In PCB / PWB boards we are following 50ohm impedance for single ended lines and 100ohm impedance for Differential lines? is it a standard value for all device input and output impedance? Each and every IC internal architecture done by millions of transistors so this impedance values determined by the IC pins connected to which pin of the transistors (BJT/FET)? Could you please help me to clarify this? thanks in advance.
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2This has been asked (and comprehensively answered) before [here](http://electronics.stackexchange.com/questions/68801/why-characteristic-impedance-must-be-50-ohms), and in many other sites on the internet, e.g. [why 50 ohms](http://www.microwaves101.com/encyclopedias/why-fifty-ohms) – Mark Ch Oct 12 '15 at 08:29
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It's a de facto standard, and it doesn't apply to everything. For example, the USB standard specifies 90 ohm differential impedance instead of 100. Maintaining the proper impedance is important for minimizing reflections at high frequencies, so controlled impedance connections are usually only required for high speed/high frequency signals such as RF and high speed serial.
alex.forencich
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The values in that range are achievable with reasonable dimensions and standard dielectric materials, and match non-printed conductors such as twisted wires (Ethernet cable) and coaxial cables. Usually values are in the range of 50 ohms (single-ended) to 120 ohms (differential) but old TV antennas used to use 300 ohm 'twin lead'. Coaxial cable is usually 50, 75 or 95 ohms.
The important thing is that the termination impedances agree with the cable impedances to minimize reflections.
Spehro Pefhany
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