How Close is Close Enough?
Let's say you can't fit your series termination in the ideal location, next to the driver. There isn't room. You have to place it a little further away than you'd like. Will it still work?
A series termination resistor is supposed to absorb high-frequency energy, damping reflections on the net. To perform at its best, it must be directly connected to a very low impedance source, presumably your driver. Anything placed in series with the termination resistor changes its value, making it less effective. That includes the short PCB trace, or connection stub, that hooks the driver to the termination resistor. Applications that need very accurate termination (like clock lines) should take this effect into account. Fortunately, we can easily calculate the degradation due to a connection stub. As long as the stub delay is less than 1/3 of the signal risetime, the approximations given below will be accurate to within ±25 percent.
The connection stub, because it is connected at one end to a low-impedance driver, acts like a little inductor LSTUB. This stub inductance acts in series with the termination resistor, adding to the impedance of the termination. If you add the impedance of the stub, (j2πf)LSTUB, to the resistor value, RT, you get a reasonable model for the combined termination impedance. Any step waveform hitting this type of termination impedance will generate a short reflected pulse, even if the value of RT exactly matches the characteristic impedance of the line. This first reflected pulse amplitude will be approximately 1/2(VSTEP)(LSTUB/Z0)(1/TR), where VSTEP is the step amplitude, Z0 is the transmission line impedance and TR is the 10-90% risetime of the step waveform.
That's the theory, except for these embellishments:
- The stub inductance may be calculated as LSTUB = DLY×Z0, where DLY is the delay of the stub in seconds and Z0 is the stub impedance.
- Add to the stub inductance the parasitic series inductance of the driver package, LPACKAGE.
- The stub affects the risetime of the first incident waveform by a tiny amount. Keep the stub delay less than 1/3 of the risetime and you will hardly see this effect. (Thanks to Tom Giovannini and Joe Cahill for reminding me to mention this).
Example: BGA package, LPACKAGE = 6000 pH, with an ideal 70-Ω series terminator located 1/2 inch (microstrip trace) from the driver package. Assume we have a 3.3-v driver with a 1-ns risetime. In this case:
LSTUB= 1/2(145 ps/in)(70 Ω ) = 5075 pH
The total inductance:
L TOTAL = LPACKAGE + LSTUB = 11075 nH
The reflected signal:
Refl. = 1/2(3.3) [(11075/70) / 1000 ] = 261mV
The series termination resistor, even though it had an ideal value, failed to completely damp the reflections in this example. The implication is that we may need to wait at least one roundtrip time for the ringing to decay before sampling the signal.
Pay close attention to the length of your connection stub. Stub delays less than 1/3 of the signal risetime create residual reflections that can be approximated by this simple lumped-element model. Stub delays in excess of 1/3 of the risetime of the driver can create significant resonances that grow rapidly with increasing trace length. Don't stretch your luck. If you want 20dB or more of reflected-wave attenuation, use a stub delay of no more than 1/6 the risetime, a very good low-inductance package, and an accurate carbon-composition or low-inductance, non-etched metal film resistor.