On many PCB designs, a resistor is placed in series with a signal net to control the waveform. In high-speed digital interfaces, this is commonly used for source-end impedance matching, with the goal of reducing signal reflection and improving signal quality.
A typical example is the data connection between a CPU and DDR devices, where a series resistor may be placed on each data line. The resistor changes the effective source impedance seen by the transmission line, helping control the edge and reduce ringing caused by impedance mismatch.
Figure 1 | Series resistors on CPU to DDR data lines
The effect of a series resistor is easier to understand through simulation because the resistor value directly changes the shape of the transmitted and reflected waveforms. An unsuitable value can leave significant overshoot or undershoot, while an appropriate value can produce a more stable transition without unnecessarily slowing the signal edge.
Build the Simulation Model
First, build a basic transmission-line model for the signal path. Set the transmission-line impedance to 50 Ω and replace the transmitter and receiver with 1.8 V high-speed models. This creates a simulation setup that represents a high-speed signaling case in which the source, transmission line, and receiver interact with one another.
Figure 2 | Simulation topology with a 50 Ω transmission line
Figure 3 | 1.8 V high-speed transmitter model
When a high-speed edge enters a transmission line, the signal does not simply travel from the driver to the receiver as an ideal voltage step. If the source impedance and transmission-line impedance are not well matched, part of the traveling wave is reflected. The reflected energy can return toward the driver and interact with subsequent portions of the waveform, producing ringing, overshoot, or undershoot.
A series resistor modifies the impedance seen at the source. When its value is selected appropriately, the combined output resistance of the driver and the resistor can approach the characteristic impedance of the transmission line. This reduces the magnitude of the initial reflection and helps the waveform settle more cleanly.
Different Resistor Values Change the Waveform Sharply
To examine the effect of resistor value, six values can be swept in the simulation: 0 Ω, 10 Ω, 20 Ω, 30 Ω, 40 Ω, and 50 Ω. The comparison makes the relationship between source resistance and waveform behavior clear.
Figure 4 | Series resistor values used in the reflection sweep
With no series resistor, the 0 Ω case produces an aggressive edge and severe overshoot. The low source impedance allows a strong initial wave to enter the transmission line, while the impedance mismatch causes more reflected energy to affect the waveform.
At 10 Ω and 20 Ω, the overshoot becomes smaller and the waveform improves. The additional series resistance reduces the effective drive strength seen by the transmission line and helps control the transition.
At 30 Ω, the waveform is the most stable among the simulated values. The result provides a useful balance between suppressing overshoot and maintaining a sufficiently fast edge. In this particular simulation, the 30 Ω value gives the best signal quality of the tested set.
When the resistance increases to 40 Ω and 50 Ω, the transition becomes slower and undershoot appears. Excessive series resistance can therefore reduce the aggressiveness of the edge too much. The objective is not to maximize the resistor value, but to find a value that provides appropriate source termination for the specific signal path.
Figure 5 | Overshoot and undershoot versus series resistor value
Why Series Resistance Changes Signal Reflection
The basic mechanism is the relationship between source impedance and transmission-line impedance. When a high-speed driver launches a signal into a transmission line, an impedance discontinuity causes part of the signal energy to reflect. The greater the mismatch, the more strongly the waveform can be disturbed.
A series resistor increases the effective source impedance of the driver. If the driver output resistance plus the series resistance approaches the characteristic impedance of the transmission line, the source becomes better matched to the line. The returning reflection is then better absorbed at the source instead of being strongly reflected again.
This is why the resistor can reduce ringing without changing the logic function of the signal. Its primary role is waveform control rather than DC voltage regulation. The selected value affects the trade-off between reflection suppression and edge speed.
A value that is too small may leave significant overshoot and ringing because the source remains poorly matched. A value that is too large can slow the rising edge and introduce other waveform degradation, such as undershoot in the simulated case. The useful range therefore depends on the driver characteristics and the transmission-line conditions rather than on the resistor value alone.
How to Choose the Series Resistor
A larger resistor is not automatically better, and a smaller resistor is not automatically better. The target is to make the combined source impedance equal to or approach the transmission-line impedance. In the simulation described above, the transmission line is 50 Ω, so the driver output resistance and series resistor together should approach 50 Ω.
This relationship also explains why the same resistor value should not automatically be copied into every high-speed PCB design. Different drivers have different output characteristics, and the actual interconnect can differ in impedance and topology. A resistor value that produces a clean waveform in one design may not provide the same result in another.
For the specific case discussed here, 22-30 Ω can be used as an initial range for evaluation. The final value should be confirmed through simulation, or different values can be evaluated during hardware bring-up until the waveform meets the required signal-quality condition.
Simulation is particularly useful because it allows several resistor values to be compared under the same conditions. Instead of treating a series resistor as a fixed rule, the designer can observe how the edge rate, overshoot, undershoot, and overall waveform change as the resistance varies.
Why Many Current DDR Designs Have No External Series Resistor
Some current DDR devices use on-die termination (ODT), which places the termination function inside the device and allows it to be programmed. For DDR data lines, ODT can reduce the need for a discrete external series resistor because part of the termination behavior is handled by the device itself.
However, the absence of an external resistor on the data lines does not mean that every DDR signal is automatically matched. Address, control, and clock nets can still experience reflection when their source and interconnect conditions are not appropriately matched. These signal groups therefore need to be evaluated according to their own topology and electrical behavior.
The termination strategy should also be considered together with PCB layout. A series resistor works at the source, so its physical location matters. The resistor should be placed as close to the driver as possible. If there is a long trace between the driver and the resistor, that section of trace remains part of the unmatched source path, reducing the effectiveness of the intended source termination.
PCB Placement of the Series Resistor
For a source-terminated high-speed signal, the basic connection is driver, series resistor, transmission line, and receiver. Keeping the resistor close to the driver makes the added resistance part of the source impedance from the beginning of the signal path.
This placement also helps keep the section between the driver and the termination point short. Once the signal enters the main transmission line, the source termination is already present and can interact with the line impedance as intended.
The resistor value and placement therefore need to be considered together. Selecting an appropriate resistance but placing it far from the driver can produce a different electrical structure from the one evaluated in simulation. For this reason, the simulation model should represent the intended PCB topology rather than treating the resistor as an ideal component with no physical location.
Series Resistors Are a Signal-Integrity Tool
A series resistor on a PCB signal trace is a simple component with a specific signal-integrity function. By increasing the effective source impedance, it can reduce the impedance mismatch between a driver and the transmission line, suppress reflection, and improve waveform stability.
The simulation results show why resistor selection requires balance. In the tested 50 Ω transmission-line case, 0 Ω produces excessive overshoot, moderate resistance improves the waveform, 30 Ω provides the most stable result among the tested values, and higher resistance begins to slow the edge and introduce undershoot. The correct value is therefore determined by the complete signal path rather than by the resistor alone.
For high-speed PCB design, the practical approach is to evaluate the driver characteristics, transmission-line impedance, resistor value, and physical placement as one system. Simulation can then be used to identify a suitable source-termination value before the design is finalized.
