Digital To Analog Converter (DAC) & Waveform Generation With MCU

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Digital To Analog Converter (DAC) & Waveform Generation
Introductory Level ★☆☆☆☆

 

In this tutorial, you’ll learn the basics of digital to analog converters DACs. What’s A DAC? How DACs Work? And What are the common types of DACs? And which to use for our projects.

This tutorial includes 4 practical LABs/Projects on generating analog voltages using a DAC. As well as generating analog waveforms @ any desired frequency (e.g Sawtooth, Triangular, and Sin Wave).  This is also how we generate basic sound tones using microcontrollers.

It’s going to be an enjoyable tutorial however, combining light-weight theory, many tips, notes, and interesting practice LABs. So Let’s Get Started!

Digital To Analog Converter DAC Tutorial Thumbnail


  Components Needed For This Tutorial  

Qty. Component Name Buy On Amazon.com
1 PIC16F877A Add
1 BreadBoard Add
1 Resistor Kit Add    Add
1 DSO (Digital Storage Oscilloscope) Add    Add
1 Jumper Wires Pack Add    Add
1 LM7805 Voltage Regulator (5v) Add
1 4MHz Crystal Oscillator Add
1 PICkit2 or 3 Programmer Add

The Prototyping Board Setup

Prototyping Board - Embedded Systems Tutorials With PIC MCUs


  What’s A Digital-To-Analog Converter | DAC?  

 

In Electronics, a digital-to-analog converter (DAC or D/A) is an electronic circuit that converts digital data (0’s & 1’s) to an analog signal. Functionally, it’s the inverse of the ADC (analog-to-digital converter). And A DAC is remarkably cheaper than ADC by orders of magnitude.

DAC Vs ADC

DACs are commonly used for analog waveform generation applications such as audio/music players, video players, TVs, and various electronic systems. Many ADCs include a DAC as a building block of their structures in fact.

As in ADC, there is quantization noise in the signal due to the fact that a DAC is converting a discrete digital data to corresponding analog voltage levels. So, there is inherent quantization noise at a rate equal to the sampling rate of the DAC.

As stated in Nyquist-Shannon sampling theorem, a DAC can be used to reconstruct an analog signal from digital data samples (captured by an ADC). As long as the bandwidth is less than Nyquist frequency (No violation to Nyquist sampling theorem). A reconstruction filter can be used to reduce quantization noise appearing at the analog output.

<< Don’t Forget To Check Out The ADC Tutorial >>

ADC With PIC Tutorial Thumbnail


   Why Do We Need DACs?   

 

We need digital to analog converters (DACs) in different applications of digital computers. Digital computers store, manipulate, read in, and send out Digital Data (0’s & 1’s). So there should be a way to convert the digital representation back to analog signals.

Audio files are stored as digital data in the memory of a computer and in order to play it back, it should be converted back to an analog signal. Another situation in which we need DACs is analog control systems. A digital computer can monitor a process and run the instructions of a control algorithm. But in order to send out the output of the control algorithm to actuators, it should be converted to an analog voltage and here DACs come to play.


   Applications Of DAC   

 

Digital To Analog Converters (DACs) are being used in a very wide range of applications whether it’s required to have a basic D/A conversion or a much more precise D/A conversion. Some applications demand a high-frequency sampling rate for the DAC while others are demanding high-resolution DACs. And many other applications need to balance between high-frequency and resolution trade-off.

Typical applications of DACs include, but not limited to, the following:

  • Audio/Music Players
  • Video Players
  • Control Systems
  • ADC (A/D)
  • Synthesizers
  • Radar Systems
  • Waveform Generators
  • Oscilloscopes & instruments
  • Power Supplies
  • DDS (Direct Digital Synthesizers)
  • DDS (Data Distribution Systems)
  • And Much More…

   Types Of DACs   

 

 1.  Binary Weighted DAC

The Binary-Weighted DAC contains individual electrical components for each bit of the DAC connected to a summing point. Each input in the summing has powers-of-two values with most current or voltage at the most-significant bit. These precise voltages or currents sum to the correct output value. This is one of the fastest conversion methods but suffers from poor accuracy because of the high precision required for each individual voltage or current. The most common DACs of this type are the following:

R-2R Ladder DAC, which is a binary-weighted DAC that uses a repeating cascaded structure of resistor values R and 2R. This improves the precision due to the relative ease of producing equal valued-matched resistors. It’s the cheapest, yet easiest, DAC you can build on your own.

