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TS8308500
2193A–BDC–04/03
Three possible line driving and back-termination scenarios are proposed
(assuming V
PLUSD = 0V):
1.
75
impedance transmission lines, 75 differentially terminated (Figure 33):
Each output voltage varies between -1V and -1.42V (respectively +1.4V and +1V), leading
to ±0.41V = 0.825V in differential, around -1.21V (respectively +1.21V) common mode for
V
PLUSD = 0V (respectively 2.4V)
2.
50
impedance transmission lines, 50 differentially terminated (Figure 34):
Each output voltage varies between -1.02V and -1.35V (respectively +1.38V and +1.05V),
leading to ±0.33V = 660 mV in differential, around -1.18V (respectively +1.21V) common
mode for V
PLUSD = 0V (respectively 2.4V)
3.
75
impedance open transmission lines (Figure 35):
Each output voltage varies between -1.6V and -0.8V (respectively +0.8V and +1.6V),
which are true ECL levels, leading to ±0.8V = 1.6V in differential, around -1.2V (respec-
tively +1.2V) common mode for VPLUSD = 0V (respectively 2.4V)
Therefore, it is possible to directly drive high input impedance storing registers, without
terminating the 75
transmission lines.
In the time domain, that means that the incident wave will reflect at the 75
transmission
line output and travel back to the generator (i.e.: the 75
data output buffer). As the buffer
output impedance is 75
, no back reflection will occur.
Note:
This is no longer true if a 50
transmission line is used, as the latter is not matching the buffer
75
output impedance.
Each differential output termination length must be kept identical.
It is recommended to decouple the midpoint of the differential termination with a 10 nF capaci-
tor to avoid common mode perturbation in case of slight mismatch in the differential output line
lengths.
Too large mismatches (keep < a few mm) in the differential line lengths will lead to switching
currents flowing into the decoupling capacitor leading to switching ground noise.
The differential output voltage levels (75
or 50 termination) are not ECL standard voltage
levels, however, it is possible to drive standard logic ECL circuitry like the ECLinPS logic line
from Motorola.