Contents
How do you find differential pairs?
The differential impedance value that is returned from most calculators is equal to the sum of the impedance from each trace (including contributions from coupling). Taking this value and dividing by 2 gives you the odd-mode impedance value of each trace.
What is a differential line?
Transmission lines Differential signaling is used with a balanced pair of conductors. For short cables and low frequencies, the two methods are equivalent, so cheap single-ended circuits with a common ground can be used with cheap cables. As signaling speeds become faster, wires begin to behave as transmission lines.
What is single-ended voltage?
A single-ended measurement is taken as the voltage difference between a wire and ground. The noise is only on the positive wire, and as a result, it is still measured along with the output voltage from the sensor. A sensor with a differential output can be wired for single-ended by wiring the low side to ground.
What are the different types of trace impedances?
The upper cluster of lines are microstrip (outer layer trace, ground on one side) impedances, varied by constant H=W (Height=Width, or d=A) values in mils. The lower cluster shows stripline (inner layer trace, grounds on both sides, hence two heights “H”) impedances, again with a constant A/d ratio.
When does odd mode impedance equal se impedance?
Odd-mode impedance of the loosely coupled pair equals the characteristic impedance of the SE trace. But if you route a pair of traces with close coupling, the odd-mode impedance is less than the SE impedance for the same trace width (unless you adjust the line width).
How does PCB trace impedance change over time?
As serial links became the high-speed interconnect of choice, PCB traces became predominately “differential”. In practice, signal connections now required two routes instead of one; and that changes the impedance situation a bit.
How is trace impedance related to dielectric constants?
Spanning the range of common dielectric constants (on the X axis), we note that trace impedance (on the Y axis) decreases ~linearly. The upper cluster of lines are microstrip (outer layer trace, ground on one side) impedances, varied by constant H=W (Height=Width, or d=A) values in mils.