Introduction to Signal Integrity for PCB Design

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Introduction to Signal Integrity for PCB Design

this webinar this is gonna be a part of a new set that we’re trying introduction to signal integrity for PCB design my name is Jorge Garcia I’m a product support specialist with autodesk Segal and have been working with eagle for about the last 10 years so it’s a pleasure to have you all here on this webinar and to start off we really want to say that simul integrity is a very broad and vast topic you know there’s no way that a 30-minute webinar could even hope to begin to scratch the surface of cylinder Gertie and it’s nuances and everything that can be involved in a design where you have to take similar integrity into account so to start off we really want to kind of define when should you start worrying about signal integrity so make that definition and and before we continue if anybody has any questions feel free to post them we’ll be leaving them for the end at the end we’ll answer any questions that pop up during the webinar so feel free to post them either for Q&A or through the chat at the end of the webinar we’ll be answering them ok so don’t worry we’ll make sure to go through all of your questions so we kind of wanted to find is when do we have to start worrying about signal integrity the technical answer is that signal integrity is always at play you know the the effects that we’re going to consider in this series of webinar are always there but at low enough frequencies they’re so small that they can be ignored and this is why many of you maybe have never ever had to worry about signal integrity I’ve never had to really consider some of the topics were going to analyze because your designs have been been in a regime where the rise times and clock frequencies are high enough where this becomes an issue so if you’re thinking I’ve made a ton of boards I’ve never thought about any of this stuff why is it important to me well if you have never needed to think about it it’s because you’re designed so far haven’t required but as technology continues to advance clock frequencies continue to go up rise times continue to go down modern products you know high-speed USB PCI buses things like that in those situations it is critical to be aware of your signal integrity it is critical to design for it because if you don’t you can have products that don’t work and that’s going to create delays in your schedule it’s going to create issues in your ability to go to market so that’s why this is important you need to take these into account and being aware of them now can help you make better design decisions later so we’re gonna go ahead and get started the first thing we want to determine is when do I need to start to worry about cylinders let me get out of this shot we’re gonna go to Eagle this is a little presentation that I’ve used before and we’re gonna use aspects of it in this webinar okay signal integrity becomes a very big deal generally when you’re dealing with what’s known as the high speed regime now the definition of a high speed signal is a little bit gray okay it’s not it’s not a it’s not a clear-cut definition but it can give you an idea of when you need to start taking these effects into account and one of the key things of dealing with the high speed signal and we’re sill integrity has a big role is that when you’re in this regime when you’re in dealing with a high speed signal your wires are no longer transparent to the signal in other words they can’t be ignored and treated as a perfect conductor rather as you can see in this Eagle sheet your wires in the high frequency regime because these types of circuits okay they have to be treated as a transmission line and this is an approximation of a transmission line using what’s known as a lumped a parameter model okay well you’re modeling being the impedance of the transmission line with resistors capacitors and inductors okay so when does life become hard for us when can we no longer ignore the wires in our circuit and have to start treating every single interconnect as a transmission line well what you want to do is you need to look at the frequencies in your design okay this particular metric is coming from a high-speed layout

