Theoretically, a differential pair signal will be distorted, if the reflection signal loses significant amplitude before the next bit is sampled by the receiver. But that defeats the purpose of differential pair cable, which is supposed to provide reliable communications over long distance without sacrificing speed. The bottom line is, make it a practice to install termination resistors on differential pair transmission lines. The cost of the resistors and installation is insignificant compared to the potential problems caused by signal rebound.
You will not need a termination resistor if the particular hardware is not the first or the last on the transmission line. It halves the value and creates a potential signal rebound.
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Another way to match the load and trace impedances is to use two load-end resistors whose parallel combination equals the trace impedance. This variation of the parallel termination can also perform well in distributed loads at the expense of a constant current leakage from Vcc to ground.
It can also be difficult to find optimum combination of resistor values for a given driver. Yet another way to match impedances is to add a capacitor in series with the parallel terminating resistor.
The addition of the capacitor mitigates the power dissipation problems of other parallel termination schemes, blocks low-frequency noise, and minimizes overshoot and undershoot. The added cost for this scheme is the added complexity of managing the RC time constant of the capacitor.
In series termination, you place the resistor near the driver to increase the impedance at the source and prevent reflections on the driver end of the trace. A resistor value is selected so that the combined sum of the termination resistor and the driver output are equal to the impedance of the trace. Series termination benefits from a lower power draw at the expense of reflection at the opposite end of the trace. Data errors due to reflections are most likely to occur when the round trip propagation time of a signal is equal to or greater than the transition rise or fall time of the driver.
In this post we covered the basics behind using termination to prevent reflections in your circuits. Preventing reflection through termination is only one piece of a much larger puzzle of eliminating noise and improving signal integrity throughout a PCB design. EDA software makes it easier to manage all the variables involved in noise reduction.
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Capacitance goes up with area and down with distance. Inductance goes up with the size of the loop. If you think of of a trace above a ground plane, as your widen the trace, the area increases but distance does not.
This means that your capacitance increases while your inductance stays the same. As your distance increases, your area must increase a lot to keep the same impedance. There are many many different calculators out there. I found one instantly with a google search. Just match your impedance, add some termination, and try to avoid bad practices like bridging across a break in a ground plane No embedded traces around these signal lines. I hope this also makes the physical effects a little more clear.
You will actually get reflections, but instead of bouncing up, it will bounce down. An open will double your voltage, it will all reflect backwards. A short does the opposite, giving you zero voltage. It also greatly increases your power absorption from your driver.
Imagine a transmission line as being a bunch of hanging weights connected by springs. If everything is uniform, and one gives a weight at the north end of the line a brief southward shove and return it to its original position, a very nice wave will propagate southward down the line; the energy that gets put into each weight from one side will be perfectly delivered to the other, so that once the wave has passed by a weight that weight will be motionless in its original position.
All very good until the wave hits the end of the line. The scenario where the last weight has some resistance, but not the right amount, will behave as a combination of 1 and 3 , or 2 and 3 above.
The scenario to shoot for is 3. They match the impedance to the trace impedance. That's why there isn't a reflection. The fact that they may sink current is just a side effect. Their values should be calculated based on the trace impedance and that of the receiver and driver. Multiple smaller value resistors will attenuate the signal too much.
It also may be more current than the driver can handle. The principle behind termination resistors is to match the impedance of your inputs to the impedance of your transmission line PCB traces and your source. Typically, input pins have high input impedance, since they are CMOS. Adding a small value resistor in parallel with the high impedance input pin will effectively set the input impedance to the resistor you added. This is useful, because output impedance is usually fairly low, and it is easy to make a micro strip transmission line with low impedance.
The goal when using a termination resistor is to make it a close to the input pin as possible. Using multiple resistors would be less optimal since the resistor is less like a lumped element. The other thing is that you should know your target impedance.
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