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Four ways makers can use D/A converters

Much of our conversation in “Discrete Signals” blog has been about acquiring data with A/D converters. For this installment, let’s look at the other side: going from digital data back into analog with a D/A converter. There are at least four ways makers can use D/A converters: creating sights and sounds, projecting touch, building complex signals, and automating tests.

Let’s take a quick look at the types of things makers might take on with a D/A converter.

  1. Putting audio and video back into analog

Almost all content is moving around in digital form these days, except the venerable music album on vinyl, yet our eyes and ears are analog sensors. Most of us can’t decipher a stream of ones and zeroes into anything we’d recognize as a program, movie, or song.

Sound must get back into analog before it hits speakers or earbuds. The first step is chewing through audio encoding formats, then taking each sample to an analog waveform. Ears hear any discontinuity as a pop or crackle, but at audio sample rates like 44.1 kHz, smoothing out analog audio generated by a D/A controller isn’t tricky. The “weakest link” effect comes into play in an analog signal chain. Selecting a D/A converter may call for one specifically designed for audio with good noise, non-linearity, and distortion characteristics, matched with amplifier components.

Figure 1 Noise, non-linearity, and distortion characteristics are crucial in audio signal chain.

Video encoding is a more significant challenge. Bandwidths and sample rates are into megahertz ranges, and waveforms are more complex. Although digital displays with HDMI inputs have taken over desktops and living rooms, there are still displays with analog inputs. Driving the RGB triad—red, green, and blue analog channels—is an art form, but it can produce high-quality images with rich color.

  1. When you can’t quite put a finger on it

One way to think of a D/A converter is as a digital potentiometer. Instead of turning a small screw or a big knob and getting a resistance change, a D/A converter creates an output voltage change that can control an amplifier’s gain. Makers can program a voltage over here, send that value over Ethernet or wireless networks, and get a voltage change sometimes quite a distance away.

While control algorithms often run in digital, actuators still operate in analog. A D/A converter can electronically project the knob-touching effect anywhere needed. Maybe it’s setting a throttle or steering on an RC car, moving a rudder on an RC plane, or positioning a robotic arm. These aren’t data streams as much as command settings. Again, some digging may be required to uncover the details of the control interface for an actuator. The good news is that physical actuators are relatively slow on the sample rate scale; so, tens of Hz out of a D/A converter usually gets the job done.

  1. Modulation can get complex in a hurry

A big area for makers is software-defined radio or SDR. On the receive side, demodulation is digital. If transmit capability is a feature, data needs modulation before converting to analog at the transmitter. It might be simple amplitude or frequency modulation. Digital schemes increasingly use some form of quadrature modulation—points on a two-dimensional grid, with channels labeled I and Q.

Figure 2 The SDR conceptual diagram highlights both hardware and software parts. Source: VIAVI Solutions

Bandwidth is one factor driving D/A selection into the hundreds of MHz for SDR applications. Without getting too far into the theory, specifications like error vector magnitude (EVM), showing how far points are off their ideal spots in the modulation constellation, become very important. Here, higher D/A resolution is also needed. Much modulation work already exists and open-source waveforms for commercial spectrum come in many formats.

  1. Automating tests with digital precision

D/A converters also serve a significant role in automating signals in test equipment or self-test circuits. A D/A converter can easily produce many classic waveforms: sine, square, or sawtooth. It can also create a voltage adjusting gain or offset for an amplifier circuit, handy in zeroing a sensor output.

One often discussed topic is the calibration of an A/D converter. As a coarse concept, it works. A D/A converter could step through inputs and see how the A/D converter responds. At a more advanced level, though, there’s a recursive problem. How accurate is the D/A output? It’s tough to calibrate anything without a very high accuracy standard as a basis for measurements. Think more along the lines of self-test. One can quickly verify operation using a D/A stimulus.

The synergy between digital and analog

Once upon a time, there were these hot messes of wires called analog computers. They seem like a stone-age technology now. The power of digital computation, even in a small microcontroller, outpaces even a large analog-only setup. But, like A/D converters on the sensor end, D/A converters are essential for controlling actuators or putting signals on airwaves. Understanding how to get out of the microcontroller and back into the analog world makes for exciting projects.

These four ways show how makers can use D/A converters to trigger some ideas.

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