Binary Summing Amplifier DAC is basically an op-amp in summing configuration. With multiple resistors connected to one input. The junction where the resistors meet is called the summing junction or the virtual ground. The binary input goes into the resistors and the analog output is obtained on the output of the op-amp. The values of these resistors are chosen carefully to produce a binary-weighted summation for the digital inputs.

 2.  PWM DAC

The pulse width modulating DAC is typically a PWM generator running @ pretty high-frequency range. The analog voltage level at the output could be easily obtained by averaging the PWM signal. In technical terms, we’ll filter out the high-frequency component of the PWM signal to leave only the average DC component. A simple low-pass filter could do the job. All in all, it’s an efficient way to create a DAC but requires more effort to get clean output.

We’ll discuss this method of creating DACs in much more detail in the future. So, stay tuned for that!

 3.  Successive Approximation (Cyclic) DAC

Cyclic DAC which successively constructs the output during each cycle. Individual bits of the digital input are processed each cycle until the entire input is accounted for.

 


   Using A DAC With Microcontrollers   

 

In this section, we’ll see how can you use a DAC with your microcontroller. There are many different ways in which you can get a DAC depending on your application’s needs. The available options to add a DAC output to your microcontroller-based system include the following:

 1.  R-2R Ladder DAC

This is probably the easiest & cheapest way to get a DAC and connect its inputs to your microcontroller. Building an R-2R ladder is rather simple, all you need is a few resistors of R and 2R (e.g. 1k&2k or 5k&10k, and so on). Here is the schematic diagram for the R-2R ladder (assuming 4-bit DAC). You can continue connecting resistors in the same manner to get 10-Bit or 12-Bit DAC.

R2R Ladder DAC Circuit Diagram - Schematic - DAC Tutorial

 2.  Binary Weighted-Summing DAC

You can also create a simple DAC with an Op-Amp in weighted-summing configuration and get a resolution up to 8-Bit. Here is a schematic diagram for a binary weighted-summing op-amp 4-Bit DAC

Binary Weighted-Summing DAC With OP-Amp - DAC Tutorial

 3.  Serial/Parallel DAC IC

Another option is to get a high-precision (resolution) DAC ic chip and interface it with your microcontroller. There are serial DACs (typically SPI bus) for reduced required pin count of MCU. And there are also parallel DAC ICs which you can use if it fits well in your application’s design specifications.

DAC ic chip - DAC Tutorial

 4.  PWM As DAC

Another considerable option is to use one of your microcontroller’s PWM output pins as a DAC. By varying the duty cycle of the PWM, the average output voltage varies correspondingly. In order to get an “averaged” output or filtered, you’ll need to set a high-frequency PWM and filter it out with a low-pass filter. The result is a quite clean DC voltage level which corresponds to the Duty Cycle % of the PWM.

Varying the PWM’s Duty Cycle following a digital sine table will result in an analog sinusoidal waveform as shown in the figure below. Please, note that this method won’t be completely discussed in this tutorial. Instead, we’ll make a simple R2R ladder and PWM DAC will be discussed separately in the future!

PWM as a DAC

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 5.  Integrated DAC Module

Well, in fact, nowadays there are so many microcontrollers are being manufactured with a DAC module integrated within! it’s a cool feature which provides you the flexibility to configure it internally without needing to connect anything outside the microcontroller and reduce the overhead as long as you need a moderate DAC output. Unless you need some specific requirements for the DAC output, then you might consider searching for a convenient separate DAC IC.

Here is the block diagram of a DAC Module within a relatively new PIC microcontroller.

Digital To Analog Converter DAC Module Block Diagram - DAC Tutorial

 


  Generating Adjustable Analog Voltage – LAB1  

 

Lab Name Generating Adjustable Analog Voltage With DAC
Lab Number 24
Lab Level Beginner
Lab Objectives Learn how to configure The ADC Module and read analog signals. And Transfer the digital output of the ADC to the input of a DAC to reconstruct the analog signal once again. And build the R-2R ladder DAC circuit on breadboard & test out everything

This is The Functional Description Of The System We’ll Be Building

Digital To Analog Converter DAC Tutorial - LAB1

 

       1. Coding       

 

Open the MPLAB IDE and create a new project name it “Analog_Out_DAC”. If you have some issues doing so, you can always refer to the previous tutorial using the link below.

timer-preloading

Set the configuration bits to match the generic setting which we’ve stated earlier. And if you also find troubles creating this file, you can always refer to the previous tutorial using the link below.

timer-preloading

Now, open the main.c file and let’s start developing the firmware for our project.

Here is the Full Code Listing For This LAB

 

       2. Simulation       

 

Here is an animation for the running simulation tests.

