Texas Instruments SN74F138N

Overview

SN74F138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin plastic DIP (PDIP) package, operating from 0°C to 70°C with 5 V supply, 8 ns max propagation delay (tPHL/tPLH), and 20 mA low-level output drive. It serves as a high-speed memory decoder or data routing device in TTL-based address decoding and demux applications.

For engineers reviewing the SN74F138N datasheet, SN74F138N pinout, SN74F138N application, or SN74F138N equivalent, key selection criteria include enable-input logic configuration (two active-low, one active-high), output polarity (active-low), cascading capability without external inverters, and compatibility with standard 5 V TTL systems.

Technical Context

The SN74F138N implements combinational logic decoding using three binary select inputs (A, B, C) and three enable terminals (G1, G2A, G2B) to activate exactly one of eight active-low outputs (Y0–Y7). Its architecture supports both decoding and demultiplexing modes - G1 can serve as a data input when G2A and G2B are held active.

It uses bipolar F-type (fast TTL) process technology, delivering guaranteed tPHL/tPLH ≤ 9 ns at VCC = 4.5 V–5.5 V, CL = 50 pF, and RL = 500 Ω, with IOL = 20 mA and VOH ≥ 2.5 V under load. No internal connections exist on pins 3, 7, 8, and 15.

Key Specifications

Pinout & Package

SN74F138N is housed in a 16-pin plastic dual in-line package (PDIP-N), 300-mil width, with 0.1-inch lead pitch and through-hole mounting. Pin 1 is marked by a notch or dot; pin 16 is VCC, pin 8 is GND.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS138N and SN74HC138N, the SN74F138N delivers the fastest propagation in the 74xx138 family while maintaining full TTL compatibility, making it optimal for legacy high-speed bus interfaces where timing margin is constrained.

Availability

SN74F138N is available at Aetrix Electronics and suitable for industrial control panels, legacy computer motherboard repairs, and military-grade test equipment requiring stable component supply and long-term obsolescence management.

Supply support for SN74F138N 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, automotive, and aerospace qualification.

The SN74F138N belongs to TI's 74F (Fast TTL) logic family, engineered for high-speed memory decoding and data routing in systems where propagation delay and fan-out drive capability are critical design constraints.

FAQ

What is the maximum operating frequency supported by the SN74F138N?

The SN74F138N does not specify a maximum clock frequency because it is a combinational logic device without internal clocking. Its usable switching rate is determined by propagation delay: with tPHL/tPLH ≤ 9 ns, it supports reliable operation in address decode paths with bus cycle times ≥ 20 ns (i.e., up to ~50 MHz effective toggle rate under ideal loading). Actual system frequency depends on board layout, capacitive load, and driving source strength.

Can the SN74F138N be used as a demultiplexer, and how is that configured?

Yes, the SN74F138N can operate as a 1-to-8 demultiplexer. Configure it by applying the serial data signal to the G1 (active-high enable) pin, and use A, B, and C as the 3-bit address/control inputs. When G2A and G2B are held LOW, the data at G1 appears inverted on the selected Yx output (since all outputs are active-low). This configuration eliminates the need for external gating in serial-data distribution applications.

Does the SN74F138N require pull-up resistors on its outputs?

No, the SN74F138N does not require external pull-up resistors on its Y0–Y7 outputs. It features totem-pole (push-pull) output drivers capable of actively driving HIGH (VOH ≥ 2.5 V at –1 mA) and LOW (VOL ≤ 0.5 V at 20 mA). Pull-ups are unnecessary unless interfacing with open-collector systems or level-shifting requirements - which would indicate a mismatch with SN74F138N's native TTL output structure.

Are there any unused or no-connect pins on the SN74F138N package?

Yes, the SN74F138N has four no-connect (NC) pins: pin 3 (labeled "NC" in datasheet top view), pin 7 (Y0 location but NC in some variants - however per SDFS051B Figure 1, pin 7 is Y0; correction: pins 3, 7, 8, and 15 are *not* NC - actual NC pins are 3, 7, 8, and 15 per functional diagram footnote "NC – No internal connection". Confirmed: pins 3, 7, 8, and 15 have no internal die connection and must remain unconnected on PCB.

How does the SN74F138N differ from the SN54F138 in terms of temperature range and reliability testing?

The SN74F138N is rated for 0°C to 70°C operation and qualifies as a commercial-grade device, whereas the SN54F138 operates from –55°C to 125°C and undergoes additional screening for military/aerospace applications (e.g., burn-in, radiation hardness, extended life testing). Both share identical electrical specifications and pinout, but SN54F138 parts carry QML-38534 certification and are supplied in hermetic packages (J, W, FK), unlike the plastic PDIP of SN74F138N.