SN74LS138 - 3-to-8 Line Decoder/Demultiplexer | Texas Instruments
MPN: SN74LS138 β Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0.89 | $0.89 |
| 10 | $0.78 | $7.80 |
| 100 | $0.62 | $62.00 |
| 500 | $0.55 | $275.00 |
| 1,000 | $0.48 | $480.00 |
Drop-in alternatives for SN74LS138 β same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool β Select alternative parts for side-by-side comparison:
SN74LS138N
β Drop-Inπ Reference alternative (not in catalog)
SN74LS138D
β Drop-Inπ Reference alternative (not in catalog)
SN74LS138NSR
β Drop-Inπ Reference alternative (not in catalog)
SN74HC138N
β Drop-Inπ Reference alternative (not in catalog)
SN74HCT138N
β Drop-Inπ Reference alternative (not in catalog)
MC74LS138N
β Drop-Inπ Reference alternative (not in catalog)
74LS138N
β Drop-Inπ Reference alternative (not in catalog)
SN74LS138 Maximum Ratings & Electrical Characteristics
| Function | 3-to-8 line decoder/demultiplexer |
| Number of Inputs | 3 (A, B, C) |
| Number of Outputs | 8 (Y0-Y7, active-low) |
| Enable Inputs | 3 (G1 active-high, G2A and G2B active-low) |
| Supply Voltage (VCC) | 4.75 V to 5.25 V |
| Operating Temperature Range | 0Β°C to 70Β°C |
| Propagation Delay Time (tpd) | 20 ns (typical) |
| Output Current (IOL) | 8 mA (sink) |
| Output Current (IOH) | -0.4 mA (source) |
| Power Dissipation | 32 mW (typical) |
| Input Voltage (VIH) | 2 V (min) |
| Input Voltage (VIL) | 0.8 V (max) |
| Package Type | 16-PDIP (N), 16-SOIC (D), 16-SOP (NS) |
| Technology | Schottky-clamped TTL (LS) |
| RoHS Status | Compliant |
SN74LS138 Pin Configuration
| Pin 1 | A β Select input A (LSB) |
| Pin 2 | B β Select input B |
| Pin 3 | C β Select input C (MSB) |
| Pin 4 | G2A β Enable input (active-low) |
| Pin 5 | G2B β Enable input (active-low) |
| Pin 6 | G1 β Enable input (active-high) |
| Pin 7 | Y7 β Output 7 (active-low) |
| Pin 8 | GND β Ground |
| Pin 9 | Y6 β Output 6 (active-low) |
| Pin 10 | Y5 β Output 5 (active-low) |
| Pin 11 | Y4 β Output 4 (active-low) |
| Pin 12 | Y3 β Output 3 (active-low) |
| Pin 13 | Y2 β Output 2 (active-low) |
| Pin 14 | Y1 β Output 1 (active-low) |
| Pin 15 | Y0 β Output 0 (active-low) |
| Pin 16 | VCC β Positive supply voltage (5V) |
Safe Operating Area (SOA) & Thermal Characteristics
No official SOA curve available for this digital IC. Always operate within absolute maximum ratings specified in the datasheet. Ensure adequate cooling and derate as needed.
Typical Applications
SN74LS138 is suitable for 6 applications: Memory Address Decoding, Data Routing and Demultiplexing, Address Decoding in Peripheral Interfaces, Chip Select Generation in FPGA/CPLD Systems, Educational Digital Logic Experiments, Industrial Control and Automation.
Memory Address Decoding
The SN74LS138 is ideal for memory address decoding in microprocessor systems. Its 3-to-8 decoding function allows selecting one of eight memory chips based on a 3-bit address. With a typical propagation delay of 20 ns, it ensures fast access times. The active-low outputs directly drive chip select pins on SRAM or EPROM devices. The enable inputs (G1, G2A, G2B) allow cascading to create larger decoders, such as 4-to-16, for expanded memory maps. This application benefits from the device's high output sink current (8 mA) and TTL compatibility.
Recommended
Data Routing and Demultiplexing
The SN74LS138 can be used as a demultiplexer to route a single data line to one of eight outputs. By connecting the data input to one of the enable pins (e.g., G1) and using the select inputs (A, B, C) to choose the output, the device distributes data to multiple destinations. This is useful in communication systems and peripheral interfaces. The short propagation delay (20 ns) ensures high-speed data routing. The active-low outputs are compatible with most logic families. This application leverages the device's versatility as both a decoder and demultiplexer.
