PWM-Based Digital-to-Analog Converter

Introduction

Now you have some experience designing a combinational circuit to be implemented by a programmable logic device, the GAL. This week you will design a sequential circuit to be implemented by the GAL. The specific circuit you will make is a low-cost digital-to-analog converter based on pulse-width modulation (PWM).

Objectives

Parts List

Equipment

Prelab

1. In addition to data sheets, IC manufacturers also produce application notes (“app notes”) to provide you with ideas to use their products in your own design. Review Microchip Technology’s Application Note AN538, Using PWM to Generate Analog Output, to learn more about the background theory of pulse-width modulation (PWM) as a method of digital-to-analog conversion.
    
2. The diagram below illustrates the complete PWM DAC system:
   
   
   
   A GAL PLD implements the digital portion of the system. A four-bit digital input is compared to the current "timeslice" indicated by the free-running four-bit counter to determine when the PWM output should be high or low. The RC analog lowpass filter extracts the average value of the PWM signal to create the final analog output of the converter.
    
3. Choose the specific values of R and C to satisfy the following specifications:
   (a) First-order RC filter
   (b) 100 Hz bandwidth for analog output signal
   (c) At least 40 dB attenuation of fundamental PWM frequency component
   (d) PWM frequency is minimum possible permitted by other specifications
   
   The design of your lowpass filter is closely related to the PWM frequency. State the PWM frequency that you will use. State the master clock frequency that you will use.
    
4. Review Section 7.11.1 in Wakerly to learn about sequential circuits in ABEL. Also review the counter example in Section 8.4.5.
   
   TIP: An up-counter can be implemented using a construct similar to “count := count+1;”, and the comparator can be implemented using relational operators such as “<”, “<=”, etc.
    
5. Implement the digital portion of the PWM DAC using the ABEL language. The input pattern 0000 causes the average value of the PWM waveform to be zero, the pattern 0001 causes the average value to be proportional to 1/16, 0010 causes an output average value of 2/16, and so on.
   
   The counter must be implemented using four output pins of the GAL, while the PWM output is a fifth output. The counter output should be observed during verification in simulation and in hardware.
    
6. Simulate and verify your design using the test vector ABEL code for the four-bit PWM DAC. Attach hardcopy of your waveforms to a lab book page.
   
   NOTE: Do not proceed to hardware implementation until your simulation is 100% correct!
    
7. A photocopy of your prelab pages is due at the beginning of the class the day before lab.

Lab

Please use the Lab Help Queue to request assistance in lab.

1. Build your PWM DAC circuit. Set up the 74HC4040 counter circuit to generate a four-bit input to your DAC. Use the Agilent 33120A function/arbitrary waveform generator to generate the squarewave clock signal needed by your counter circuit. The clock frequency should be selected to fit within the bandwidth of your PWM converter.
   
   Use a 555-based circuit to create your PWM master clock. Here are some resources: schematic and design equations, data sheet, and on-line design tool. NOTE: R₁ and R₂ in the on-line design tool are the same as R_A and R_B, respectively, in the schematic.
    
2. Make hardcopy of the MSO display of the analog output and all digital outputs of the PLD during one PWM period. Clearly label the traces.
    
3. Adjust the MSO to display the analog output voltage as the digital input number varies from zero to fifteen. Try to make the waveform fill the display so as to get the most amplitude and time resolution. This waveform should look like a stairstep ramp.
    
4. Evaluate the performance of your circuit as a digital-to-analog converter based on your measurement from Step 3. Do the noise characteristics of the analog output depend on the specific digital input? To what degree is the output linear?
    
5. Demonstrate your finished circuit to the instructor.

All done!