How Signals Interact With Series or Parallel Termination

Series and parallel termination are intended to provide impedance matching looking into a transmission line, specifically when a digital interface does not require a specific impedance value. Although many interfaces use a standardized impedance, the use of a long transmission line brings the need for termination to some target impedance, even if the interface is not standardized to a specific impedance value. Both termination methods are intended for interfaces that do not have a design to a specific impedance value.

There is also a perception that both series and parallel methods should be used at the same time. When we look at the actual signal dynamics of both methods, it becomes clear that it is not necessary or desirable to always use them at the same time. Instead, if we understand the signal behavior in each case, we can see when to use series termination and when to use parallel. Let's look at each case specifically for digital signals.

Series or Parallel Termination?

Series terminations and parallel terminations are applied with discrete resistors and should always be applied as chip resistors. These chip resistors are used as follows:

• Series termination is ideally placed very close to the driver pin

• Parallel termination must be placed very close to the receiver pin

The question of when to use series versus parallel termination depends on what you need the signal to do. Only specialty cases will use both at the same time. If you're a digital designer working with unspecified single ended interfaces or specialty logic, then the table below should help you decide which to use.

This table summarizes the specific instances where series and parallel termination should be used independently of each other.

As mentioned in the table, there are variations on these termination techniques that involve the use of pull-up or pull-down methods, both for single-ended and differential lines.

Now let's look at exactly what happens to a signal in cases of series and parallel termination.

Series Termination

On a bus that uses fast I/Os (such as push-pull), series termination is most commonly used. The reason for the use of series termination is to ensure the voltage seen at the load end of a trace has the correct value and does not create overshoot/undershoot after reflection. The overshoot/undershoot is prevented by suppressing reflection off the unterminated load; this works because the series resistor absorbs the signal portion that is reflected from the load end.

To see how this works, take a look at the image below. The fast I/O shown in this image has a low output impedance. Terminating this I/O to the trace means placing a series resistor right at the driver pin such that the equation below is satisfied.

Digital buffers, such as a push-pull buffer used in CMOS ICs, excite a signal by essentially charging and discharging the capacitances in the buffer. The capacitance combined with the on-die resistance in the buffer channel, the logic propagation delay, and trace impedance effectively create a complex RC circuit. So if we add a series resistor, it lengthens the rise time for the output signal.

The other effect of series resistance is to act like a voltage divider on the output. Without the series resistor, The IO is attempting to source a voltage amplitude of V. With the series resistor, there is now a resistor divider, and the output voltage is:

Voltage that is put onto the transmission line with a series resistance R.

When the series resistance and trace impedance are equal, the output is half of what the buffer is attempting to source, or V/2.

In these buses, the load is essentially just a capacitance, so it has high impedance up to high frequencies. This means it reflects the signal coming to the load, and the reflected and incoming signals superimpose on each other. This means the voltage at the receiver is:

Voltage that is put onto the transmission line with a series resistance R.

It is when the series resistor is too large or too small that we get undershoot or overshoot, respectively. The reflection off of the receiver is needed because it sets the signal level to the desired value of V. To see this in action, just note that ZL is a very large number up until the load capacitance dominates the load impedance.

This is why we don't also apply parallel termination at the receiver when the V(load) and V need to be equal. We also do not want re-reflection off of the driver, which is why the series resistor must match the trace impedance.

Parallel Termination

Instead of applying series termination at the driver, we can also apply parallel termination at the receiver. Parallel termination provides a different function, which is to completely suppress a reflection. In this case, we want all of the voltage that is sourced at the drive end of the interconnect to be absorbed by the parallel resistor.

The parallel resistor is in parallel with the load’s input capacitance, so the load impedance is essentially equal to the parallel resistance (Rp), and the received signal amplitude is:

Voltage that is put onto the transmission line with a series resistance R.

It’s quite easy to see that when Rp = Z0, the load absorbs all of the signal. Now because all of the voltage has been absorbed by the parallel resistor, the load voltage equals exactly the signal that is put onto the transmission line (assuming no losses). The reason this works is because the buffer has a low impedance output in the ideal case.

In reality, the source side of the trace could have any source impedance. What matters is that the voltage placed onto the trace (Vsignal) is totally absorbed at the load via its parallel resistor. Because the parallel resistor totally absorbs the incoming signal, there is no reflection and it does not matter if the source impedance does not match the trace impedance.

Differential Termination

Differential termination relies on the exact same concepts listed above. When the differential signal is being injected into the transmission line, each individual signal level is set by matching on the transmission line. Normally series is not used when DC coupling is needed, or if there is a series resistor it is built into the semiconductor die. If termination is needed, typically only parallel termination is used at the load with Rp = Z(differential).

When to Use Any of These Methods

In summary, we can use the series method in these instances:

• The signal rise time needs to be slowed down

• The aren't worried about the backwards-traveling reflection off of the load end of the trace

Series termination provides lower power consumption from the output buffer, and this is one reason why there is always a little bit of series resistance in outputs from I/Os. For parallel termination, we care about the following instances:

• Series resistance would slow down the signal too much and create a timing violation

• It is desirable to avoid the backwards traveling wave, which might create additional crosstalk

• We aren’t worried about the power consumption in the parallel resistor

There are also alternatives to parallel termination, such as Thevenin termination or pull-up termination as mentioned in the above table. Both of these set the matching impedance and adjust the received signal level so that reflection is suppressed from the load.

These two methods are not normally used with discrete components. Instead, if they are used, they are probably built into the die for your load component.

In standardized interfaces, you don't need to pick a method, the method is already standardized and is most likely built on to the die of the component. The exception is with some components like line drivers, certy's, multiplexers, or even connectors, which may not obey any standard and could accept a range of voltages.If you're unsure, make sure to check the standards or data sheet for your component.

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