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Optimizing PWM DAC Filter Design and PCBA Layout

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • What is a PWM DAC filter?

  • What are the PWM DAC filter design objectives?  

  • How to best design and lay out your PWM DAC filter board.

A glass of water

A simple glass of water.

When I walk into a store today, intent on getting something to drink, I am sometimes in awe of the sheer number of options available. First, there are natural juices in familiar flavors such as orange, apple, grape, grapefruit, pomegranate, cranberry, and many others. And, we have milk in various percentages of homogenization, or chocolate, if you prefer.  There are also the sodas that may contain low or no calories, be composed of one fruit flavor or several competing ones, or be designed to extol the unique taste of a spice, such as ginger. 

Although the task of deciding from the abundance of choices can be almost paralyzing, when I am most in need of a thirst quencher, a simple glass of water always does the trick.  

When designing a circuit board, choosing the best option from the vast number of different components that perform the same function can also present a significant dilemma. This may include deciding whether to use a system-on-a-chip (SoC), IC, or a custom design to perform the needed circuit operation. Custom designs, which utilize passive and/or active components, may offer the best alternative. Fixed modules, like SoCs, typically are designed for flexibility over a range of operation. On the other hand, custom designs can hone in on the specific criteria for the application at hand, which maximizes performance and simplifies the choice of components. 

Let’s take a look at how customization makes board layout simpler for the conversion of a pulse-width modulation (PWM) signal into an analog output, which is the function of a PWM DAC filter. 

What Is a PWM DAC?

To put it simply, 

A PWM DAC is a circuit or sub-circuit that takes a digital PWM signal and converts it into an analog signal. Typically, this is done for low-frequency applications or applications where power loss or consumption is not a critical operation parameter.

This operation, shown in the figure below, relies on a low-pass filter to generate the analog output.

PWM to analog signal conversion graphic

A PWM DAC filter’s functional objective.

This filter is often referred to as a PWM DAC filter to identify its specific function. As this circuit includes both digital and analog signal processing, there are signal integrity (SI) and power integrity (PI) implications for your board layout, which are discussed below.

Design Considerations for Your PWM DAC

In today’s world, where it seems almost all devices and products use digital signals, a reasonable question may be, “Why convert a digital PWM signal to analog at all instead of simply using a digital pulse train?” Well, the fact is that there are many uses for analog signals. They are primarily used when there is a need for precise control that depends upon the amount of power, voltage, or current applied to dictate the speed, luminosity (or perceived brightness), volume, or some other parameter. In those cases, a PWM DAC offers a simple means of meeting the load demand.

The low-pass filter is the key to the operation of your PWM DAC circuit. And, there are several passive and/or active designs that can be used depending upon your circuit requirements. The simplest of these is the first-order RC filter shown below.

first-order PWM DAC filter design schematic

Simple PWM DAC filter. 

The circuit above is commonly used to pass signals with frequencies below a defined cutoff frequency while blocking those above the same threshold. However, for the PWM DAC, the objective is to generate a continuous analog output. The equation that defines the PWM signal is given in Eq. (1): 

Vac = A x duty cycle                                    (1)

        Vac is the average or DC component of the PWM signal

A is the amplitude of the PWM signal

        duty cycle is the pulse width (PWM signal period ratio)

From Eq. (1), we see that the magnitude of the analog signal is directly proportional to the PWM amplitude and pulse width. The important analog signal parameters are the peak, DC offset--which should be equivalent to Vac from Eq. (1)-- and the amount of ripple or, inversely, the smoothness of the analog signal. Therefore, PWM DAC filter design consists of selecting the appropriate components such that most or all of the following are satisfied:

PWM DAC Filter Objectives

  • Choose a cutoff frequency that eliminates unwanted high-frequency signal components.

  • Achieve the peak desired by the load circuit.

  • Eliminate any DC offset.

  • Minimize the analog signal ripple.

Achieving these goals simultaneously can be a design challenge, but with the right tools, an optimal design can be readily determined. 

Optimal PWM DAC Filter Design and Board Layout

When designing a circuit where circuit optimization requires simulation over a range of conditions for comparison, such as a PWM DAC filter, there is no better tool than Cadence’s PSpice. PSpice can also aid in manufacturability and eliminate unnecessary board respins. The results of simulations and analyses can also be integrated into your design in real-time to improve your design and promote first-time-right manufacturing of your PWM DAC filter PCBAs. 

With the industry-leading PCB Design and Analysis Software platform from Cadence, you can leverage the advanced In-Design integration capabilities of Allegro PCB Editor to quickly design your PWM DAC filter. Additionally, you get the ability to examine the electrical behavior of linear and nonlinear circuits as you prepare to create your PCB layout, enabling faster board design. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.