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 transforms an input voltage into a frequency output that is linearly proportional to the input. This component is designed for applications requiring high accuracy, low cost, and ease of use. Its monolithic design ensures reliability and consistent performance.

• Analog-to-digital conversion (ADC) systems
• Frequency modulation and demodulation
• Signal isolation and transmission
• Data acquisition systems
• Industrial process control
• Remote sensing and telemetry

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

The AD654 is available in an 8-pin PDIP package. The pinout and descriptions are as follows:

1. Power Supply: Connect the V+ pin (Pin 1) to a regulated power supply between +5 V and +36 V. Connect the GND pin (Pin 2) to the ground of the circuit.
2. Input Voltage: Apply the input voltage signal (0 V to +10 V) to the VIN pin (Pin 3). Ensure the input voltage does not exceed the specified range to avoid damage.
3. Frequency Setting:
   • Connect an external resistor (RIN) to Pin 4 to set the input current.
   • Connect an external capacitor (C1) to Pin 5 to determine the frequency range.
   • Optionally, connect a capacitor to Pin 6 (C2) for improved frequency stability.
4. Output Signal: The frequency output is available at the FOUT pin (Pin 7). This output can be connected to a microcontroller, frequency counter, or other signal processing devices.

• Input Resistor Selection: Choose RIN to scale the input voltage to the desired input current. For example, with a 10 V input and a 1 mA current, RIN = 10 kΩ.
• Capacitor Selection: Use high-quality capacitors with low temperature coefficients for C1 and C2 to ensure stable frequency output.
• Decoupling Capacitor: Place a 0.1 µF decoupling capacitor between V+ and GND to minimize power supply noise.
• Output Loading: Avoid excessive loading on the FOUT pin to maintain signal integrity. Use a buffer if necessary.
• Temperature Effects: Operate the AD654 within the specified temperature range (0°C to +70°C) for optimal performance.

The AD654 can be interfaced with an Arduino UNO to measure the frequency output. Below is an example code snippet:

// Example: Reading frequency output from AD654 using Arduino UNO
// Connect AD654 FOUT (Pin 7) 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), countPulse, 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");
}

// Interrupt service routine to count pulses
void countPulse() {
  pulseCount++;
}

1. No Output Signal:

   • Verify the power supply voltage is within the specified range (+5 V to +36 V).
   • Check the connections to the VIN, RIN, and C1 pins.
   • Ensure the input voltage is within the 0 V to +10 V range.

2. Inaccurate Frequency Output:

   • Confirm the external resistor (RIN) and capacitor (C1) values are correct.
   • Use high-quality components with low temperature coefficients.
   • Check for noise or instability in the power supply.

3. Output Signal Distortion:

   • Avoid excessive loading on the FOUT pin. Use a buffer if necessary.
   • Ensure proper grounding and minimize noise in the circuit.

Q1: Can the AD654 handle negative input voltages?
A1: No, the AD654 is designed for input voltages in the range of 0 V to +10 V. Negative voltages may damage the device.

Q2: What is the maximum frequency output of the AD654?
A2: The maximum frequency output is 500 kHz, depending on the input voltage and external components.

Q3: Can I use the AD654 with a 3.3 V power supply?
A3: No, the minimum supply voltage for the AD654 is +5 V. Using a lower voltage may result in improper operation.

Q4: How do I improve frequency stability?
A4: Use high-quality capacitors for C1 and C2, and ensure a stable power supply with minimal noise.

This concludes the documentation for the AD654. For further details, refer to the official datasheet provided by Analog Devices.