guidelines from TI I have the the document number reference there in the side there in the the sheet okay and what they recommend is find the highest frequency in your design now the highest frequency in your design is determined by the fastest rise time in your design so you can have a 10 kilohertz signal okay and let’s say you know you look 10 kilohertz I should be more than fine 99.9% of time you would be if that 10 kilohertz signal if the edge as you can see here has a very quick rise time and it’s a very quick edge something on on the order of one nanosecond then you need to treat it as a transmission line you need you’re in the high speed regime so don’t let just the clock frequency dictate whether you’re you have a high speed signal or not you have to make sure that you look at the rise time that’s generally the more subtle point where people don’t realize that they are dealing with the high speed signal okay if the rise time is very quick and by quick we’re talking on the order of a nanosecond you know 1/2 nanoseconds you need to start kind of being careful you just start thinking about you know is the signal a high speed single and something I need to be extra careful with so we can see an example here okay if you notice we have a square wave and the rise time of that edge is a hundred nanoseconds now based on what I just said we know that this is probably not an issue not a high speed signal okay but if we look if go ahead and take the reciprocal of the rise time we’re looking at about a 50 megahertz signal we’re gonna look at the length of that signal so the length of that waveform so first thing we need to do is calculate how fast this signal is moving through the PCB okay and the speed is basically calculated by the speed of light divided by the square root of the relative permittivity of the material so that sounds like a whole bunch of jargon basically the velocity of the signal through your board is the speed of light divided by two okay because for fr4 material the relative permittivity is four square root of four is two speed of light divided by two gives you approximately one and a half times ten to the eighth meters per second okay pretty pretty straightforward and the method if it if it seems like a lot just realize that this is kind of a definition of derivation of where we make this determination as soon as we finish going through this math I’ll give you a rule of thumb that is easier to remember than going through this math but this derivation is insightful so we’re gonna go ahead and finish it so the wavelength of this Tepes in was basically the speed that it’s moving through your board it’s propagating divided by the frequency so if we notice you got the frequency is 50 megahertz we got the speed of light the speed of the signal traveling is one and a half times ten to the eighth the wavelength if you do the math works out to be three meters okay three meters so if your traces are considerably shorter than three minutes three meters you can you can ignore you can ignore the the interconnect you can treat it as you always have as a completely transparent so what is a lot less than three meters well TI defines it as one-tenth the wavelength so one tenth of three meters thirty centimeters about 12 inches so if your traces are less than a foot you’re golden you don’t have to deal with any of this okay but if now you’re dealing with traces that are much longer which obviously in most cases that wouldn’t be the case you know 10 feet 12 feet you do have to take this into you you’re basically dealing with signal integrity issues okay so what is the good rule of thumb what basically can simplify all this and just something that if you run into it you know you can you can say ah I need to be careful with this okay the rule of thumb as defined by civil integrity simplified by Eric Baggesen which is a very good book if you’re going to be dealing with a lot if you’re working in usb IOT anything with high-speed communications definitely I encourage you to get that book very good practical application of single integrity concepts um the rule of thumb he gives is basically if you have clock frequencies above 100 megahertz or rise time shorter

than one nanosecond then you’re dealing with a high-speed signal okay so clock frequency higher than 100 megahertz rise times shorter than 1 nanosecond you’re dealing with a high-speed signal okay and if you’re dealing with a high-speed signal you have to treat every interconnection as a transmission line and that’s where things become more delicate as you’re working through this so let’s take a moment and kind of discuss the rules of thumb okay in signal integrity every every equation that is used everything that you see here is either a definition or an approximation okay see the definition an approximation some are very good approximations but they are approximations nonetheless okay what’s the point of saying that okay if you’re working at a company and you’re gonna start making PCBs and you have high-speed signals okay you don’t want to lose a ten thousand dollar running boards okay so the best way to confirm the operation of your design before you manufacture it in the signal integrity world is you have to do simulation so simulation is a big part of signal integrity design and analysis there’s just no way around it basically those are that that’s the most accurate way to verify your design approximations a rule of thumb can give you a good answer now but if you need an excellent answer if you need high accuracy you have to go with the simulators you can’t really do these types of analyses by hand it just becomes intractable so luckily there’s a lot of tools that have become available and maybe another webinar we’ll go into into talking about them initially though spice is a very good way to start and you can cover a lot of ground with spice so in future webinars we’ll be going over that but this is a key concept you want to have in mind is you know do I have high-speed signals in my design once you can make that determination then you know if you have to go through the concepts that we’re gonna deal with now or if the design is basically transparent and pretty much any route will work okay from the scene integrity point of view obviously high power those there’ll be other considerations but from the signal integrity point of view your your the first thing you need to do is determine do I have high speed signals am i in the high speed regime if I’m not then I can I can relax a little if I am then I have to be very careful and again if you’re working for a corporation and you have products I need to go to market simulation is your best friend you are going to have to to go into simulation because all of in general the all of the equations all we’re gonna run into our on some level in approximation so keep that in mind so what’s the next concept that we want to kind of introduce your single integrin that we’ll be talking about a lot in future webinars okay next concept that we want to discuss is impedance okay impedance is the key parameter in signal integrity it’s the it basically gives you every important electrical property about an interconnect so you need to become very familiar with the idea of impedance okay for most of us who’ve worked with electronics in any for any amount of time it should be very familiar it’s basically the relationship between voltage and current it’s the ratio of voltage to current in a resistor it’s a steady value it’s very simple it’s it’s basically always whatever the resistor value is but now when you’re dealing with inductors and capacitors you have a frequency dependent impedance okay so impedance is more general than resistance you can always talk about impedance you can’t always talk about resistance okay so that is the key term and you’re gonna see it pop up a lot and to further that discussion on impedance let’s look at this sheet okay so why is impedance important basically impedance dictates how your signal is going to interact with an interconnect okay we have a very simple example here okay we have a driver going into a transmission line with a characteristic impedance into a load okay very simple now in order for signal integrity to be preserved the signal wants to see a constant impedance

