There’s something awesome about being able to lớn use the lachạy thử technologies & interfaces in your PCB thiết kế. We’re talking about things lượt thích USB 3.0, HDXiaoMi MI, Ethernet; the danh mục goes on. Anything that adds some modern day functionality lớn your device and makes it stand out. But there is a tradeoff when adding some of these advanced technologies onlớn your board; they suddenly throw you into lớn the world of high tốc độ design. It’s in this world where you have sầu lớn pay attention to lớn more variables than ever, like signal integrity (SI), electromagnetic interference (EMI), & most importantly, differential signaling. While you might have been using single-line traces in the past, if you want lớn work with the lakiểm tra technologies then prepare to add a paired trace to lớn the set. So what is differential signaling all about, và why would you ever need khổng lồ use it on your high tốc độ PCB design? Let’s find out.
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What You’re Used To
To underst& differential signaling, you first have sầu lớn underst& its opposite – single-ended signaling. Don’t let the fancy name throw you off; this is exactly the kind of signaling you’ve been working with on any PCB design that isn’t considered high speed. As the name suggests, single-ended signaling is all about sending a signal from a transmitter to a receiver with one trace. That’s it.
An example of single-ended signaling on a schematic, notice the single signal line from Data Into Data Out. (Image source)
This means that you’ll have sầu a single copper trace carrying your signal khổng lồ its over destination, & from there it heads to your common ground và bachồng lớn your source. This is a comtháng practice in every standard PCB layout you might have worked on in the past. Anytime you draw a trace in longky.mobi EAGLE and connect it from one pin khổng lồ another; then you’re working with a single-ended signal.
When you start cramming in a bunch of traces & components inkhổng lồ really tight spaces, then you need a way lớn work around potential electromagnetic interference (EMI) issues. Because if there’s anything to lớn know about EXiaoMI issues, it’s that it will do an excellent job of messing up the unique of the signals that you’re sending. Here’s an example:
Say you need to lớn store a piece of data inlớn a particular spot in DDR memory, so you send a signal from point A lớn point B. What happens along this signal’s journey if it encounters some electromagnetic interference? The data inside the signal may be affected by the interference. Turning our nice beautiful square wave sầu into lớn a fuzzy mess.And before you know it, the signal you sent ends up all jumbled & unrecognizable.To help protect the integrity of signals along their journey in a high tốc độ thiết kế, you need a more robust way to keep your information-in-transit secure than what single-kết thúc signaling can provide. And this is exactly what differentials signaling help shield.
What Differential Signaling Is All About
Unlike single-ended signals, differential signals take advantage of not one, but two traces that work in tandem together. Here’s how it works – you’ve sầu got two traces that both carry the same signal, one of which is considered the positive signal, và the other trace is considered the negative sầu signal.
Here’s differential signaling (bottom) and single-end signaling (top) side by side. (Image source)
When information is transmitted along this dual-trace arrangement and arrives at its destination, a receiver can then extract the data by analyzing the potential difference between the positive và negative sầu signal. And by analyzing this dual signal và its voltage difference, your receiver can go about understanding whether this signal is transmitting a 1 or a 0, or a high or low voltage.
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So for every differential signal that you need to lớn add on your board, you’ll need to have two traces lined up side-by-side. For example, if we have a board with trăng tròn different nets that need to be connected, we’d need a total of 40 individual traces khổng lồ get the job done.
We know what you might be thinking now – why in the world would I ever want to double the amount of traces on my board layout? That’s going to take up some valuable real estate on a PCB that could be used for component placement và making my routing job easy. At a glance you’re right, differential signals vày take up more space on your PCB, but they have sầu some convenient benefits in high speed thiết kế applications, such as:
Keeping nguồn Systems Separate
Because differential signals are equal & opposite, they don’t necessarily skết thúc a return signal lớn ground; then you can make something like an analog signal going khổng lồ a digital device without having to lớn worry about crossing over power boundaries. This makes it a lot easier lớn keep power systems separate. One thing khổng lồ bear in mind though, if you’re working with USB or RS-485 technologies, then you’ll most likely need a shared ground so that your differential signals stay within the required voltage threshold.
