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I know the basic function of resistors and capacitors. I know filters, oscillators, one-shot, differentiator/integrator, ADC/DAC, power supplies, etc. But in a motherboard why are there so many external resistors and capacitors? How many of these analog circuits are there? Or does it have something to do with the copper transmission lines?

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  • \$\begingroup\$ Pull ups and pull downs. Try googling them. \$\endgroup\$ – Andy aka Apr 2 at 14:16
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    \$\begingroup\$ ... and filter capacitors, bypass capacitors, switching voltage regulator feedback resistors etc etc etc. \$\endgroup\$ – Michael Karas Apr 2 at 14:20
  • \$\begingroup\$ How are you telling the difference between SMD resistors and caps anyways? Are you just including resistors because you cannot tell the difference? Because most are probably decoupling caps, by far. I would expect most resistors to be termination resistors...though I would expect these to be arrays or integrated which you probably would not recognize as discretes. \$\endgroup\$ – DKNguyen Apr 2 at 14:24
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    \$\begingroup\$ @DKNguyen - Arrays are rarely used on high density / high frequency applications like motherboard for computers and servers. Main reason is that arrays twist the layout where individual components can go closer to where needed and not add extra length to critical traces. Another is component value optimization for signal integrity tuning if needed. An array pretty much means a group of values are the same where optimization may require one to be different. Lastly I have studied this for high volume manufacturing and the costs of arrays are little different from the cost of (continued) \$\endgroup\$ – Michael Karas Apr 2 at 14:29
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    \$\begingroup\$ (continued from above) the equivalent discrete components. The big factories place more emphasis on reducing BOM line count than total BOM item count. \$\endgroup\$ – Michael Karas Apr 2 at 14:31
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The vast majority of caps should be decoupling caps. Termination resistors should account for many (most? but am hesitant to say so) of the resistors.

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  • \$\begingroup\$ Is there any rule on how you should place these discrete smd components? Such as to not bring about interference or any other unwanted effects that will cause the circuit to behave in an unpredictable manner. \$\endgroup\$ – Joey Apr 2 at 16:38
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    \$\begingroup\$ @Joey There is but its broader than what I think you have in mind. For general circuit design you should get Henry Ott's book Electromagnetic Compatibility. It's really good. His website under the "Tech Tips" is also really good but not as complete as the book. As you get into RF, analog, and transmission line stuff (like really high speed digital switching that requires terminations) you have to start just following the physics. \$\endgroup\$ – DKNguyen Apr 2 at 17:21
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They provide filtering, DC smoothing, adjustment (voltage, current, frequency of an IC), pull-ups and pull-downs, impedance matching, terminations, current limiting, DC blocking, basically all the functions that are included as options for ICs, that can't be placed inside ICs or that can be done cheaper or occupy less space than an additional IC.

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