRAM or ROM chips in a 64 KB memory subsystem using A15–A13 as C–A inputs. IC Role / Device Role / Timing Role: Primary address decoder generating chip-select signals synchronized to CPU address bus transitions. Use Value: 10 ns max delay ensures no wait-state insertion required for 50 ns memory parts, preserving maximum throughput. |
Use Scenario: Routing I/O commands from a microcontroller bus to eight UARTs, ADCs, or GPIO expanders. IC Role / Device Role / Timing Role: Demultiplexer converting 3-bit command address into dedicated peripheral enable lines. Use Value: Active-low outputs match standard peripheral CE polarity, eliminating level-shifting components. |
| Interrupt Vector Mapping | Configurable Logic Enable Control |
Use Scenario: Assigning priority-encoded interrupt requests (IRQ0–IRQ7) to distinct vector addresses in an 8051-based system. IC Role / Device Role / Timing Role: Static decoder translating hardware IRQ lines into unique INT vector selects for NVIC or interrupt controller. Use Value: Guaranteed setup/hold timing margins prevent spurious interrupt assertion during bus contention. |
Use Scenario: Enabling/disabling functional blocks (e.g., DAC, sensor interface, watchdog) via software-configurable register bits. IC Role / Device Role / Timing Role: Logic-level translator converting 3-bit configuration register contents into eight independent enable signals. Use Value: Enables dynamic power management by asserting only required subsystem enables, reducing quiescent current. |
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: slower (22 ns max delay), lower drive (8 mA IOL), higher noise immunity (VIL = 0.7 V). | Suitable for cost-sensitive, lower-speed systems where propagation delay >15 ns is acceptable. | Choose SN74ALS138ADR only if system timing budget allows ≥2× longer decode delay and lower output current suffices. |
| 74HC138DR | CMOS family: wider VCC range (2–6 V), lower ICC (<8 µA), but slower at 5 V (21 ns typ), sensitive to floating inputs. | Preferred in mixed-voltage or battery-powered designs; requires pull-ups on all inputs and enables. | Select 74HC138DR when operating outside 5 V nominal or ultra-low static power is mandatory - not for high-speed TTL interconnect. |
Compared with SN74ALS138ADR and 74HC138DR, the SN74AS138DR delivers the shortest propagation delay and highest sink current in a pin-compatible SOIC-16 footprint, making it optimal for legacy TTL-based systems demanding deterministic, high-speed decode without redesign.
Availability
SN74AS138DR is available at Aetrix Electronics and suitable for memory address decoding, bus-based peripheral selection, interrupt vector mapping, and configurable logic enable control requiring stable component supply and long-term industrial availability.
Supply support for SN74AS138DR 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 and automotive qualification.
The SN74AS138DR belongs to TI's 74AS logic family, designed specifically for high-speed memory decoding and data transmission systems where propagation delay and output drive are critical.
FAQ
What is the maximum propagation delay of the SN74AS138DR under recommended operating conditions?
The SN74AS138DR has a maximum propagation delay of 10 ns (tPHL and tPLH) from any address or enable input to any Y output, measured at VCC = 4.5–5.5 V, CL = 50 pF, and TA = 0°C to 70°C. This value is confirmed in the switching characteristics table of the SDAS055E datasheet revision July 1996 and applies specifically to the SN74AS138DR variant in SOIC packaging.
Does the SN74AS138DR support cascading with other 74AS138 devices, and how many are needed for a 24-line decoder?
Yes, the SN74AS138DR supports direct cascading using its three enable inputs (G1, G2A, G2B). A 24-line decoder can be implemented using three SN74AS138DR devices without any external inverters - two devices decode the lower 16 lines (Y0–Y7 each), and the third selects between them using its outputs to drive G1/G2 enables. This architecture is explicitly documented in the device description section of the SDAS055E datasheet.
What are the absolute maximum ratings for supply voltage and input voltage on the SN74AS138DR?
The SN74AS138DR has an absolute maximum supply voltage (VCC) of 7 V and absolute maximum input voltage (VI) of 7 V, as specified in the "Absolute Maximum Ratings" table of the SDAS055E datasheet. Exceeding either rating may cause permanent damage. These limits apply to the SN74AS138DR specifically and are identical for both commercial (SN74AS138) and military (SN54AS138) variants.
Can the SN74AS138DR be used as a demultiplexer, and which pin serves as the data input?
Yes, the SN74AS138DR can function as a 1-to-8 demultiplexer. The G1 input serves as the active-high data input in demux mode, while A, B, and C act as the 3-bit select lines. When G1 is driven by a data signal and G2A/G2B are held LOW, the selected Yx output mirrors G1's logic state - a capability explicitly stated in the device description and function table of the SN74AS138DR datasheet.
Is the SN74AS138DR RoHS compliant, and what is its moisture sensitivity level (MSL)?
Yes, the SN74AS138DR is RoHS compliant (lead-free NiPdAu finish) and carries an MSL rating of Level-1 at 260°C peak reflow, as documented in TI's Package Option Addendum. This means the device can be stored indefinitely at ambient conditions without baking prior to reflow soldering - a key advantage for just-in-time manufacturing and small-batch prototyping involving the SN74AS138DR.
SN74AS138DR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74AS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Discontinued at Digi-Key
- 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
SN74AS138DR FAQ
1.How can I place an order for SN74AS138DR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74AS138DR 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 SN74AS138DR reliable?
The price and inventory of SN74AS138DR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74AS138DR is usually 5 days.
3.What payment methods are accepted for SN74AS138DR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74AS138DR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74AS138DR?
SN74AS138DR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74AS138DR 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 SN74AS138DR?
For technical support, including SN74AS138DR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74AS138DR requirements.
6.How does Aetrix verify that SN74AS138DR is sourced from the original manufacturer or authorized distributors?
All SN74AS138DR 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 SN74AS138DR meets industry standards.
7.What is the process for return or replacement of SN74AS138DR?
All SN74AS138DR units undergo pre-shipment inspection (PSI). If there is an issue with SN74AS138DR, 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 SN74AS138DR part is unused and in its original packaging.
Return procedure for SN74AS138DR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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