Part A
74HC138D
Toshiba Semiconductor và lưu trữ
- Lifecycle
- Active
- Stock
- 2131 PCS
- Package
- SOIC-16
- Series
- Logic - Signal Switches, Multiplexers, Decoders
Replacement verdict
Not a drop-in replacement
The 74HC125D and 74HC138D are not functionally interchangeable. The 74HC125D is a quad non-inverting buffer with 3-state outputs, while the 74HC138D is a 3-to-8 line decoder/demultiplexer. They differ in logic function, channel count, output type, and pinout, so substituting one for the other is not feasible without redesigning the circuit.
Different logic function
Different channel count
Different output type
Different pinout
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC125D | 74HC138D | Why it matters |
|---|---|---|---|
| Logic Function | Quad buffer/line driver with 3-state outputs | 3-to-8 line decoder/demultiplexer | Defines the core function; these devices are not functionally interchangeable. |
| Number of Channels / Outputs | 4 buffers | 8 outputs | Determines how many signals can be handled simultaneously. |
| Output Type | 3-state | Active-low (inverting) outputs | Affects bus interfacing and output polarity in a circuit. |
| Control Inputs | Output enable (active-low) per channel | Three enable inputs (two active-low, one active-high) | Determines how the device is enabled or cascaded in a design. |
| Number of Inputs | 4 data inputs plus 4 enable inputs | 3 address inputs plus 3 enable inputs | Impacts pin count and signal routing requirements. |
| Propagation Delay (typical at 5 V) | Approximately 12 ns | Approximately 20 ns | Affects maximum operating speed and timing margins. |
| Supply Voltage Range | 2 V to 6 V | 2 V to 6 V | Determines compatibility with logic levels and power rails. |
| Package | SO14 (D package) | SO16 (D package) | Affects PCB footprint and assembly compatibility. |
| Operating Temperature Range | -40 °C to +125 °C | -40 °C to +125 °C | Ensures reliability across industrial and automotive environments. |
| Logic Family | HC (High-speed CMOS) | HC (High-speed CMOS) | Indicates compatible input thresholds and drive characteristics. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC125D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Type | Decoder/Demultiplexer | - |
| Circuit | 1 x 3:8 | - |
| Independent Circuits | 1 | - |
| Current - Output High Low | 5.2mA, 5.2mA | 7.8mA, 7.8mA |
| Voltage Supply Source | Single Supply | - |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 125°C (TA) |
| Mounting Type | Surface Mount | Surface Mount |
| Logic Type | - | Buffer, Non-Inverting |
| Number of Elements | - | 4 |
| Number of Bits per Element | - | 1 |
| Output Type | - | 3-State |
Frequently asked questions
What are the primary functions of the 74HC125D and 74HC138D?
The 74HC125D is a quad non-inverting 3-state buffer/line driver with active-LOW output enable pins, used for signal buffering and bus isolation. The 74HC138D is a 3-to-8 line decoder/demultiplexer with active-LOW outputs, used for address decoding and memory chip selection.
Can the 74HC125D and 74HC138D be used interchangeably in a design?
No. They perform fundamentally different logic functions. The 74HC125D provides four independent 3-state buffers, while the 74HC138D decodes a 3-bit binary input into eight mutually exclusive active-LOW outputs. Substitution requires a complete redesign of the surrounding logic.
What are the key electrical specifications for both devices?
Both are 74HC-series CMOS devices typically operating from 2.0 V to 6.0 V VCC. The 74HC125D provides ±6 mA output drive at 4.5 V and features 3-state outputs with high-impedance when disabled. The 74HC138D provides ±4 mA output drive at 4.5 V and includes two active-LOW enable inputs (E1, E2) and one active-HIGH enable input (E3) for cascading.
Do the 74HC125D and 74HC138D share the same package and pinout?
Both are commonly offered in SO14 (D) packages with 14 pins, but their pinouts differ. The 74HC125D has four buffers with individual output enable pins, while the 74HC138D has three address inputs, three enable inputs, and eight outputs. Pin-to-pin compatibility does not exist between the two.
Which device is better suited for address decoding in a microprocessor system?
The 74HC138D is specifically designed for address decoding, converting a 3-bit address into eight active-LOW chip-select signals. The 74HC125D is not intended for decoding; it is used for buffering or isolating signals on a bus.
What are the propagation delay characteristics of these devices?
At 4.5 V VCC and 25 °C, the 74HC125D typically exhibits a propagation delay of approximately 10 ns to 15 ns from input to output. The 74HC138D typically exhibits a propagation delay of approximately 15 ns to 20 ns from address input to output. Actual values vary by manufacturer and load conditions.
Can the 74HC125D be used to drive the enable inputs of the 74HC138D?
Yes, provided the fan-out and logic polarity are respected. The 74HC125D can buffer control signals to the 74HC138D enable inputs, but the designer must ensure the 3-state buffer is enabled and that signal levels and current drive meet the 74HC138D input requirements.
Are both devices compatible with 3.3 V logic systems?
Yes, both the 74HC125D and 74HC138D are specified for operation from 2.0 V to 6.0 V, making them compatible with 3.3 V logic systems. However, input thresholds and output drive strength should be verified against the specific manufacturer datasheet for the target VCC.