DAC Technology: Digital to Analog Conversion and Waveform Generator Design

DAC Technology: The Mystery from Digital to Analog – Practical Design of Waveform Generators

Hello everyone, I’m Daodao. Today we will talk about DAC technology, which stands for Digital to Analog Conversion. In simple terms, it allows a microcontroller to output various voltage values, such as sine waves and triangular waves, which are analog signals. Many friends find this difficult, but once you understand a few key points, it’s actually quite easy to get started.

What is DAC?

Imagine using numbers to represent volume levels, like from 0 to 100. However, actual sound is not stepped but continuously varying. DAC is responsible for converting these numbers into smooth voltage signals. A real-life example is adjusting the volume on your phone: even though it displays discrete numbers, the actual output is a continuously varying sound level, and DAC is what makes this happen.

Choosing the Hardware Circuit Scheme

Let’s take a look at several common DAC schemes:

1. Built-in DAC

VDD -------- DAC Module -------- Output Pin

               |

             GND

Advantages: Easy to use, convenient

Disadvantages: Average precision, not all microcontrollers have this

1. External DAC Chip (e.g., DAC8552)

Microcontroller ---- SPI/IIC ---- DAC8552 ---- Output

                           |

                          GND

Advantages: High precision, strong anti-interference

Disadvantages: Occupies communication interface, a bit expensive

1. R-2R Resistor Network

D7 ----R---- R ----R---- R ----R---- Output

      |      |      |      |

      2R     2R     2R     2R

      |      |      |      |

     GND    GND    GND    GND

Advantages: Low cost, easy to understand

Disadvantages: Precision affected by resistor errors

Key Points for Code Implementation

Taking the built-in DAC of STM32 as an example, let’s implement a simple sine wave generator:

#define DAC_POINTS 256  // Number of sample points

uint16_t SineWave[DAC_POINTS];  // Store sine wave data


// Initialize DAC and timer
void DAC_Init(void) {

    // Enable DAC clock

    RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);
    
    // DAC configuration

    DAC_InitTypeDef DAC_InitStructure;
    DAC_InitStructure.DAC_Trigger = DAC_Trigger_T2_TRGO;  // Timer 2 trigger
    DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
    DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Enable;
    DAC_Init(DAC_Channel_1, &DAC_InitStructure);
    
    // **Note: Don’t forget to configure the DAC output pin to analog mode**
    GPIO_InitTypeDef GPIO_InitStructure;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AIN;
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;  // PA4 is DAC channel 1
    GPIO_Init(GPIOA, &GPIO_InitStructure);
}

Practical Waveform Generation

The key to generating a sine wave is to first establish a lookup table:

// Generate sine wave lookup table
void Generate_SineWave(void) {
    for(uint16_t i = 0; i < DAC_POINTS; i++) {
        // Calculate sine value, scale to DAC range (0-4095)
        SineWave[i] = (sin(2 * PI * i / DAC_POINTS) + 1) * 2047;
    }
}

Analysis of Practical Application Cases

In an audio signal generator project, we encountered a problem: the output waveform had noticeable glitches. After investigation, we found two reasons:

1. Improper timer interrupt priority settings, leading to untimely DAC output

2. Output buffer not enabled, resulting in insufficient load capacity

Solution:

// Increase timer interrupt priority
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;

// Enable DAC output buffer
DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Enable;

Common Issues and Solutions

1. Incorrect output voltage range

• Check reference voltage settings

• Confirm data calculations do not exceed DAC range (0-4095)

1. Waveform distortion

• Reduce output frequency

• Increase sample points

• Check timer configuration

1. Output noise

• Add filter capacitor (0.1μF)

• Pay attention to ground connections

• Keep signal lines away from switching power supplies

Precautions

1. Never ignore power supply ripple, it is recommended to add 5-10μF electrolytic capacitor and 0.1μF ceramic capacitor for filtering

2. Grounding is very important! Analog ground and digital ground should be separated, and finally star-ground at the power supply

3. Remember to add protection circuits at the output, at least a current-limiting resistor to prevent short circuits

4. Keep interrupt handling concise, to avoid affecting DAC output timing

Practical Recommendations

Choose a suitable experimental board, preferably a development board with oscilloscope functionality. Start practicing with the simplest square wave, gradually transitioning to sine waves and other complex waveforms. When issues arise, use the oscilloscope to observe and develop the ability to analyze waveforms.

Debugging Tips:

• Use a multimeter to measure output voltage to confirm basic DAC functionality

• Use an oscilloscope to observe waveforms and identify problems

• When output is abnormal, first check timing, most issues arise from this

Timer cycle calculation formula:

Timer_Period = SystemCoreClock / (PSC + 1) / Sample Rate

With this, we have successfully built a waveform generator. Doesn’t it feel rewarding? Remember: knowledge gained from books is shallow; true knowledge comes from practice. Get hands-on and try it out!