Texas Instruments SN74F138NG4

Overview

SN74F138NG4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer IC used for high-speed memory decoding and data routing in TTL-compatible digital systems. It features three enable inputs (G1, G2A, G2B), eight active-low decoded outputs (Y0–Y7), propagation delays as low as 2.7 ns (tPLH), and operates at VCC = 4.5–5.5 V over 0°C to 70°C. It supports demultiplexing via enable-as-data functionality.

For engineers reviewing the SN74F138NG4 datasheet, SN74F138NG4 pinout, SN74F138NG4 application, or SN74F138NG4 equivalent, key selection criteria include its 16-pin PDIP package, active-low output logic, cascading capability using dual active-low and single active-high enables, and compatibility with legacy 74F-series timing and drive requirements.

Technical Context

The SN74F138NG4 implements positive-logic binary decoding with three select inputs (A, B, C) and three independent enables (G1 = active-high; G2A, G2B = active-low). Its internal architecture uses NAND-based gating to generate eight mutually exclusive active-low outputs, enabling direct interface with memory chip selects or LED drivers without external inversion.

It supports hierarchical expansion: two SN74F138NG4 devices can form a 24-line decoder with no external inverters; a 32-line configuration requires only one inverter. The device's enable inputs double as data paths in demux mode, allowing serial-to-parallel conversion when G2A or G2B serves as the data input while A/B/C act as address lines.

Key Specifications

Pinout & Package

SN74F138NG4 is supplied in a 16-pin plastic dual in-line package (PDIP-N), with 0.3-inch body width and through-hole mounting. Pin 1 is marked by a notch or dot; leads are tin-lead (NIPDAU) RoHS-compliant per TI's production release.

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 SN74F138NG4 delivers the shortest propagation delay and highest drive strength at 5 V, making it optimal for legacy high-speed TTL systems where timing closure is critical and voltage is fixed at 5 V.

Availability

SN74F138NG4 is available at Aetrix Electronics and suitable for memory decoding, peripheral selection, LED multiplexing, and interrupt vectoring applications requiring stable component supply and long-term obsolescence management.

Supply support for SN74F138NG4 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 communications reach.

The SN74F138NG4 belongs to TI's 74F family of fast TTL logic devices, engineered specifically for high-performance memory decoding and data-routing systems where propagation delay and drive capability are primary constraints.

FAQ

What is the maximum clock or address transition frequency supported by SN74F138NG4?

The SN74F138NG4 does not operate on a clock signal-it is combinational logic. Its usable input transition rate is governed by propagation delay and system setup/hold timing. With tPHL/tPLH ≤ 9 ns and clean edges, it reliably supports address bus toggling up to ~50 MHz in well-designed layouts. For SN74F138NG4, ensure input rise/fall times remain under 5 ns to avoid metastability or false outputs.

Can SN74F138NG4 drive LEDs directly without current-limiting resistors?

No-SN74F138NG4 outputs must always be used with external current-limiting resistors. Although it can sink up to 20 mA per output, connecting an LED directly to VCC and Yx would exceed absolute maximum ratings and cause latch-up or permanent damage. For SN74F138NG4, a typical design uses a resistor (e.g., 220 Ω at 5 V) between VCC and the LED anode, with the cathode tied to Yx.

Is SN74F138NG4 pin-compatible with SN74LS138N?

Yes-SN74F138NG4 and SN74LS138N share identical 16-pin PDIP pinouts, terminal functions, and logic behavior. However, SN74F138NG4 draws more supply current (13–20 mA vs. ~2 mA) and delivers faster switching. For SN74F138NG4, verify that the PCB layout and power delivery network can accommodate higher dynamic current and potential EMI from faster edges.

Does SN74F138NG4 support mixed-voltage operation (e.g., 3.3 V inputs with 5 V VCC)?

No-SN74F138NG4 is a 5 V-only TTL device. Its input thresholds (VIH = 2.0 V, VIL = 0.8 V) are optimized for 5 V signaling. Applying 3.3 V logic levels may result in marginal or undefined switching behavior. For SN74F138NG4, use level translators or buffer drivers when interfacing with 3.3 V controllers.

How many SN74F138NG4 devices are needed to decode a 5-bit address to 32 outputs?

Four SN74F138NG4 devices are required: two for the lower 3 bits (generating 8 lines), and two additional units enabled by the upper 2 bits (A4,A3) to select which group of 8 is active. This yields 4 × 8 = 32 outputs. For SN74F138NG4, G1 of each pair is driven by A4/A3 combinations, while G2A/G2B are tied LOW and A/B/C carry the LSBs-no external inverters needed beyond one for complementing A4 or A3 if required.