How to Choose a DAC for Waveform Generation

When you look at MCU/SoC specs, you don’t always see DACs listed as a prominent feature. I’m not sure when the DAC lost the popularity contest to its cousin the ADC, but the result is that a DAC often has to be chosen as a separate component. There is plenty of advice on choosing ADCs for different applications, data rates, and bandwidth. In contrast, most of the advice on how to choose a DAC that I’ve seen focuses on audio reproduction. If you’re designing for fields like industrial automation, integrated test and measurement equipment, software-defined radio, or other specialty analog application, here’s what you should consider when selecting a DAC.

DAC vs. ADC Selection Criteria

ADC and DAC processes are inverse of each other, but both processes and both types of converters are important for interfacing between the digital and analog worlds. Although the specifications for each conversion process need to be considered in the correct context, many of the same specifications apply in both processes. There are even some standard tricks that are used to ensure low-noise acquisition and reproduction of analog signals that are applicable to DAC selection.

If you’re good at choosing ADCs, you’re probably also good at choosing DACs. A thorough understanding of Nyquist’s theorem (also known as the sampling theorem) is an important place to start when learning how to choose a DAC. If you can think of things in terms of the Nyquist frequency and its relationship to data rate, then you’re well on your way to choosing a DAC. Let’s look at the relevant specs in DAC selection and how they affect the performance of waveform generation.

DAC Specifications for Waveform Generation

The specifications that govern waveform generation are similar to those needed for an ADC. Here are some of the important specifications to consider when selecting an DAC for these waveform generation tasks:

• Interface. You’ll need to get data into your DAC to generate an analog signal. Typical interfaces are SPI for serial input, parallel, or PWM.

• Resolution and monotonicity. The resolution determines both the level of noise a DAC can tolerate and the accuracy of analog signal reproduction. Monotonicity is a related accuracy specification that defines the ability of DAC to maintain an analog output that follows the direction of the input data. The DAC output shouldn’t spike upward before trending downward when the input level decreases.

• Sampling rate. All DACs and ADCs have bandwidth that is defined by the sampling rate. The sample rate determines the maximum frequency that can be reproduced accurately (Nyquist frequency). However, the impulse train used in a DAC introduces additional frequency content beyond the bandwidth as defined by the sampling rate. Therefore, bandwidth is not well-defined for a DAC; I’ll look at this in more detail below.

• Dynamic range. All analog components have a well-defined dynamic range (measured in dB). This specifies the difference between the maximum and minimum output signal levels.

The two major specifications involved in selecting a DAC for waveform generation are resolution and sampling rate as these are the foundation for reconstructed signal accuracy. Note that sample rates can reach many Gsps in higher-end DAC. These specifications then need to be compared to the reconstructed signal bandwidth to ensure accurate regeneration of an analog signal. However, due to the signal reconstruction process, additional circuitry is needed for accurate signal reconstruction that is not found in ADC circuits.

Spurious Images in DAC Waveform Generation

While ADCs and DACs perform inverse processes, they do not reproduce the exact same waveforms. The inaccuracy in an analog signal that is introduced in a digital-to-analog conversion process is shown below. Due to quantization of the reconstructed analog signal, the output signal from a DAC has some signal images that appear at higher frequencies than the Nyquist frequency. 

In the above image, the sinc envelope on the DAC output is due to the use of an impulse train for signal regeneration, which has a sinc power spectrum. The use of an impulse train generates higher-order images of the reconstructed analog signal; think of these images as containing higher-order harmonics of the reconstructed analog signal’s Fourier spectrum. The amplitude of these images is weighted by a sinc envelope, as shown above.

Oversampling

Just like oversampling spreads noise out over a larger bandwidth and reduces the overall noise floor in an ADC, oversampling spreads the image content out over a higher bandwidth, as shown above. In other words, using a higher sampling rate pushes images of the reconstructed signal up to higher frequencies. This eases requirements for filtering the output signal as a lower-order filter can be used for smoothing.

Filtering

To remove images, you need to pass the output analog signal through a lowpass or bandpass filter with high rolloff. The high-end cutoff should be near the edge of the desired bandwidth to suppress any unwanted images. Higher order active filters can be purchased as ICs with standard form factor, or a filter can be designed from discrete components. The overall process involved in sampling and filtering during waveform generation is shown below.

Other Components for Analog Signal Processing

When you’re looking for a DAC, you can find a range of components from major manufacturers. The bit resolution on these components tends to be larger than that used in ADCs with similar sample rates, even though these two components could accurately sample and reproduce the same signal. This is due to the use of dithering on modern ADCs to artificially compensate for low resolution and increase the sampling accuracy upon reconstruction.

If you’re working with analog signal processing systems, there are plenty of other important components you’ll need for signal acquisition, manipulation, and reconstruction. Here are some other components you might need for your system:

• DSP integrated circuits

• An MCU or FPGA

• Passive components

Learning how to choose a DAC is the first step in accurate waveform generation and signal reconstruction, and you can find a range of components for your new product with the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to current distributor pricing data, parts inventory, and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our DAC integrated circuits page to find the components you need.

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