Recommended
Address Decoding in Peripheral Interfaces
In microcontroller systems, the SN74LS138 is used to decode address lines to enable peripheral devices such as UARTs, timers, and I/O ports. Its three enable inputs provide flexibility for enabling the decoder only when a specific address range is active. The device operates from 0Β°C to 70Β°C, making it suitable for industrial control applications. The 16-pin PDIP package is easy to use in through-hole designs. This application benefits from the device's low power consumption (32 mW) and high noise immunity of TTL logic.
Recommended
Chip Select Generation in FPGA/CPLD Systems
The SN74LS138 can be used to generate chip select signals in systems with FPGAs or CPLDs, where external decoding is needed to interface with legacy TTL peripherals. Its active-low outputs are ideal for driving active-low chip select inputs. The device's 5V TTL compatibility ensures seamless integration with mixed-voltage systems. The short propagation delay (20 ns) minimizes address-to-chip-select latency. This application is common in retro computing and educational projects.
Recommended
Educational Digital Logic Experiments
The SN74LS138 is widely used in educational settings to teach digital logic design. Its simple 3-to-8 decoding function helps students understand binary-to-decimal conversion, enable logic, and demultiplexing. The 16-pin DIP package is breadboard-friendly, and the device operates at standard 5V TTL levels. The active-low outputs and enable inputs provide hands-on experience with real-world logic design. This application benefits from the device's low cost and wide availability.
Recommended
Industrial Control and Automation
In industrial control systems, the SN74LS138 is used for address decoding in PLCs and motor controllers. Its operating temperature range of 0Β°C to 70Β°C covers most industrial environments. The device's TTL outputs can drive indicator LEDs or relays through appropriate buffers. The enable inputs allow for safety interlocks by gating the decoder with control signals. This application benefits from the device's robustness and long-term availability.
Recommended
Recommended Products Summary
Engineering reference data for SN74LS138 β comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | SN74LS138N | SN74LS138D | SN74HC138N | SN74HCT138N | MC74LS138N |
|---|---|---|---|---|---|---|
| Package | 16-PDIP (N) | 16-PDIP | 16-SOIC | 16-PDIP | 16-PDIP | 16-PDIP |
| Brand | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments | onsemi |
| Technology | Schottky TTL (LS) | Schottky TTL (LS) | Schottky TTL (LS) | High-speed CMOS (HC) | TTL-compatible CMOS (HCT) | Schottky TTL (LS) |
| Propagation Delay (typical) | 20 ns | 20 ns | 20 ns | 10 ns | 15 ns | 20 ns |
| Power Dissipation (typical) | 32 mW | 32 mW | 32 mW | 0.5 mW | 0.5 mW | 32 mW |
| Output Sink Current (IOL) | 8 mA | 8 mA | 8 mA | 4 mA | 4 mA | 8 mA |
| Supply Voltage Range | 4.75V - 5.25V | 4.75V - 5.25V | 4.75V - 5.25V | 2V - 6V | 4.5V - 5.5V | 4.75V - 5.25V |
| Operating Temperature | 0Β°C to 70Β°C | 0Β°C to 70Β°C | 0Β°C to 70Β°C | -40Β°C to 85Β°C | -40Β°C to 85Β°C | 0Β°C to 70Β°C |
Key Differentiators
- Higher output drive current (vs SN74HC138N)
- TTL compatibility (vs SN74HC138N)
- Wider operating temperature range (vs SN74LS138N)
Design Notes
Ensure that the enable inputs are correctly biased: G1 must be high, and G2A and G2B must be low for the decoder to be active. If any enable is in the wrong state, all outputs will be high (inactive). Tie unused enable inputs to the appropriate rail (G1 to VCC, G2A/G2B to GND) to avoid floating inputs, which can cause erratic behavior.
Place a 0.1uF decoupling capacitor close to the VCC and GND pins to reduce power supply noise. For high-speed operation, keep traces short and avoid long parallel runs to minimize crosstalk. The SN74LS138 has a typical propagation delay of 20 ns, so ensure that address and enable signals are stable before the outputs are expected to change.
The SN74LS138 dissipates about 32 mW under typical conditions, which is low. However, if driving multiple outputs at maximum sink current (8 mA each), total power can increase. Ensure adequate airflow or copper area for heat dissipation in dense designs. The operating temperature range is 0Β°C to 70Β°C, so keep the ambient temperature within this range.
Compliance Information
RoHS compliant per TI product page. Not AEC-Q100 qualified. Lead-free per TI's green packaging.