as it travels down the trace okay if there’s ever discontinuity if there’s ever a difference in impedance then your signals going to distort you’re going to get what are called reflections okay and that’s what we’re showing here so as you can see we have just a simple square wave coming in be sick into the transmission line okay if the load matches in other words is the same impedance as a transmission line and then V out is going to be pretty much pristine okay or is ideally going to be pristine and that’s what you see here however if it’s mismatched you’re going to get reflections and those reflections are going to manifest as squiggles you know as higher harmonic components in the signal as you can see here or distortions now if you’ve noticed I’ve drawn a little dotted line through the middle okay and this is basically representing a detector threshold okay so imagine digital logic anything above that threshold is a high anything below that threshold is alone okay if you get single Distortion because you don’t have the impedances matched or if you notice these flexions are very close to that threshold in other words if the impedance mismatch is large enough you can get your data corrupted okay and your product will not work so it shows you what a fine line how critical this can be here okay so if you’re in the high speed regime one of the big fundamental rules is to maintain constant impedance and how do we do that basically you make sure that your trace width is consistent the whole way through okay if you can avoid vias you totally should because the vo will affect the impedance it will be a discontinuity in impedance it will be a difference there you will get some reflection now obviously you can’t always avoid vias and in future webinars we’ll analyse well when can I via you know what can you do to make the vs safer to make the via as transparent as possible to the signal and those are things that have to be really taken into account with high you know with high speed boards where now you have different materials different things that you have to deal with okay so that’s what we would kind of keep in mind here that we’re discussing maintain the thickness of your traces in doing so you maintain a constant impedance and you can get cleaner signals now obviously depending on the type of driver the driver may have an impedance that it wants to match to but your transmission line will have a different impedance and in those cases you’ll have to do things to equalize them but having it constant is going to make your life easier okay so what are things that can create impedances continuities if you have to ultra with halfway through your trace that can create reflections okay depending on links and things like that they may be serious it may be not so again another webinars will go into more detail about this but that’s something you want to keep in mind here with signal integrity impedance matching is vital to maintaining signal integrity any impedance mismatch will create distortion in your signal and that can lead to your product not functioning properly okay so let’s show you a common example here okay as we know op amps in general have very high input impedance now I just chose a random op amp in case any one here is a very much’ signal integrity expert I know a 741 would never be used for such an application I just had to put in a an op amp okay so if you notice if you just take the transmission line straight to the op amp okay you have whatever the impedance of the transmission line is going into an extremely large impedance okay there are going to be reflections now obviously if you have low speed traces those speed signals you don’t have to sweat it you know that’s okay because we’ve all done this you know if somebody tells me they have it they’re lying you know because we’ve all done this we’ve all just taken connections straight from component a into the op amp okay we’ve all done it we all do it it’s okay but in a high-speed situation that’s not gonna work okay and the reason is you’re gonna have that impedance mismatch and you’re gonna get 200 distortion on the signal so by using

you know and this is one way to terminate there are many ways to do termination or depending on the situation but this is possibly the simplest one and okay is to put in that that load resistor there in parallel cause it’s gonna happen is you know a high impedance in parallel with a lower one the effective impedance is going to be the lower one basically okay and that will help minimize distortions so keep that in mind okay these are just very fundamental things we’re going through over here and this webinar in future webinars will go into more details ok Waianae specific concepts and things that we can do but this this webinar is intended to be just a general overview now what’s the next concept that we want to discuss in the last one we’re going to discuss for a webinar today and that is ground ok we’re all used to ground we all talk about ground you have to be very careful with the word ground in signal integrity okay and really you always should be but now once you’re dealing with high-frequency signals the understanding of ground needs to change a little bit okay and we need to kind of expand on it become a little bit more nuanced and a little bit more specific with it okay so what we want to keep in mind here is that most of the time when we talk about ground it puts our mind into thinking that ground is basically just a huge current sink you know all the girl all of our signal ground currents they just go into this one big place okay and that is completely and utterly false that is not true okay if you were to if if you need very expensive equipment to illustrate to see this physically but when you when you start analyzing okay and we all know this fundamentally but because of you know Pratt daily practice and and working sometimes we lose sight of this fact electricity current always flows in a loop okay for every signal you have the current going to through the signal path and then coming back through a return path okay every signal on your board does this it goes out and the current flows back okay for most of us we just let it flow to ground okay but if you have the proper equipment what you’re gonna notice is that when the current goes back through the ground plane it’s not evenly distributed through the ground plane it actually wants to follow the signal path as closely as possible so what you’re gonna see is that currents in a ground plane do everything they can to flow right under the signal path that originally created them okay and the reason for this is to minimize inductance is to minimize impedance water flows through the path of least resistance current flows through the path of least impedance always okay and the path of least impedance occurs closest to the signal trace right underneath it okay so you can imagine that in a situation like this if you have a lot of signals and a lot of traces there’s gonna be current that’s kind of kind of overlap or run through each other and that’s gonna create noise it’s going to create additional issues okay and these are things that you have to become very aware of when dealing with the high speed regime so what’s the key don’t think of things in terms of a ground you know you really shouldn’t ever just think of an infinite current sink it’s a good it’s a good useful approximation when you’re not in the high speed regime but if you just think about every trace that goes out of my Icee for example out of my Arduino has to come back to the battery has to come back to the ground has to come back you know you have a return you have a signal path and you have a return path and that happens for every single net if you think in those terms then you can become more aware of how your signal is behaving on the board and that can help you troubleshoot issues okay so those are the the key concepts we kind of want to talk about here okay and if we look at it you know since we’re talking about ground we’re gonna go ahead and talk about this side okay the idea of ground and why just thinking in terms of a current sink is a problem okay of just this one big plane and putting everything through there everything’s gonna be okay all right if you look these three

