How to Use SN74AHCT125N : Examples, Pinouts, and Specs

The SN74AHCT125N is a quad buffer/driver with 3-state outputs, manufactured by Texas Instruments. It is designed for high-speed operation and low power consumption, making it ideal for use in digital circuits. Each of the four independent buffers features a 3-state output, which can be controlled via an enable pin. This component is commonly used for signal buffering, level shifting, and driving high-capacitance loads in digital systems.

• Signal buffering in digital circuits
• Level shifting between different voltage domains
• Driving high-capacitance loads
• Bus interface and control in microcontroller-based systems
• Data communication and signal isolation

• Manufacturer Part ID: SN74AHCT125N
• Operating Voltage Range: 4.5 V to 5.5 V
• Input Voltage Range: 0 V to 5.5 V
• Output Voltage Range: 0 V to Vcc
• High-Level Input Voltage (VIH): 2.0 V (minimum)
• Low-Level Input Voltage (VIL): 0.8 V (maximum)
• Output Current (IO): ±25 mA (maximum per output)
• Quiescent Current (ICC): 8 µA (maximum)
• Propagation Delay: 6 ns (typical at 5 V)
• Operating Temperature Range: -40°C to 125°C
• Package Type: 14-pin PDIP (Plastic Dual In-line Package)

The SN74AHCT125N is a 14-pin IC with the following pinout:

1. Power Supply: Connect the VCC pin (Pin 14) to a 5 V power supply and the GND pin (Pin 7) to ground.
2. Input Signals: Apply the input signals to the A pins (Pins 2, 5, 9, and 12) of the respective buffers.
3. Output Enable Control: Use the OE pins (Pins 1, 4, 10, and 13) to control the 3-state outputs:
   • Drive the OE pin LOW to enable the corresponding output.
   • Drive the OE pin HIGH to place the output in a high-impedance (Hi-Z) state.
4. Output Signals: The buffered output signals will appear on the Y pins (Pins 3, 6, 8, and 11) when the corresponding OE pin is LOW.

• Ensure that the input voltage levels are within the specified range (0 V to 5.5 V).
• Avoid exceeding the maximum output current of ±25 mA per output to prevent damage to the IC.
• Use decoupling capacitors (e.g., 0.1 µF) near the VCC pin to stabilize the power supply and reduce noise.
• When using the 3-state outputs, ensure that no two outputs are connected directly to avoid bus contention.

The following example demonstrates how to use the SN74AHCT125N to buffer a digital signal from an Arduino UNO.

1. Connect the VCC pin (Pin 14) to the Arduino's 5 V pin and the GND pin (Pin 7) to the Arduino's GND.
2. Connect an Arduino digital output pin (e.g., Pin 8) to the 1A pin (Pin 2) of the SN74AHCT125N.
3. Connect the 1OE pin (Pin 1) to GND to enable the output.
4. Connect the 1Y pin (Pin 3) to the desired load or circuit.

// Example code to demonstrate the use of SN74AHCT125N with Arduino UNO

void setup() {
  pinMode(8, OUTPUT); // Set Pin 8 as an output to drive the SN74AHCT125N
}

void loop() {
  digitalWrite(8, HIGH); // Send a HIGH signal to the buffer input
  delay(1000);           // Wait for 1 second
  digitalWrite(8, LOW);  // Send a LOW signal to the buffer input
  delay(1000);           // Wait for 1 second
}

1. No Output Signal:

   • Ensure that the OE pin for the corresponding buffer is connected to GND (LOW).
   • Verify that the input signal is within the specified voltage range.

2. Output Signal is Distorted:

   • Check for excessive load capacitance on the output pin. If necessary, reduce the load or use a series resistor.
   • Ensure proper decoupling of the power supply using capacitors near the VCC pin.

3. High Current Consumption:

   • Verify that no two outputs are shorted together or connected to conflicting signals.
   • Check for any floating input pins and connect them to a defined logic level (HIGH or LOW).

4. Component Overheating:

   • Ensure that the output current does not exceed the maximum rating of ±25 mA per output.
   • Verify that the power supply voltage is within the specified range (4.5 V to 5.5 V).

Q1: Can the SN74AHCT125N be used for level shifting?
A1: Yes, the SN74AHCT125N can be used for level shifting from TTL (3.3 V) to CMOS (5 V) logic levels, as it is designed to be compatible with TTL input levels.

Q2: What happens if the OE pin is left floating?
A2: If the OE pin is left floating, the output state is undefined. Always connect the OE pin to a defined logic level (HIGH or LOW).

Q3: Can multiple outputs be connected together?
A3: Yes, but only if the outputs are in a high-impedance (Hi-Z) state. Otherwise, connecting active outputs together can cause bus contention and damage the IC.

Q4: Is the SN74AHCT125N suitable for driving LEDs?
A4: The SN74AHCT125N can drive LEDs, but ensure that the current through the LED does not exceed the maximum output current rating of ±25 mA. Use a current-limiting resistor in series with the LED.