If you design PCBs, you have probably seen a small resistor placed in series on some signal traces. That inconspicuous component helps control the signal and prevent it from misbehaving. Technically, it provides damping and assists impedance matching to reduce signal reflections.
Data lines between a CPU and DDR memory are a common example. Each line often includes a series resistor. The resistor helps the signal behave and reduces reflections that cause overshoot and ringing. The same structure appears on other point-to-point high-speed nets when the driver is stronger than the line it feeds.
What the Series Resistor Is Doing
A high-speed driver launches a voltage step onto a PCB trace. Once the edge is fast enough that the trace behaves as a transmission line, the step travels toward the receiver and reflects wherever the impedance changes. An open or high-impedance receiver, a connector, or a mismatch between driver strength and trace impedance all send part of that step back toward the source.
The returning wave adds to the forward wave. At the receiver that addition shows up as overshoot, undershoot, and ringing. Overshoot can stress input structures and reduce noise margin. Ringing can cross the receiver threshold more than once and close the sampling window. A series resistor does not remove the transmission line. It raises the source impedance so less of the incident step is launched into a mismatch, and it damps the energy that does reflect back to the driver.
The useful condition is simple. The sum of the transmitter's source resistance and the series resistor should equal or approximate the transmission-line impedance. For a 50 Ω line, that sum should be near 50 Ω. The external resistor is only the adjustable part of that sum. The driver already has some output resistance, so the board resistor is usually smaller than the line impedance.
A Simple Simulation of the Effect
To make the effect clear, a simulation was set up. The transmission-line impedance was set to 50 Ω, and the transmitter and receiver were modeled as 1.8 V high-speed drivers and receivers to mimic a realistic signal path. Six series-resistor values were tested: 0 Ω, 10 Ω, 20 Ω, 30 Ω, 40 Ω, and 50 Ω. The comparison is the impact of each value on reflections at the receiver.
The results show a clear progression, not a case in which any added resistor is an improvement.
- 0 Ω, no series resistor: the signal drives too hard and exhibits severe overshoot.
- 10 Ω and 20 Ω: overshoot is progressively reduced and the waveform looks cleaner.
- 30 Ω: the waveform is the most stable, with minimal overshoot and good edge behavior.
- 40 Ω and 50 Ω: the signal rise becomes slower and undershoot appears, which is undesirable.
The 0 Ω case is the unmatched driver. Source impedance is well below the 50 Ω line, so the launched step is large relative to what the line and the receiver can absorb. The reflected wave returns and stacks on the edge. That is the severe overshoot in the simulation.
At 10 Ω and 20 Ω the source is closer to the line, but not yet matched. Overshoot falls because less energy is reflected, and the remaining reflection is partly absorbed when it returns to the driver. The edge is still fast enough to look clean.
At 30 Ω the combination of driver resistance and series resistor is closest to the 50 Ω line in this model. Minimal overshoot and a well-behaved edge are the signature of that match. This 30 Ω result belongs to this simulation. It is not a universal DDR resistor value.
At 40 Ω and 50 Ω the source impedance has gone past the useful match for this driver. The line is over-damped. Rise time stretches, and undershoot appears. A slower edge can miss timing even though the ringing has been suppressed. Matching is a balance between reflection control and edge rate, not a contest to add as much resistance as possible.
How to Choose the Series Resistor
The series resistor should be neither as small as possible nor as large as possible. The key is for the sum of the transmitter's source resistance and the series resistor to equal or approximate the transmission-line impedance—for example, 50 Ω—to suppress reflections.
In practice, designers often start with 22–30 Ω. That range assumes a driver whose own output resistance is already a meaningful fraction of a 50 Ω line, which is why the external part is not itself 50 Ω. The exact value should be validated with simulation, or adjusted during board bring-up and tuning, until the signal meets the required margins.
Simulation should use the actual driver strength, the trace impedance, and the receiver model for that net. A 30 Ω result on a 1.8 V, 50 Ω path does not transfer unchanged to a different I/O voltage, a 40 Ω DDR trace, or a driver with a different output impedance. Bring-up tuning checks the same waveform on the finished board, where package, via, and connector discontinuities are present and the simulation model is no longer the whole channel.
Too small a resistor leaves the overshoot seen at 0–20 Ω in the simulation. Too large a resistor produces the slow edge and undershoot seen at 40–50 Ω. The starting range of 22–30 Ω is a place to begin that comparison, not a value to copy onto every net.
Why Some Modern DDR Designs Omit the External Resistor
Many modern DDR designs use on-die termination (ODT), which integrates termination resistors inside the memory or controller and allows them to be adjusted. ODT typically applies to data lines. Address, control, and clock lines may still require external termination or series resistors to avoid reflections.
ODT changes where the match is made. Instead of relying only on a series resistor at the driver, the receiver presents a controlled termination that absorbs the incident wave. Data lines, which are often bidirectional, are the usual place for that on-die network because both ends can be configured. Address, control, and clock are commonly unidirectional and may still need an external series resistor or an external termination if the driver and the line are not already matched.
Absence of a series resistor on a DDR data net is therefore not evidence that reflections no longer matter. It usually means the match has moved on-die. Nets that do not have ODT still need the external treatment.
Place the Resistor at the Driver
Place any series resistor as close to the transmitter as possible. If it is placed too far from the source, it will not effectively improve the signal.
The resistor is part of the source impedance. It has to sit where the wave is launched. A long trace between the driver pin and the resistor is itself a transmission-line segment with the driver's native impedance. Reflections form on that segment before the resistor can damp them. The resistor then sees a waveform that has already rung, and it cannot repair the launch.
Keeping the part next to the driver pin makes the source look like driver resistance plus series resistance for the rest of the route. That is the condition the 0–50 Ω comparison was built on. A 30 Ω resistor at the far end of a long unmatched stub is not the same circuit as a 30 Ω resistor at the pin.
Series damping is a small layout choice with a direct effect on overshoot, ringing, and edge rate. The simulation on a 50 Ω, 1.8 V path shows why the value has a useful middle: no resistor drives too hard, 30 Ω is the most stable case in that model, and 40–50 Ω slows the edge and introduces undershoot. Start near 22–30 Ω, confirm against the real driver and line, keep the part at the transmitter, and do not assume ODT has removed the need for external series resistance on every DDR net.
