Over the years, signal integrity concerns have moved from RF and esoteric high-speed designs into the mainstream. Rare is the project these days that doesn’t have to consider transmission lines and controlled impedanceThe total opposition that a circuit offers to the flow of alternating current or any other varying current at a particular frequencyThe number of complete cycles or vibrations per unit of time. Rate of alternation in an AC current. Expressed in cycles per second or hertz (Hz).. It is a combination of resistance R and reactance X, measured in ohms. PCBs.
Hit the Web, and, as with most other topics, you can find yourself absorbed for days researching transmission lines and terminations of all sorts. Here is just a tiny sampling:
Endpoint distortion, by the good Doctor Johnson
Practical hints from Texas Instruments
By far, the most commonly encountered termination type is series, where a driver’s output impedanceThe total opposition that a circuit offers to the flow of alternating current or any other varying current at a particular frequencyThe number of complete cycles or vibrations per unit of time. Rate of alternation in an AC current. Expressed in cycles per second or hertz (Hz).. It is a combination of resistance R and reactance X, measured in ohms. is matched to the transmission line (PCB trace or cable), either by virtue of its inherent output characteristics or by the addition of a resistor placed near the driver output. The load at the other end of the line is assumed to be an open circuit, but, in reality, there will be some capacitanceThat property of a system of conductors and dielectrics which permits the storage of electricity when potential difference exists between the conductors. Its value is expressed as the ratio of a quantity of electricity to a potential difference. A capacitance value is always positive. and possibly some input current (bear in mind I’m thinking of digital design; RF inputs are usually 50Ω). The line takes a full round trip to settle, and you can’t generally have more than one load unless they’re grouped at the end. However, the load sees a clean signal, and the termination doesn’t dissipate any excess power.