Digital To Analog Converter DAC LAB1 Simulation

Yes, it seems like a kind of trolling around to feed in an analog input voltage to an ADC and reconstruct it back once again using a DAC. We’re not doing something useful and It’s not the best way to get an adjustable Vout. But it’s just the beginning to make you familiar with the operation of a Digital To Analog Converter (DAC)! The next LABs are much more interesting, however.

 

       3. Prototyping       

 

Wiring up this schematic on a breadboard should be an easy task. Just upload your firmware hex file to the microcontroller chip. And hook up the input power rails and start testing it out!

 

Here is a short video for the results of this LAB & verification with AVOmeter.

 

Download Project

 


 Setting Sampling Period For Desired Frequency 

 

While generating analog waveforms using a microcontroller & a DAC, it’s desirable to have a controllable output frequency for the analog waveform. The frequency of the output signal is determined by the following formula

Digital To Analog Converter DAC Waveform Frequency

Where Ns is the number of sample points in a single cycle of the waveform. And Ts is the sample period for the DAC. In fact, we control both of those parameters in order to achieve the desired output frequency with an acceptable amount of “Quantization Noise” due to sampling.

Let’s consider a quick practical numeric example to make this more clear.

Example 1

Let’s assume it’s desired to get a sawtooth waveform swinging between (0-5v) @ frequency of 10 Hz.

Well, in order to get a ramp-up in output voltage, the digital output going to the DAC should go from 0 up to 255 which corresponds to a voltage ramp from 0 to 5v.

So, let’s now settle for 256 levels (sample points) in each cycle of the waveform. This can be reduced for increasing the maximum output frequency while sacrificing the “Harmonic Distortion” due to the reduced resolution which increases the quantization noise. All in all, now we’ve Ns = 256

For the desired Fout = 10Hz, the period of each cycle is T = 100ms

The only unknown now is the sampling period Ts. And using the formula shown above, it ends up being

Ts = 100ms/256 = 390μsec

Now, we’re almost done! The C-Code which will generate the desired waveform will be something like this

 


   Generating Sawtooth (Ramp) Waveform – LAB2   

 

Lab Name Generating Sawtooth Waveform With DAC  @ 10Hz
Lab Number 25
Lab Level Beginner
Lab Objectives Learn how to generate analog waveform (sawtooth) with a microcontroller and a DAC. And how to control the frequency of the output waveform.

 

       1. Coding       

 

Open the MPLAB IDE and create a new project name it “Sawtooth_Out_DAC”. If you have some issues doing so, you can always refer to the previous tutorial using the link below.

timer-preloading

Set the configuration bits to match the generic setting which we’ve stated earlier. And if you also find troubles creating this file, you can always refer to the previous tutorial using the link below.

timer-preloading

Now, open the main.c file and let’s start developing the firmware for our project.

Here is the Full Code Listing For This LAB

 

       2. Simulation       

 

Here is the simulation result

Sawtooth Generator With Microcontroller And DAC

 

       3. Prototyping       

 

Wiring up this schematic on a breadboard should be an easy task. Just upload your firmware hex file to the microcontroller chip. And hook up the input power rails and start testing it up!

Here is a snapshot on my oscilloscope for the output waveform

Digital To Analog Converter - Sawtooth Waveform Generator With Microcontroller

 

Download Project

 


   Generating Triangular Waveform – LAB3   

 

Lab Name Generating Triangular Waveform With DAC @ 10Hz
Lab Number 26
Lab Level Beginner
Lab Objectives Learn how to generate analog waveform (triangular) with a microcontroller and a DAC. And how to control the frequency of the output waveform.

 

In this example, Ns = 256 (ramp-up) + 255 (ramp-down) = 511 sample points. So, the Ts = 100ms/511 = 196μsec

 

       1. Coding       

 

Open the MPLAB IDE and create a new project name it “Traingular_Out_DAC”. If you have some issues doing so, you can always refer to the previous tutorial using the link below.

timer-preloading

Set the configuration bits to match the generic setting which we’ve stated earlier. And if you also find troubles creating this file, you can always refer to the previous tutorial using the link below.

timer-preloading

Now, open the main.c file and let’s start developing the firmware for our project.

Here is the Full Code Listing For This LAB

 

       2. Simulation       

 

Here is the simulation result.

Triangular Waveform Generator With Microcontroller And DAC

 

       3. Prototyping       

 

Wiring up this schematic on a breadboard should be an easy task. Just upload your firmware hex file to the microcontroller chip. And hook up the input power rails and start testing it up!

