Digital Signal Processing Reference
In-Depth Information
v
D
+
v
D
−
v
diff
=
v
D
+
−
v
D
−
1.0
0.6
0.8
0.4
0.6
0.2
0.4
0.0
0.2
0.2
0.0
−
0.4
−
0.2
−
0.4
−
0.6
0.0
1.0
2.0
3.0
4.0
5.0
0.0
1.0
2.0
3.0
4.0
5.0
Time, ns
Time, ns
(b)
(a)
Figure 7-4
Example of how common-mode noise is eliminated with differential signal-
ing: (a) single-ended waveforms at each leg of a differential receiver showing common-
mode noise; (b) differential waveform.
7.2 DIFFERENTIAL CROSSTALK
Although crosstalk noise on a differential pair has a significant common-mode
component, it also has a differential component because the distance between an
aggressor and each side of the pair differs (in some cases, if the aggressor is
on another layer, the crosstalk could be 100% common mode, but this is rare).
Consequently, each leg will experience slightly different crosstalk, which will not
be rejected by the differential amplifier. Nonetheless, under certain conditions,
differential signaling can reduce crosstalk significantly.
If the crosstalk between single-ended lines is compared to the crosstalk
between differential pairs, the differential crosstalk will be less if the spacing is
similar. The drawback is that the differential pair will occupy significantly more
board area. Additionally, if the single-ended pairs are spaced far enough apart
so that the same board area as the differential pairs is occupied, the single-ended
crosstalk is typically lower simply because the signals are spaced so far apart.
For example, consider Figure 7-5, which shows three cases:
1. Two single-ended coupled transmission lines spaced 10 mils apart
(Figure 7-5a).
2. Two differential pairs with an interpair spacing of 10 mils (Figure 7-5b).
Note that the spacing between pairs is identical to the spacing between
single-ended signals in case 1.
3. Two single-ended coupled transmission lines spaced 28 mils apart
(Figure 7-5c), which occupies the same board area as the differential pairs
of case 2.
Figure 7-6 shows the far end crosstalk for each case when the aggressors
are driven with 100-ps-wide pulses and line voltages of 0.5 V. The differential
crosstalk is calculated as the difference between
V
+
and
V
−
:
v
differential
=
(V
+
)
−
(V
−
)
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