How to Use AD9835: Examples, Pinouts, and Specs

The AD9835 is a low-power, programmable waveform generator manufactured by Analog Devices. It is capable of producing sine, triangle, and square waveforms with a frequency resolution of up to 28 bits. The device is controlled via a serial interface, making it highly versatile and suitable for a wide range of applications.

• Signal generation for testing and measurement equipment
• Frequency modulation (FM) and phase modulation (PM) systems
• Function generators
• Audio signal synthesis
• Communications systems

Below are the key technical details of the AD9835:

The AD9835 comes in a 20-pin TSSOP package. Below is the pin configuration and description:

1. Power Supply: Connect the VDD pin to a stable power supply (2.3 V to 5.5 V) and connect DGND and AGND to ground.
2. Clock Input: Provide a stable clock signal to the MCLK pin. The clock frequency determines the maximum output frequency.
3. SPI Communication:
   • Use the SCLK, SDATA, and FSYNC pins to configure the device via an SPI-compatible interface.
   • Write to the frequency and phase registers to set the desired waveform parameters.
4. Output: Connect a load resistor to the IOUT pin to generate a voltage output. Optionally, use a low-pass filter to smooth the output waveform.

• Clock Stability: Ensure the master clock (MCLK) is stable to avoid jitter in the output waveform.
• Output Filtering: Use an external low-pass filter to remove high-frequency components from the output signal.
• Reset: Use the RESET pin to initialize the device before programming it.
• SPI Timing: Follow the SPI timing requirements specified in the datasheet to ensure reliable communication.

Below is an example of how to interface the AD9835 with an Arduino UNO to generate a sine wave:

#include <SPI.h>

// Pin definitions for AD9835
#define FSYNC 10  // Connect to FSYNC pin of AD9835
#define SCLK  13  // Connect to SCLK pin of AD9835
#define SDATA 11  // Connect to SDATA pin of AD9835

void setup() {
  // Initialize SPI
  SPI.begin();
  pinMode(FSYNC, OUTPUT);
  digitalWrite(FSYNC, HIGH); // Set FSYNC high initially

  // Reset AD9835
  writeRegister(0x2100); // Reset command
  delay(10);

  // Set frequency to 1 kHz (example value)
  setFrequency(1000);

  // Exit reset mode
  writeRegister(0x2000); // Clear reset bit
}

void loop() {
  // The waveform will continue to generate without further code
}

// Function to write a 16-bit value to the AD9835
void writeRegister(uint16_t data) {
  digitalWrite(FSYNC, LOW); // Enable communication
  SPI.transfer(highByte(data)); // Send high byte
  SPI.transfer(lowByte(data));  // Send low byte
  digitalWrite(FSYNC, HIGH); // Disable communication
}

// Function to set frequency (example: 1 kHz)
void setFrequency(uint32_t frequency) {
  uint32_t freqWord = (frequency * 268435456UL) / 25000000UL; // Calculate frequency word
  writeRegister(0x4000 | (freqWord & 0x3FFF)); // Write lower 14 bits
  writeRegister(0x4000 | ((freqWord >> 14) & 0x3FFF)); // Write upper 14 bits
}

• Replace 25000000UL with the actual clock frequency (MCLK) in Hz.
• Use appropriate resistors and capacitors for the output stage to match your application requirements.

1. No Output Signal:

   • Ensure the power supply is within the specified range (2.3 V to 5.5 V).
   • Verify that the SPI communication is functioning correctly.
   • Check that the RESET pin is properly initialized.

2. Distorted Waveform:

   • Use a low-pass filter to remove high-frequency noise.
   • Ensure the load resistor on the IOUT pin is correctly chosen.

3. Incorrect Frequency Output:

   • Verify the master clock (MCLK) frequency.
   • Double-check the frequency word calculation in your code.

Q: Can the AD9835 generate arbitrary waveforms?
A: No, the AD9835 is limited to sine, triangle, and square waveforms. For arbitrary waveforms, consider using a DAC with waveform memory.

Q: What is the maximum output frequency?
A: The maximum output frequency is 25 MHz, but the actual usable frequency depends on the master clock and application requirements.

Q: How do I improve waveform accuracy?
A: Use a stable clock source, minimize noise in the power supply, and apply proper filtering to the output signal.

Q: Can I use the AD9835 with a 3.3 V microcontroller?
A: Yes, the AD9835 operates with a supply voltage as low as 2.3 V, making it compatible with 3.3 V systems. Ensure proper logic level matching for SPI communication.