Part A
74HC138D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 2131 Stückzahl
- Package
- SOIC-16
- Series
- Logic.Signal Schalter, Multiplexer, Decoder
Replacement verdict
Not a drop-in replacement
The 74HC151D and 74HC138D are not functionally interchangeable despite sharing the same 74HC logic family and SOIC-16 package. The 74HC151D is an 8-to-1 multiplexer with complementary outputs, while the 74HC138D is a 3-to-8 line decoder/demultiplexer with active-low outputs. Their logic functions, channel configurations, and output characteristics differ fundamentally, making direct substitution impossible without circuit redesign.
Different logic function
Different channel configuration
Different output polarity
Different pin function assignment
Parts at a glance
Part B
74HC151D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 3388 Stückzahl
- Package
- SOIC-16
- Series
- Logic.Signal Schalter, Multiplexer, Decoder
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC151D | 74HC138D | Why it matters |
|---|---|---|---|
| Function | 8-to-1 multiplexer | 3-to-8 line decoder/demultiplexer | Defines the core logic operation; these devices are not functionally interchangeable. |
| Number of Select Inputs | 3 (S0, S1, S2) | 3 (A0, A1, A2) | Determines how many control bits are needed to address the inputs or outputs. |
| Number of Data Inputs | 8 (D0–D7) | 1 (E1/E2/E3 enable inputs) | Affects the number of signal sources that can be routed or decoded. |
| Number of Outputs | 1 (Y, plus W complementary) | 8 (Y0–Y7, active LOW) | Determines how many destinations can be driven simultaneously. |
| Output Polarity | True and complementary (Y, W) | Active LOW (Y0–Y7) | Polarity must match the downstream logic to avoid inversion errors. |
| Enable/Strobe Inputs | 1 active LOW enable (E) | 3 enable inputs (two active LOW, one active HIGH) | Enables cascading and chip-select control in larger systems. |
| Supply Voltage Range | 2.0 V to 6.0 V | 2.0 V to 6.0 V | Both support standard HC-family logic levels for 3.3 V and 5 V systems. |
| Propagation Delay (typical at 5 V) | ~14 ns | ~14 ns | Similar timing allows comparable speed in high-frequency designs. |
| Package | SO16 (SOIC-16) | SO16 (SOIC-16) | Identical footprint allows drop-in PCB compatibility without layout changes. |
| Operating Temperature Range | -40 °C to +125 °C | -40 °C to +125 °C | Both are suitable for industrial and automotive-grade environments. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC151D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Type | Decoder/Demultiplexer | Multiplexer |
| Circuit | 1 x 3:8 | 1 x 8:1 |
| Independent Circuits | 1 | 1 |
| Current - Output High Low | 5.2mA, 5.2mA | 5.2mA, 5.2mA |
| Voltage Supply Source | Single Supply | Single Supply |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 125°C |
| Mounting Type | Surface Mount | Surface Mount |
Frequently asked questions
What are the primary functional differences between the 74HC151D and the 74HC138D?
The 74HC151D is an 8-to-1 multiplexer that selects one of eight data inputs based on a 3-bit select code and routes it to a single output. The 74HC138D is a 3-to-8 line decoder/demultiplexer that takes a 3-bit binary address and activates one of eight mutually exclusive outputs. In short, the 74HC151D concentrates eight inputs to one output, while the 74HC138D distributes one address to eight outputs.
Are the 74HC151D and 74HC138D pin-compatible?
No. Both devices are typically offered in a 16-pin SOIC package (the "D" suffix in Nexperia nomenclature), but their pin functions differ. The 74HC151D has eight data inputs, three select inputs, a strobe, complementary outputs (Y and Y-bar), and supply pins. The 74HC138D has three address inputs, three enable inputs, eight active-LOW outputs, and supply pins. They cannot be interchanged without redesigning the PCB connections.
Can the 74HC151D and 74HC138D be used together in the same logic design?
Yes. Because they perform complementary functions, they are often used together. For example, a 74HC138D can decode a 3-bit address to select one of eight devices, while a 74HC151D can multiplex eight data sources onto a single line. Their compatible HC-family logic levels and supply voltage ranges allow them to interface directly in the same system.
What is the difference in output logic between the 74HC151D and the 74HC138D?
The 74HC151D provides both a true (Y) and complementary (Y-bar) output, and the selected data input appears at Y while its inverse appears at Y-bar. The 74HC138D provides eight active-LOW outputs; the selected output goes LOW while all unselected outputs remain HIGH. This difference affects how each device drives downstream logic or indicators.
Do the 74HC151D and 74HC138D operate at the same supply voltage and logic levels?
Yes. Both are 74HC-series CMOS devices and are specified for a supply voltage range of 2.0 V to 6.0 V, with standard HC-family input thresholds and output drive characteristics. They are compatible with other 74HC devices and with many 5 V TTL systems when used within the recommended operating conditions.
Which device is better for expanding a microcontroller's I/O?
It depends on the direction of expansion. The 74HC138D is typically used to expand outputs by decoding a small number of microcontroller pins into eight select or chip-enable signals. The 74HC151D is typically used to expand inputs by allowing a microcontroller to read one of eight digital signals through a single input pin. Neither is universally better; the choice depends on whether more outputs or more inputs are required.
Are the 74HC151D and 74HC138D available in the same package type?
Both are commonly available in the SO16 (16-pin small outline) package, which corresponds to the "D" suffix in many manufacturer part numbers. However, package availability can vary by manufacturer and distributor, so the specific orderable part number and datasheet should always be checked before design release.
Can the 74HC138D be used as a demultiplexer, and how does that compare to the 74HC151D?
Yes. The 74HC138D can function as a 1-to-8 demultiplexer when one of its enable inputs is used as the data input and the three address inputs select the output. The 74HC151D performs the opposite function: 8-to-1 multiplexing. So the 74HC138D can route one signal to one of eight destinations, while the 74HC151D routes one of eight signals to a single destination.