Resisting Incoming Electromagnetic Interference
Differential signaling also has the added benefit of reducing any incoming electromagnetic interference or crosstalk from other noisy traces. Any interference that a differential signal soaks in gets distributed evenly between the positive sầu and negative traces, which reduces any change in amplitude that external EMI issues can cause.
How vì chưng you determine what’s on và off in this noisy digital signal? (Image source)
Resisting Outgoing Electromagnetic Interference
Your differential signals will also generate their own EMI while transmitting information, just like single-ended signals. However, because the positive sầu and negative sầu signals in a differential share the same polarity & distance, this effectively cancels out any EXiaoMi MI emissions.
Another great example of how EMI in the form of noise can affect a signal along its journey. Thankfully differential signals were used here. (Image source)
Lower Voltages
Differential signals also have the added benefit of being able to lớn operate at lower voltages than single-ended signals, all while maintaining their signal-to-noise ratio (SNR). And with lower voltages, you get the benefit of being able to use lower supply voltages, reduced power consumption, & reduced EMI emissions.
Timing Precision
Single-ended signals have a bunch of factors lớn consider khổng lồ determine what kind of logic state they might be in, lượt thích the power supply voltage, reference voltage, etc. But with differential signals, this is a lot easier khổng lồ determine. If the negative sầu trace in a differential signal is a higher voltage than the positive trace, then you have a high xúc tích và ngắn gọn state, và if it’s the other way around, then you have sầu a low xúc tích và ngắn gọn state.
Logic states have sầu both a high and a low range to signal a 1 or a 0 being transmitted. (Image source)
Using Differential Signals In Your Design
Now that you underst& all of the great benefits that come with using differential signaling on your high tốc độ thiết kế, you might be wondering what kind of constraints they require. As you can probably imagine, all of the benefits of differential signaling weighs heavily on being able khổng lồ keep those traces at a consistent length and space from each other at all times, otherwise, you’ll mess up the equal voltage & polarity benefits between the two. Here are three quichồng tips when setting up the kiến thiết rules for your differential signals in longky.mobi EAGLE:
Rule 1 – Keep your traces at equal lengths. If you don’t do this, then you ruin the entire benefit of two traces being coupled together along their entire transmitter to receiver journey. And losing that means dealing with some nasty EXiaoMI emissions that can corrupt your data. In most devices, you can have a trace length difference of up lớn 500 mils tolerance but keep them as close as possible.Rule 2 – Route differential traces cthua thảm together. Called coupling, this again comes down to a concern about EXiaoMI. The closer that you route your differential signals together, the smaller you make the loop area of your induced current, which has a direct impact on how much EMI your traces will emit. By keeping two traces close together, your ability lớn cancel out EXiaoMI issues is greatly enhanced.Rule 3 – Keep your impedance consistent. It’s important lớn always keep your differential trace impedance constant over the entire length of its venture from the transmitter to lớn the receiver. Your impedance will depend on many things, lượt thích the width of your traces, the thickness of your copper, & the materials you’re using in your layer staông chồng up. Dial down these variables, calculate exactly what you’re impedance needs khổng lồ be, và stick with it.Staying With the Times
If you’re going to lớn be working with the lathử nghiệm technologies on your PCB design lượt thích USB 3.0, HDXiaoMi MI, DDR, Ethernet, etc. then differential pairs are going khổng lồ be your new best friend. Not only vị these tightly coupled traces help khổng lồ reduce incoming and outgoing EMI, but they also make it easy lớn keep power systems separate, and can lower the overall voltage needed khổng lồ power your project. Rethành viên though, lớn get all of the benefits of differential signaling you need to strictly define your thiết kế rules lớn keep your traces at equal lengths with tight spacing & exact impedances. If you don’t, you’ll ruin their beneficial balance!
Ready to get started with differential signaling on your first high speed thiết kế project? Try longky.mobi EAGLE for không lấy phí today!