circuits look identical right because they’re all going to ground okay but depending on how they’re routed you may run into issues okay so let’s take a look at the physical implementation okay so on one end you have power coming into the the component obviously and then you have it coming back to where it came from okay so in my case this illustration I should probably update to have two points you know for power coming in so just assume for now that power is coming in through the rightmost capacitor on each of these okay so power comes in if you were to daisy-chain the connections there isn’t impede their you know here we’ve modelled it as just a simple resistor but it does have an impedance that has a capacitance as an inductance to it okay as you can see here if it’s daisy chained then this reference potential is going to be different from this reference potential is going to be different from this reference potential okay and that can cause a circuit to not work okay in these good examples what you notice is that we’re thinking about a return path and that’s what’s reflected in the PCB okay so in the schematic I’m just making it really obvious but if we go to the PCB you’re gonna see that how the connections are made reflexes okay and the reason a ground plane is good is that it can make it easy it’s giving lots of paths for the current to come back to its origin okay and because it’s just a plane of copper most you know unless the current is going through large distances these impedances are going to be small okay so that’s what’s really going on you can so think of a copper plane as making it easy for all the return path to go back to their to their origin okay but don’t think of it as just a sync of current and as long as it’s connected to the ground plane everything is going to be okay because that may not be the case if we have a lot of signals going around here you have to be careful on how you route them because you may cause return paths to run over each other to intersect and that can increase noise so keep that in mind okay think signal path return path don’t think so much in terms of a ground that’s going to help avoid issues in the future okay so let’s let’s kind of summarize what we’ve gone over first we’ve determined how we can see if we have high speed signals or not okay the simple rule of thumb if you have clock frequencies higher than 100 megahertz and raw or or rice times shorter than about one nanosecond you have a high-speed signal and you need to start thinking about signal integrity very very carefully okay impedance is the language of signal integrity impedance is simply the relationship between a voltage and current in a conductor through a component okay that relationship can be very complex especially if you have inductor or capacitor you’re gonna have phase shifting you obviously have a frequency dependence so impedance is more general than just resistance okay so we’re gonna be talking the rest of this webinar series as we do them we’re going to be talking about impedance keep that in mind relationship between voltage and current very simple okay for your signal integrity to be preserved for your signal not to distort it needs to see a constant impedance as it travels through your board okay and those impedances should be matched to minimize distortion finally the last concert that we’ve covered is ground okay we’ve enhanced our understanding of what ground is okay most of the time you can think of it as an infinite sync but that’s not technically accurate okay every signal has a return path if you think in terms of signal path and return path then things are going to be easier because you’re going to pay as much attention to the return path as you do to a single path and if you have a ground plane okay the current close to the ground plane doesn’t just uniformly distribute in a future webinar we’re gonna see that in order to

minimize impedance because the signal is going to flow through the minimum impedance it wants to minimize it and that occurs when it’s right below the signal path or right next to it as close as possible to it that’s when you get minimum impedance okay and that’s how currents want to flow in a ground plane they don’t just distribute evenly like if you were throwing up a drop of water into a bucket okay it’s actually taking a specific section of the ground plane to travel through and if there’s anything in the way then that’s going to change the impedance gonna add a little bit of noise so if you have a via in the way of that clean return path then the currents gonna have to go around the V and that’s gonna change your impedance okay so those are the three fundamental concepts we want to take away from the webinar today and those are gonna form a building block for future webinars on signal integrity