Datasheet

SN65HVD20, SN65HVD21
SN65HVD22, SN65HVD23, SN65HVD24
www.ti.com SLLS552E –DECEMBER 2002–REVISED MAY 2010
NOISE CONSIDERATIONS FOR EQUALIZED RECEIVERS
The simplest way of overcoming the effects of cable losses is to increase the sensitivity of the receiver. If the
maximum attenuation of frequencies of interest is 20 dB, increasing the receiver gain by a factor of ten
compensates for the cable. However, this means that both signal and noise are amplified. Therefore, the receiver
with higher gain is more sensitive to noise and it is important to minimize differential noise coupling to the
equalized receiver.
Differential noise is crated when conducted or radiated noise energy generates more voltage on one line of the
differential pair than the other. For this to occur from conducted or electric far-field noise, the impedance to
ground of the lines must differ.
For noise frequency out to 50 MHz, the input traces can be treated as a lumped capacitance if the receiver is
approximately 10 inches or less from the connector. Therefore, matching impedance of the lines is accomplished
by matching the lumped capacitance of each.
The primary factors that affect the capacitance of a trace are in length, thickness, width, dielectric material,
distance from the signal return path, stray capacitance, and proximity to other conductors. It is difficult to match
each of the variables for each line of the differential pair exactly, but a reasonable effort to do so keeps the lines
balanced and less susceptible to differential noise coupling.
Another source of differential noise is from near-field coupling. In this situation, an assumption of equal
noise-source impedance cannot be made as in the far-field. Familiarly known as crosstalk, more energy from a
nearby signal is coupled to one line of the differential pair. Minimization of this differential noise is accomplished
by keeping the signal pair close together and physical separation from high-voltage, high-current, or
high-frequency signals.
In summary, follow these guidelines in board layout for keeping differential noise to a minimum.
• Keep the differential input traces short.
• Match the length, physical dimensions, and routing of each line of the pair.
• Keep the lines close together.
• Match components connected to each line.
• Separate the inputs from high-voltage, high-frequency, or high-current signals.
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