Texas Instruments SN74F138NSR

Overview

SN74F138NSR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer IC designed for high-speed memory decoding and data routing. It features three enable inputs (G1, G2A, G2B), eight active-low decoded outputs (Y0–Y7), propagation delays as low as 2.7 ns (tPLH), operates at 4.5–5.5 V, and is rated for 0°C to 70°C industrial temperature range.

For engineers reviewing the SN74F138NSR datasheet, SN74F138NSR pinout, SN74F138NSR application, or SN74F138NSR equivalent, key selection criteria include enable-input logic configuration (two active-low, one active-high), output drive capability (20 mA sink), TTL-compatible input thresholds, and SOP-16 packaging for space-constrained PCB layouts.

Technical Context

The SN74F138NSR implements combinational logic decoding using standard TTL circuitry with Schottky-clamped transistors to achieve high speed and noise immunity. Its three enable inputs allow hierarchical cascading without external inverters - e.g., a 24-line decoder requires no external gates, while a 32-line implementation needs only one inverter.

Each of the eight outputs is individually controlled by the binary-select inputs (A, B, C) and all three enables. An enable input (e.g., G1) may serve as a data input in demultiplexing mode, converting the device into a 1-to-8 data distributor with address-controlled output selection.

Key Specifications

Pinout & Package

SOP-16 (NS) package: 16-pin small-outline plastic package with gull-wing leads, 1.27 mm pitch, 10.4 mm × 5.3 mm body size, maximum height 2.00 mm, and moisture sensitivity level (MSL) Level-1 (unlimited floor life at ≤30°C/60% RH).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with SN74LS138N and SN74HC138N, the SN74F138NSR delivers optimal balance of speed, drive strength, and TTL compatibility for legacy 5 V embedded control systems - especially where decoder delay must remain below memory access time.

Availability

SN74F138NSR is available at Aetrix Electronics and suitable for memory decoding, bus arbitration, LED multiplexing, interrupt prioritization, and industrial control logic requiring stable component supply and long-term manufacturability.

Supply support for SN74F138NSR 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, automotive, and communications reach.

The SN74F138NSR belongs to TI's 74F family of FAST TTL logic devices, engineered specifically for high-speed memory decoding and data-routing applications where propagation delay and output drive are critical.

FAQ

What is the maximum operating frequency supported by SN74F138NSR in a memory decoding application?

The SN74F138NSR does not specify a maximum clock frequency, as it is a combinational logic device. However, its worst-case propagation delay is 9 ns (tPHL/tPLH), enabling reliable operation in systems with memory access times >9 ns - effectively supporting CPU bus frequencies up to ~100 MHz when integrated into tight-timing memory subsystems. The SN74F138NSR performance is validated under 50 pF load and 500 Ω termination per TI SDFS051B.

Can SN74F138NSR be used as a demultiplexer, and how is that configured?

Yes, the SN74F138NSR can operate as a 1-to-8 demultiplexer. To configure it, hold the binary select inputs A, B, and C static (e.g., grounded), then apply the data signal to G1 (active-high enable). When G2A and G2B are held LOW, the data appears inverted on the selected Yx output (e.g., G1 HIGH → Y0 LOW). This mode leverages the SN74F138NSR's inherent enable-input flexibility without external components.

What are the absolute maximum ratings for SN74F138NSR supply voltage and input voltage?

The SN74F138NSR has an absolute maximum supply voltage (VCC) of –0.5 V to 7 V and absolute maximum input voltage (VI) of –1.2 V to 7 V. Exceeding these limits risks permanent damage. Recommended operating VCC is strictly 4.5 V to 5.5 V, and input voltages should stay within VIH ≥ 2.0 V (high) and VIL ≤ 0.8 V (low) for guaranteed TTL compatibility. These values are specified in the SN74F138NSR datasheet SDFS051B Rev July 1996.

Is SN74F138NSR pin-compatible with SN74LS138 or SN74HC138?

No, the SN74F138NSR is not pin-compatible with SN74LS138 or SN74HC138 in the same package variant. While all three share identical pin numbering and function assignment in PDIP-16 (N) and SOIC-16 (D), the SN74F138NSR uses SOP-16 (NS) packaging - a distinct physical outline with different land pattern and solder mask requirements. Electrical behavior (speed, drive, thresholds) also differs significantly, requiring schematic and layout review before substitution.

Does SN74F138NSR support cascading for larger decoder configurations, and how many devices are needed for a 4-to-16 decoder?

Yes, the SN74F138NSR supports cascading via its three enable inputs. For a 4-to-16 decoder, two SN74F138NSR devices are required: the fourth address bit (A3) selects between them - driving G1 of one device and G2A/G2B of the other through inversion if needed. TI documentation confirms that 24-line decoding is possible without external inverters, and 32-line requires only one inverter - validating robust hierarchical expansion capability in the SN74F138NSR design.