Are there any length limits to series termination ? If I have a trace that's 12 in or a trace that is 100 in, can the same series termination resistor be used (assuming that Zo = 50 ohm) ?
Trace termination length limits
signal integritytermination
Related Solutions
Talking about signal termination is like opening a can of worms. This is a HUGE subject that is difficult to summarize in just a couple hundred words. Therefore, I won't. I am going to leave a huge amount of stuff out of this answer. But I will also give you a big warning: There is much misinformation about terminating resistors on the net. In fact, I would say that most of what's found on the net is wrong or misleading. Some day I'll write up something big and post it to my blog, but not today.
The first thing to note is that the resistor value to use for your termination must be related to your trace impedance. Most of the time the resistor value is the same as your trace impedance. If you don't know what the trace impedance is then you should figure it out. There are many online impedance calculators available. A Google search will bring up dozens more.
Most PCB traces have an impedance from 40 to 120 ohms, which is why you found that a 1k termination resistor did almost nothing and a 100-ish ohm resistor was much better.
There are many types of termination, but we can roughly put them into two categories: Source and End termination. Source termination is at the driver, end termination is at the far end. Within each category, there are many types of termination. Each type is best for different uses, with no one type good for everything.
Your termination, a single resistor to ground at the far end, is actually not a very good. In fact, it's wrong. People do it, but it isn't ideal. Ideally that resistor would go to a different power rail at half of your power rail. So if the I/O voltage is 3.3v then that resistor will not go to GND, but another power rail at half of 3.3v (a.k.a. 1.65v). The voltage regulator for this rail has to be special because it needs to source AND sink current, where most regulators only source current. Regulators that work for this use will mention something about termination in the first page of the datasheet.
The big problem with most end-termination is that they consume lots of current. There is a reason for this, but I won't go into it. For low-current use we must look at source termination. The easiest and most common form of source termination is a simple series resistor at the output of the driver. The value of this resistor is the same as the trace impedance.
Source termination works differently than end termination, but the net effect is the same. It works by controlling signal reflections, not preventing the reflections in the first place. Because of this, it only works if a driver output is feeding a single load. If there are multiple loads then something else should be done (like using end termination or multiple source termination resistors). The huge benefit of source termination is that it does not load down your driver like end termination does.
I said before that your series resistor for source termination must be located at the driver, and it must have the same value as your trace impedance. That was an oversimplification. There is one important detail to know about this. Most drivers have some resistance on it's output. That resistance is usually in the 10-30 ohm range. The sum of the output resistance and your resistor must equal your trace impedance. Let's say that your trace is 50 ohms, and your driver has 20 ohms. In this case your resistor would be 30 ohms since 30+20=50. If the datasheets do not say what the output impedance/resistance of the driver is then you can assume it to be 20 ohms-- then look at the signals on the PCB and see if it needs to be adjusted.
Another important thing: when you look at these signals on an o-scope you MUST probe at the receiver. Probing anywhere else will likely give you a distorted waveform and trick you into thinking that things are worse than they really are. Also, make sure that your ground clip is as short as possible.
Conclusion: Switch to source termination with a 33 to 50 ohm resistor and you should be fine. The usual caveats apply.
You may want to read about the impedance and capacity of a wire/transmission line. I will try my best to translate to English as I learned most of this in German ;)
Every wire not only has a resistance, but also and impedance and capacity. These add up to the [electrical impedance] (https://en.wikipedia.org/wiki/Electrical_impedance). If you look at the effects of impedance and capacity, you will notice that these scale with frequency.
You can always use a serial matching end, it just depends on the configuration. If you have a wire with a impedance of 50 Ohm (typical HF wire) or 100 Ohm (like CAT5 Network cable), you need a matching end. This end is a 'network', wich will also get the frequency, so a normal resistor 50Ohm will work at 1MHz, but will have a mismatch (and a reflection!) at 1Ghz (which is why extra expensive frequency tolerant resistors exist). To counter this, you can measure the values of your resistor (C and L) and attach additional resistors/inductors/capacitys to counter the effect.
So your network on the end of the cable has to match your cables impendance at the given frequency. How you archive this matching is your choice. A series resistor is the common choice for 'household' electronics in the sub GHz area, above there are special solutions.
Best Answer
You may have to change the series terminator when trace length changes. A lot of the time, series terminators are just really damping resistors, and don't really match impedance. The driver impedance is extremely low, and the load impedance is extremely high (and slightly capacitive) and the trace is short enough that its impedance is mostly irrelevant. Think about it, if the source impedance is 50 Ohms, and the transmission line is 50 Ohms, the output voltage will initially be divided in half while the signal is in flight to the load.
The actual purpose of the series terminator is often just to damp out the overshoot, or slow the edges to help with EMC. This requires tuning (or simulation with accurate models), and line length can effect it, causing you to change the value of the series terminator.
When you get to long transmission lines, termination becomes pretty important. In theory, if your long transmission line is terminated at the load, you don't need driver series termination. There may be a mismatch between driver and transmission line, but it doesn't matter, because there will be no reflection, and no damping needed. In other words, you have a low impedance source driving a 50 Ohm (or whatever load). This presents no problem and allows the voltage to slew immediately to VCC. However, if there is any chance that the load is mismatched, then the best situation is to have the source impedance be the same as the transmission line. This way, any reflections coming back from the load will encounter a matched termination at the source and will not reflect again. But, as noted before, this situation cuts the drive voltage in half.
I would like to make clear that I recommend to always put series termination in any high-speed signal if you have room to place the resistor, especially clocks. You may need to tune the resistor to pass radiated emissions.
One other thing. If the chip has variable drive strength settings, start with 0 Ohm series termination and use the weakest drive setting that will meet setup and hold times. Usually reducing drive strength gets you more radiation reduction with less degradation of signal integrity than adding series resistance.
I believe this is all theoretically sound, and it is also based on my practical experience.