How to Use AD654 - Low Cost Monolithic Voltage-to-Frequency Converter: Examples, Pinouts, and Specs

The AD654 is a precision voltage-to-frequency converter manufactured by Analog Devices. It is designed to convert an analog input voltage into a frequency output that is linearly proportional to the input. This component is widely used in applications requiring high accuracy, low cost, and efficient signal processing.

• Analog-to-digital conversion (ADC) systems
• Frequency modulation and demodulation
• Data transmission in noisy environments
• Signal isolation and long-distance transmission
• Industrial process control and instrumentation
• Pulse-width modulation (PWM) systems

The following table outlines the key technical specifications of the AD654:

The AD654 is an 8-pin device. The pinout and descriptions are as follows:

1. Power Supply: Connect the V+ pin to a DC power supply within the range of +4.5 V to +36 V. Connect the GND pin to the ground of the circuit.
2. Input Voltage: Apply the analog input voltage (0 V to 10 V) to the VIN pin. Ensure the input voltage does not exceed the specified range to avoid damage.
3. Frequency Setting:
   • Connect an external resistor to the RSET pin to set the full-scale frequency. The recommended value is typically in the range of 10 kΩ to 1 MΩ.
   • Connect an external capacitor to the CSET pin to set the frequency range. Typical values are in the range of 100 pF to 10 nF.
4. Output Signal: The frequency output is available at the FOUT pin. This signal is TTL/CMOS compatible and can be directly interfaced with digital systems.
5. Reference Voltage: The VREF pin provides a stable 1.2 V reference voltage, which can be used for calibration or as a reference for other components in the circuit.

• Use precision resistors and capacitors for RSET and CSET to ensure high accuracy and stability.
• Keep the input voltage free from noise to avoid frequency jitter in the output.
• Decouple the power supply with a 0.1 µF ceramic capacitor close to the V+ pin to minimize power supply noise.
• Avoid leaving unused pins floating. For example, ensure the NC pin is left unconnected as specified.

The AD654 can be used with an Arduino UNO to measure an analog voltage and convert it to a frequency signal. Below is an example code snippet:

// Example: Reading frequency output from AD654 using Arduino UNO
// Connect AD654 FOUT pin to Arduino digital pin 2 (interrupt pin)

const int freqPin = 2; // Pin connected to AD654 FOUT
volatile unsigned long pulseCount = 0; // Variable to store pulse count

void setup() {
  pinMode(freqPin, INPUT); // Set freqPin as input
  attachInterrupt(digitalPinToInterrupt(freqPin), countPulses, RISING);
  Serial.begin(9600); // Initialize serial communication
}

void loop() {
  delay(1000); // Wait for 1 second
  noInterrupts(); // Disable interrupts to read pulseCount safely
  unsigned long frequency = pulseCount; // Frequency in Hz
  pulseCount = 0; // Reset pulse count
  interrupts(); // Re-enable interrupts
  Serial.print("Frequency: ");
  Serial.print(frequency);
  Serial.println(" Hz");
}

void countPulses() {
  pulseCount++; // Increment pulse count on each rising edge
}

• Ensure the FOUT pin is connected to a digital input pin on the Arduino that supports interrupts.
• The code calculates the frequency by counting pulses over a 1-second interval.

1. No Output Signal:

   • Verify the power supply connections to the V+ and GND pins.
   • Check the external resistor and capacitor values connected to RSET and CSET.
   • Ensure the input voltage at VIN is within the specified range.

2. Inaccurate Frequency Output:

   • Use precision components for RSET and CSET to minimize errors.
   • Ensure the input voltage is stable and free from noise.
   • Verify the calibration of the circuit using the VREF pin.

3. High Frequency Jitter:

   • Add a decoupling capacitor (e.g., 0.1 µF) close to the V+ pin to reduce power supply noise.
   • Shield the input signal to prevent external interference.

Q: Can the AD654 operate with a single supply voltage?
A: Yes, the AD654 is designed to operate with a single supply voltage ranging from +4.5 V to +36 V.

Q: What is the maximum output frequency of the AD654?
A: The maximum output frequency is 500 kHz, depending on the values of RSET and CSET.

Q: Is the output signal compatible with microcontrollers?
A: Yes, the output signal at the FOUT pin is TTL/CMOS compatible and can be directly interfaced with microcontrollers like Arduino.

Q: Can I use the AD654 for bidirectional voltage-to-frequency conversion?
A: No, the AD654 is designed for unidirectional voltage-to-frequency conversion only.