MODELING AND BUDGETING OF TIMING JITTER AND NOISE - Advanced Signal Integrity for High-Speed Digital Design - page 577

TABLE 13-5. Noise Budget Analysis for Example 13-6
Source Single-Ended Differential

Supply noise (V) 0.250 0.003

Crosstalk (V) 0.200 0.020

ISI (V) 0.200 0.040

Rx offset + sensitivity (V) 0.045 0.045

Gaussian noise (V) 0.000 0.001

V ref noise (V) 0.140 —

Equalizer resolution (V) — 0.005

Total noise (V) 0.835 0.114

Signal swing (V) 2.000 0.400

Noise margin (V) 1.165 0.286

Noise immunity 2.4 : 1 3.5 : 1

where v NMl and v NMh are the noise margins for the low and high sides, v ol, max
and v oh, min the minimum and maximum transmitter output levels, and v ih, min and
v ih, max the minimum and maximum required levels at the receiver. Recognizing
that the difference between minimum and maximum output levels is equal to
the signal swing and that the difference between minimum and maximum input
levels at the receiver is equivalent to the worst-case noise, we can express the
system noise margin as

v NM = v swing − v noise (13-35)

Since the single-ended system swing is five times greater than for the differ-
ential system, it has a much higher noise margin (1165 versus 286 mV). From
these figures we might be tempted to conclude that the single-ended system
offers superior noise performance to the differential system. However, the noise
margin metric has a serious limitation; it does not account for the relative dif-
ference between the signal swing and the noise. Several of the biggest noise
sources (e.g., crosstalk, supply noise, ISI) increase in proportion to the signal
swing, while the low swing differential system typically reduces the nonpro-
portional sources (e.g., offset, sensitivity, common-mode rejection). This implies
that despite potentially higher noise margins, increasing signal swing does not
guarantee improved noise performance. An alternate metric, the noise immunit y,
addresses this limitation by taking the ratio of the signal swing to the worst-case
noise:

v swing
v noise

r NI = (13-36)

If we take the worst case noise to be a measure of the minimum signal swing
required for successful operation, the noise immunity expresses the ratio of the
actual signal to the required swing. The differential system has a noise immunity