Texas Instruments CD74HC138E

Overview

CD74HC138E from Texas Instruments is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-low outputs, operating from 2 V to 6 V, featuring 13 ns typical propagation delay at 5 V/15 pF, -55°C to +125°C temperature range, and three enable inputs (G2, G1, G0) for cascading in memory address decoding or I/O port selection applications.

For engineers reviewing the CD74HC138E datasheet, CD74HC138E pinout, CD74HC138E application, or CD74HC138E equivalent, this device serves as a logic-level address selector in industrial control systems, microcontroller peripheral expansion, and digital instrumentation where low-power, wide-voltage operation and precise output enable control are required.

Technical Context

The CD74HC138E implements a standard 3-input binary address decoder with three independent strobe inputs (G2, G1, G0) that collectively gate all eight outputs to high when any is asserted - enabling hierarchical decoding without external logic. Its HC-series CMOS architecture ensures rail-to-rail output swing and high noise immunity (30% of VCC).

It operates in two functional modes: normal decoding (all strobes inactive) selects exactly one of Y0–Y7 as low while others remain high; and disabled mode (any strobe active) forces all outputs high. No internal latching or clocking is present - it is purely combinational.

Key Specifications

Pinout & Package

CD74HC138E is supplied in a 16-pin plastic dual in-line package (PDIP-N), measuring 25.40 mm × 6.35 mm, with through-hole mounting and industry-standard lead pitch (2.54 mm). The package includes internal ESD protection and is RoHS-compliant with NiPdAu lead finish.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HCT138E and SN74LS138N, the CD74HC138E offers broader supply flexibility, lower static power, and superior noise immunity - making it optimal for new designs targeting energy efficiency, mixed-voltage compatibility, and extended temperature operation.

Availability

CD74HC138E is available at Aetrix Electronics and suitable for memory address decoding, I/O port expansion, digital instrumentation multiplexing, and industrial control logic requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HC138E 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 company headquartered in Dallas, Texas, delivering analog and embedded processing solutions for industrial, automotive, personal electronics, and communications markets.

The CD74HC138E belongs to TI's legacy high-speed CMOS logic family, designed specifically for low-power, wide-voltage, and high-reliability digital control applications in harsh environments.

FAQ

What is the maximum clock or address transition frequency supported by CD74HC138E?

The CD74HC138E is a combinational logic device with no internal clock; its usable frequency depends on propagation delay and system setup/hold timing. With 13 ns typical tpd at 4.5 V, it supports reliable operation in address decode paths up to approximately 30 MHz when driving moderate capacitive loads (≤50 pF). For higher frequencies, verify timing margins using worst-case tpd (38 ns at -40°C to +85°C) and actual board capacitance.

Can CD74HC138E be used with a 3.3 V microcontroller without level shifting?

Yes - the CD74HC138E operates down to 2 V and accepts 3.3 V logic levels directly. Its VIH(min) is 1.35 V at VCC = 3.3 V (per HC electrical specs), well below typical 3.3 V MCU VOH(≥2.4 V). No level shifter is needed for control inputs; outputs swing rail-to-rail and are compatible with 3.3 V inputs having VIH ≥ 2.0 V.

How does the enable logic work across G0, G1, and G2 on CD74HC138E?

All three enables must be satisfied for normal decoding: G0 and G1 must be low (active), and G2 must be high (active). If G0 or G1 is high, or G2 is low, all outputs (Y0–Y7) are forced high - regardless of A0–A2 state. This allows hierarchical decoding: e.g., G2 from a higher-order decoder selects the CD74HC138E block, while G0/G1 manage sub-block enables.

Is CD74HC138E pin-compatible with CD74HCT138E?

Yes - CD74HC138E and CD74HCT138E share identical PDIP-16 pinout, function table, and package dimensions. They differ only in input threshold specifications and supply voltage range; substitution is electrically safe if VCC remains within both parts' overlapping range (4.5–5.5 V) and input drive meets HCT's stricter VIH/VIL requirements.

What decoupling capacitance is recommended for CD74HC138E in high-noise environments?

Texas Instruments recommends a 0.1 µF ceramic capacitor placed as close as possible to the VCC pin (Pin 16) and GND (Pin 8), with short, low-inductance traces. For systems with fast-switching loads or high EMI, add a parallel 1 µF tantalum or ceramic capacitor. This ensures stable supply during output transitions and suppresses ground bounce induced by simultaneous switching of multiple Yx outputs.