Issue link: https://iconnect007.uberflip.com/i/1539283
SEPTEMBER 2025 I DESIGN007 MAGAZINE 27 • Short circuit: Now, imagine the signal sud- denly hits a direct short, like a wire touching another wire with virtually no resistance. Here, the reflection coefficient becomes negative one (-1). This means 100% of the signal volt- age bounces back, but it's inverted. It's like the signal falls into a black hole. Everything gets sucked away, and the voltage at that point drops to zero because the reflected wave perfectly cancels the incoming one. The Impact of Reflections on Signal Quality Reflections can significantly degrade signal quality, causing issues like overshoot, undershoot, and ring- ing. Consider a simple example: a signal propagating from a driver with 4Ω output impedance down a 50Ω transmission line to a high-impedance receiver. When the signal reaches the high-impedance receiver, most of it reflects back toward the driver. This reflected signal then encounters the low-impedance driver and reflects again, but with opposite polarity. This process continues, creating a series of reflections that appear as ringing on the signal waveform. For high-speed digital systems, these reflections can cause false triggering, reduced noise margins, and timing errors, ultimately leading to data corruption. Termination Techniques Proper termination is essential for minimizing reflections and maintaining signal integrity. Several termination strategies can be employed: 1. Series Termination Series termination involves placing a resistor at the source end of the transmission line. The value of this resistor, combined with the output impedance of the driver, should match the characteristic impedance of the transmission line. For example, if a driver has an output impedance of 4Ω and the transmission line has a characteristic impedance of 50Ω, a series termination resistor of approximately 46Ω would be appropriate. Series termination works by creating a voltage divider with the driver's output impedance, which reduces the initial signal amplitude. When the signal reaches the high-impedance receiver, it reflects back with a positive reflection coefficient. This reflection returns to the source and brings the signal at the receiver to its full amplitude. 2. Parallel Termination Parallel termination places a resistor at the receiving end of the transmission line, matching the line's characteristic impedance. This technique immediately absorbs the incident signal, preventing reflections from occurring. There are several variations of parallel termination: • DC parallel termination: A resistor connected to ground or to a DC voltage • AC parallel termination: A resistor in series with a capacitor, which blocks DC current while terminating high-frequency components • Thevenin termination: Two resistors forming a voltage divider to provide both termination and proper DC biasing Each approach has its advantages and disadvan- tages regarding power consumption, signal swing, and DC bias requirements. These same effects apply to differential nets, where the focus there shifts to differential impedance and differential signal termination.