I've understood why impedence matching resistor terminations are usefull with fast signals in digital electronics, but I'm not sure when I should use them.

My question is, when should I, and really worth, add a termination resistor to a signal line?

I'm looking for a general tip, like: If signal is faster than 200Mhz and line path is longer that 1cm.

  • \$\begingroup\$ Always, unless you understand when not \$\endgroup\$ – PlasmaHH Oct 31 '16 at 14:58

When prop. delay exceeds rise time = transmission line

  • effects will occur in the form of LC resonant ringing and the amount of ringing depends on the source impedance matching.

  • Rise Time is ~ 0.35/f for 10-90%

  • speed ,v is 2x10^8 on FR4 with d=4 @1GHz [m/s]
  • Tp=v * x for x= path length
  • thus if path, x

    • x>20[cm/ns]/f[GHz] * 35%, you may have ringing without good termination and controlled impedances
    • most people add margin and use 10% of 20cm/ns
    • or >2mm per GHz or >20mm per 200MHz, return loss is affected
  • the same rule applies to Return Loss in Antenna >10% wavelength, although some calculate for 5% paths @ f.

For controlled impedances with a ground plane it is the track width/gap ratio that lowers Z. For co-planar it is also track width/gap ratio that lowers impedance. For track inductance it is Track length/width that determines nH/cm which is why ultra low ESL caps are 1x2 (LxW) for length between electrodes, L whereas typical caps like 603 805 1206 or 2x1(LxW) have higher ESL.

  • meanwhile \$ Z_o=\sqrt{L/C}\$ and velocity ratio \$v_c/c=1/\sqrt{LC}\$

for FR4 d=4.2 but @ 1GHz d~4 (depending on concentration of fiberglass) thus \$speed=c/\sqrt{d}\$ = c/2

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  • \$\begingroup\$ Thanks for your answer. Can I apply same considerations on connection length if on the signal path I can find 1 or 2 vias that make signal change lying plane? How this vias will affect signal line and impedence? \$\endgroup\$ – Singee Nov 2 '16 at 10:39
  • \$\begingroup\$ microvia nH values exist and multiple shunt vias reduces L.. as usual Length/Dia. ratio determines L and for any square , L is constant 1mmx1mm or 1mx1m.. Reduce dielectric thickness to reduce Zo, 50 Ohm has certain Thick/gnd gap ratio \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 2 '16 at 16:15

A general rule of thumb if the path is longer than 1/20 or 1/10 of a wavelength. Propagation speed is about 1/2 C on an FR-4 board.

See also all the answers to this question.

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  • \$\begingroup\$ Does the same rule of thumb apply, as to when to use controlled impedance traces. (microstrip or stripline) And finally for short traces do you keep the width as thin as possible to reduce capacitance? \$\endgroup\$ – George Herold Oct 31 '16 at 15:52
  • \$\begingroup\$ @GeorgeHerold I don't think it matters if it's controlled impedance or not if you don't terminate it. Narrower traces mean slightly faster propagation time but the difference between a 5mil trace and a 100 mil trace isn't much (10% or something like that). \$\endgroup\$ – Spehro Pefhany Oct 31 '16 at 15:59

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