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SN74HC138N

3-to-8 decoder/demux, active-low outputs, 3 enable inputs, 15ns, PDIP-16

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Manufacturer Part：

SN74HC138N

Package：

PDIP-16 (24.33×9.4 mm, Through-Hole)

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Description

Product Overview

The Texas Instruments SN74HC138N is a 3-line to 8-line decoder/demultiplexer in a 16-pin PDIP (N) package. Designed for high-speed memory decoding and data routing, it features three enable inputs (two active-low, one active-high) for flexible cascading, active-low outputs, and typical propagation delay of 15 ns. Operating from 2 V to 6 V, it is a workhorse logic IC for address decoding and data distribution.

Key Specifications

Function 3-to-8 Decoder/Demultiplexer
Number of Channels 1 (3 input, 8 output)
Supply Voltage 2 V ~ 6 V
Propagation Delay (tpd) 15 ns typical
Output Drive ±4 mA at 5 V
ICC (max) 80 µA
Input Current (max) 1 µA
Enable Inputs 3 (G1 active-high, G2A and G2B active-low)
Output Logic Active-low (selected output is LOW)
Package PDIP-16 (N) (24.33×9.4 mm)
Operating Temperature -40°C ~ +85°C

Features

  • Specifically targeted for high-speed memory decoding and data transmission systems
  • Wide operating voltage range: 2 V to 6 V
  • Active-low outputs simplify interface to active-low chip selects
  • Three enable inputs (G1, G2A, G2B) for cascading and data reception
  • Low power consumption: 80 µA max ICC
  • Can drive up to 10 LSTTL loads
  • Balanced outputs and positive input clamp diodes

Applications

  • Memory address decoding in microprocessor systems
  • Data demultiplexing and routing
  • Chip-select generation for multiple peripherals
    • Cascaded decoding for larger address spaces Clock and enable signal distribution

The SN74HC138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer designed for high-performance memory decoding and data routing applications requiring very short propagation delay times. The device accepts a 3-bit binary input (A, B, C) and activates one of eight active-low outputs (Y0-Y7). Three enable inputs—G1 (active-high) and G2A, G2B (active-low)—provide flexible cascading for larger decoding networks. When any enable input is inactive, all outputs are forced HIGH. The active-low output configuration directly interfaces with the active-low chip-select inputs common on memory devices and peripherals, eliminating the need for inverting buffers.

The SN74HC138N decodes a 3-bit binary input to activate one of eight output lines. The three select inputs (A, B, C) are internally decoded through AND-OR logic gates to determine which of the eight outputs (Y0-Y7) should be driven LOW. The enable logic requires G1 = HIGH AND G2A = LOW AND G2B = LOW for the decoder to be active; if any enable condition is not met, all outputs are forced to their inactive HIGH state. This enable structure makes cascading straightforward: two SN74HC138 devices can form a 4-to-16 decoder by connecting the fourth address bit to an enable input. The internal CMOS gate structure uses complementary N-channel and P-channel transistors, providing low static power consumption and balanced output drive capability. The typical propagation delay of 15 ns ensures that the decoder introduces negligible delay in memory access paths.

Pin Name Type Function
1 A Input Least significant select input
2 B Input Middle select input
3 C Input Most significant select input
4 G2A Input Active-low enable input
5 G2B Input Active-low enable input
6 G1 Input Active-high enable input
7 Y7 Output Active-low decoded output (select = 111)
8 GND Power Ground
9 Y6 Output Active-low decoded output (select = 110)
10 Y5 Output Active-low decoded output (select = 101)
11 Y4 Output Active-low decoded output (select = 100)
12 Y3 Output Active-low decoded output (select = 011)
13 Y2 Output Active-low decoded output (select = 010)
14 Y1 Output Active-low decoded output (select = 001)
15 Y0 Output Active-low decoded output (select = 000)
16 VCC Power Supply voltage (2-6V)
  • Memory decoding: Generating chip-select signals for up to 8 memory devices from a 3-bit address field
  • Peripheral selection: Activating one of multiple I/O devices or registers in microprocessor systems
  • Cascaded systems: Two devices form a 4-to-16 decoder; four devices form a 5-to-32 decoder for large address spaces
  • Data demultiplexing: Routing a single data line to one of eight destinations under select control
  • Clock distribution: Distributing a clock signal to one of eight clock domains using an enable input as the clock source
Manufacturer Part Number Package Notes
TI SN74HC138D SOIC-16 Surface-mount equivalent
TI SN74HC138PWR TSSOP-16 Smaller surface-mount package
TI SN74HCT138N PDIP-16 HCT version, TTL-compatible inputs (4.5-5.5V)
TI SN74AHC138N PDIP-16 AHC version, faster propagation delay
Nexperia 74HC138N PDIP-16 Direct equivalent from Nexperia
onsemi MC74HC138N PDIP-16 Direct equivalent from onsemi
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We have obtained a number of professional certifications and built our own professional testing laboratory.This ensures that every product we deliver to our customers meets the highest quality requirements.We conduct tests in strict accordance with procedures to ensure stable product quality and accurate parameters.To guarantee genuine original parts, we also cooperate with reliable third-party testing institutions for strict quality inspection.We always attach great importance to quality and fully comply with industry standards, relevant regulations, and ISO 9001:2015 requirements.

Shipping & Payment

All electronic components we source from our partnered supply chains go through strict incoming inspections.Through careful testing, we ensure everything delivered to customers is genuine original parts and meets quality requirements.In addition, we keep complete inspection records to make the entire supply chain process clear and traceable.

Certification
We have obtained a number of professional certifications and built our own professional testing laboratory.This ensures that every product we deliver to our customers meets the highest quality requirements.We conduct tests in strict accordance with procedures to ensure stable product quality and accurate parameters.To guarantee genuine original parts, we also cooperate with reliable third-party testing institutions for strict quality inspection.We always attach great importance to quality and fully comply with industry standards, relevant regulations, and ISO 9001:2015 requirements.

Service & Packaging

All electronic components we source from our partnered supply chains go through strict incoming inspections.Through careful testing, we ensure everything delivered to customers is genuine original parts and meets quality requirements.In addition, we keep complete inspection records to make the entire supply chain process clear and traceable.

Certification
We have obtained a number of professional certifications and built our own professional testing laboratory.This ensures that every product we deliver to our customers meets the highest quality requirements.We conduct tests in strict accordance with procedures to ensure stable product quality and accurate parameters.To guarantee genuine original parts, we also cooperate with reliable third-party testing institutions for strict quality inspection.We always attach great importance to quality and fully comply with industry standards, relevant regulations, and ISO 9001:2015 requirements.

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