For those of you who do PCB design, you've probably seen resistors connected in series on some signal lines, right? What exactly does this inconspicuous little component do? Today, I'll explain it to you in the simplest terms, and you'll understand after reading!

Take a common example: the data lines connecting the CPU and DDR chips. Each line has a resistor connected in series. The core function of this resistor is to make the signal more "obedient," preventing it from wandering off—in professional terms, this is called "impedance matching," which avoids signal reflection.

Explaining the principles is too dry, so let's use simulation software to see the actual effect; it'll be crystal clear!

Build a simulation model

The first step is to build a link model. Set the impedance of the transmission line to the commonly used 50 ohms, and then replace the transmitting end (tx) and receiving end (rx) with a 1.8V high-speed model. This is necessary to simulate a real signal transmission scenario.

Different resistance values ​​produce vastly different results!

We selected six resistance values ​​for testing: 0 ohms, 10 ohms, 20 ohms, 30 ohms, 40 ohms, and 50 ohms, specifically to examine their impact on signal reflection.

The simulation results immediately revealed the differences:

• Without a series resistor (0 ohms), the signal surged too rapidly, exhibiting severe overshoot;
• Switching to 10 ohms and 20 ohms gradually reduced the overshoot, resulting in a much better waveform;
• At 30 ohms, the waveform was the most stable, with neither overshoot nor sluggishness, demonstrating the best signal quality;
• However, at 40 ohms and 50 ohms, the signal rise became slower, and undershoot occurred, making the results less than ideal.

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How to choose a resistor?

Remember, a larger or smaller resistor is not necessarily better! The key is to ensure that the "internal resistance of the transmitting end + the resistance of the series resistor" equals or is close to the impedance of the transmission line (e.g., 50 ohms as mentioned earlier). This will eliminate reflections.

In practical designs, it's generally advisable to start with 22-30 ohms. The specific value is best verified through simulation, or different resistor values ​​can be tried during later debugging until the signal meets the requirements.

Fun fact: Why don't modern DDR memory have this resistor anymore?

Modern DDR memory uses ODT (On-Demand Technology), which integrates the resistor into the chip, and it's even adjustable! However, it's important to note that ODT only applies to data lines. Address lines, control lines, and clock lines, if not properly handled, can still experience signal reflections.

Also, series resistors should be placed as close to the transmitting end as possible; placing them too far away will negate their signal-improving effect.