Here is a snapshot for the output on my oscilloscope

Digital To Analog Converter - Triangular Waveform Generator With Microcontroller

 

Download Project

 


  Generating Sine Wave With MCU & DAC – LAB4  

 

Lab Name Generating Sine Wave With DAC @ 50Hz
Lab Number 27
Lab Level Beginner
Lab Objectives Learn how to generate analog waveform (Sine) with a microcontroller and a DAC. And how to control the frequency of the output waveform.

 

For the last couple of labs, it was sufficient to have a single variable “counter” to create up-counting RAMP waveform and also up-down-counting TRIANGULAR waveform. But for this lab, we’ll need to store a small look-up table for a single cycle SINE wave.

You can calculate it manually with your own calculator or you can also craft a MATLAB script to generate the table for you. But for the sake of simplicity, in this tutorial, I’ll show you how to use the following online calculator to get the sine table. Online Sine Table Generator

Digital To Analog Converter DAC - Sine Wave Generation With Microcontroller

For this tutorial, I’ll only need 32 sample points. The maximum amplitude for the sine wave is 5v which corresponds to 255 as a digital input for the DAC. My DAC is 8-Bit. If you’re using a 10-Bit DAC, this value would be 1023 and so on. Finally, for the last option, I’ve used for convenience and you can get all the values in a single line of code if that’s what you want! Click submit and a table like this will pop-up in your face!

Digital To Analog Converter DAC - Sine Wave Generation With Microcontroller And DAC

Copy these values to an array in your code and you’re good to go!

To get the sampling period, we’ll calculate it in the same way as the previous LABs. F=50Hz so T = 20ms

Therefore, Ts = 625μsec

 

 

       1. Coding       

 

Open the MPLAB IDE and create a new project name it “Sine_Out_DAC”. If you have some issues doing so, you can always refer to the previous tutorial using the link below.

timer-preloading

Set the configuration bits to match the generic setting which we’ve stated earlier. And if you also find troubles creating this file, you can always refer to the previous tutorial using the link below.

timer-preloading

Now, open the main.c file and let’s start developing the firmware for our project.

Here is the Full Code Listing For This LAB

 

       2. Simulation       

 

Here is the simulation’s result.

Digital To Analog Converter DAC - Sine Waveform Generator With Microcontroller And DAC

       3. Prototyping       

 

Wiring up this schematic on a breadboard should be an easy task. Just upload your firmware hex file to the microcontroller chip. And hook up the input power rails and start testing it up!

Here is a snapshot for the output on my oscilloscope

Digital To Analog Converter DAC - Sine Wave Generation With Microcontroller

 

Download Project

 


 

I know the results of all LABs in this tutorial aren’t what you might hope for, in terms of output frequency accuracy. These are just toy examples to show you how a DAC works and how can digital data in your microcontroller’s registers turn into analog waveforms!

This topic will be revisited again and in much more detail. We’ll generate analog waveforms with frequency accuracy up to 0.000001 and even more! Stay tuned for the tutorial of Direct Digital Synthesis

Digital To Analog Converter DAC With DDS


   Concluding Remarks   

 

  1  

it’s better to set up a timer to schedule Consistently Periodic Data Transfers To The DAC unit without too much of CPU intervention (time waste due to delays). The example codes provided in this tutorial are done in such a way to make it simple & easy for you to understand how DACs work and how analog waveforms could be synthesized digitally using digital microcontrollers. But, for real-world applications, you’ll have to consider this point and take it seriously.

All examples which are shown previously make CPU utilization of 100% You even can’t handle interrupts without disrupting the waveform generation. The CPU can do nothing other than transferring the data to the DAC all the time in order to get the desired analog waveform on the output.

And obviously enough, it’s not the way to do such a thing. But for now, it’s ok!

  2  

Converting data from the discrete digital domain to a discrete sampled analog signal at fixed (discrete) time intervals will introduce Harmonic Distortion at the analog output side of the DAC, due to sampling quantization noise effect. This is something you’ll have to deal with in order to get a clean analog waveform at the output.

This point will be discussed in great detail in a complete separate tutorial in the near future, so check my website frequently and click the little bill to get notified when something new is published.

  3  

Theoretically, A DAC’s output will swing between 0v And Vref (typically 5v or 3.3v). However, practical DACs don’t. There will be always a small drift above real zero and a small drop from Vref. More advanced DAC chips are being advertised to have a Rail-To-Rail output which is hopefully close enough to the theoretically ideal DAC.

 

 

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Khaled Magdy

I'm an embedded systems engineer doing both Software & Hardware. I'm an EE guy who studied Computer Engineering, But I'm also passionate about Computer Science. I love reading, writing, creating projects and Technical training. A reader by day a writer by night, it's my lifestyle. You can view my profile or follow me via contacts.

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