# When should I use impedence matching resistors in fast signals?

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.

• Always, unless you understand when not – 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

• 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? – Singee Nov 2 '16 at 10:39
• 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 – 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.