We are born into a world of purely analog signals. The sound of a stream running through a mountain meadow, the brown color of dirt, that screaming baby next to you; all of these ‘signals’ are of an analog nature. Yet, we grow up and seem to spend most of our time attempting to turn these signals into a reproducible ‘digital’ copy so we can perceive them through a digital device. Analog and digital signal performance is necessary to keep in mind.
Digital-to-analog conversions (and back again) are a hot commodity in our present culture. Simply just reading this article can be perceived to be a ‘digital’ conversion of my ‘analog’ thoughts. Listening to a song on the radio is a representation of what the original analog signal once was but now captured and reproduced over and over for years and years of continued enjoyment.
With all this ‘analog-to-digital’ and ‘digital-to-analog’ conversion happening all around us at nearly every waking moment of our existence (these days), it’s a wonder how it’s still such a mysterious topic. This is a subject that we should all strive to understand from a qualitative point of view because this technology can improve our perception of the world around us.
Signal Conversion: What Is It?
Many analog-to-digital conversions (ADC’s) and digital-to-analog conversions (DAC’s) that I hear about these days in the audiophile industry. Other than my own personal fascination with audio circuitry, the audiophile industry is the area where folks really, really, really care about signal performance. If you were planning to drop $10,000 on a new amplifier, wouldn’t you care about how signals are converted from the CD to your speakers? Of course, you would!
The process that a signal goes through from starting as an audio signal and ending as a digital signal is rather straightforward. Assuming a guitarist wanted to record himself for later listening, they would simply play their own analog signals (i.e. the song) into some sort of recording sensor; in this case, it could be as simple as a microphone. As the microphone senses the air pressure change, it moves around a small magnet based on the changes.
This magnet induces a small current (that varies in voltage as the notes change) that is sent to an ADC. This converter ‘measures’ or samples the incoming analog sound/signal at very small increments along the way (sampling rate), recording the signal amplitude at each sample (bit rate) and ultimately saving them for later use. In the end, the newly recorded signal is simply just a bar graph of amplitudes characterized by 1’s and 0’s.
The process to retrieve this information is simply just reversed, albeit not a trivial task of ‘playing back’ the digital signal. Such a signal (made up of just 1’s and 0’s) wouldn’t make any sense to our ears if replayed through an amp without a converter first converting back into analog.
The upper signal (blue) is a digital representation of the lower, analog signal (yellow).
To reverse the process, you first need to take sampling measurements along the bar graph of a song through a digital-to-analog (DAC) converter which understands the 0’s and 1’s enough to spit out a smoother, analog representation of the guitar. You can then send it to an amplifier to amplify the signal before sending it to a pair of speakers as a real-world signal (i.e. sound waves) capable of being (re)understood by our ears.
This is the basic premise behind any analog signal captured for future reproduction. Any ADC or DAC in the market follows this basic path with its voltage. But with such a common approach to signal conversion being widely used in the circuit industry, how are there so many converters ranging in performance, efficiency, and cost? What constitutes a ‘better’ converter? What attributes do you need to consider and what can you forgo?
Three Major Attributes to Consider
Like most answers, the specific attributes you need to consider depend on your application. The same is true for determining which converter is best; it depends on the specific application. Let’s take a 90° turn and assume that you aren’t, in fact, interested in the high fidelity audio marketplace and are instead looking for solutions to measure the height of the water at your local marina. Maybe you have a really nice yacht that you require measurements for, who knows?
Bridging the gap between this scenario and what we learned in the last section, we can begin to whittle down the ideal factors that will help us to determine values for our converting system. We will attempt to find a great balance between performance and cost.
Starting with the sampling rate; back in our audio example, we can often see sampling rates of up to 44kHz (that’s a lot of samples to measure in a single second!) for higher fidelity applications. However, now that we’re measuring ocean height in our very own calm marina, 44kHz sampling rate will likely be far too high for what we need. Choosing a requirement that works for us, we might decide to measure once per minute or so. This would actually convert to a 0.0166Hz sample rate (quite a jump!). Easy enough.
Bit Rate (and depth)
Now comes bit rate. Again, in our high fidelity audio examples, we’ll often see higher bit rates (1400kbps) offering greater performance, but at a much higher cost. Bit rate is the rate of data that can be exchanged each second. The higher the bit rate, the more measurement data you can transfer per second. Ensuring our data measurements can support our previously selected sampling rate will optimize the process and give us better accuracy.
Bit depth, on a similar thread, is the ability to create finer and finer measurements for each sample. For instance, if we’re assuming a changing marina height from 0 to 50 feet, we might only care about measurements down to an inch, rather than ten-thousandths of an inch.
Note that even a 4-bit sample can produce up to 16 integer values. Reading more about bit depth will make this obvious to you.
Clocking (Avoiding Jitter Effects)
Clocks are often argued to be the most crucial part of any circuit, especially if two clocks are working together in a single application. Clocks are important as they are the red light/green light gatekeepers of data being sent/received in a system. Impurities in data collection and reproduction (especially in ADC’s and DAC’s) can often be traced back to clock timing issues. If a sending clock misfires slightly (even due to EMI issues), the receiver will either not collect the data or receive somewhat skewed data.
One way modern ADC and DAC designers mitigate this issue is to internalize the clock(s) into their own local one. This ensures that all the data enters and exits at the same time so errors (or jitter effects) are minimized. This isn’t as much of a design choice as much as something to be aware of based on your application.
Sampling rate as a visual grid representation from 5% rate to 100%.
As a fast-moving society, we are rocketing towards turning an analog world into a digital one. Whether or not you are a part of the conversion process, you are certainly in the wake and will begin seeing this shift everywhere, if you haven’t already.
When it comes to making decisions about your projects based on attributes of either an analog-to-digital converter or a digital-to-analog converter, being wise about sample rate, bit rate (and depth), and clock performance will help your design attain data packets properly and accurately.
If you need to run your design through a SPICE simulator, utilizing software such as Altium Designer can alleviate any guesswork before pushing through to a prototype. With real-world simulations and comprehensive support, Altium will make your design changes a breeze. If you want to learn more about how Altium can help with your project, talk to an Altium expert today.
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