You can also reduce ringing by reducing the circuit??™s Q, which is a measurement
of the circuit??™s ability to resonate. To reduce the Q, decrease the wires??™ inductance
or increase the load??™s capacitance. To decrease the inductance, use larger
diameter wires or wires that are twisted more tightly. To increase capacitance,
use an AC termination as described below.
1
An AC, or active, termination can reduce power consumption of idle lines, and
may also reduce ringing voltages. However, this type of termination also
reduces the maximum possible cable length and bit rate. Figure 7-8 shows
examples.
In Figure 7-8A, a resistor and capacitor connect in series across the differential
lines. The capacitor prevents ringing by absorbing the high frequencies that
make up the ringing voltages. The capacitor also reduces power consumption
because the current on the lines is near zero when the capacitor has charged
after each transition. The added capacitance lowers the maximum bit rate and
cable length, so this termination is limited to shorter, low-speed links.
TIA-422 gives this formula to select the capacitor??™s value:
CT (pF) < 2 * (1-way cable delay (ps)) / (characteristic impedance (?„¦))
Assuming a propagation rate of 125 ps/in. and a characteristic impedance of
120?„¦, a 10-ft cable should use a capacitor of 250 pF or less. A 100-ft cable can
use values of up to 2500 pF.
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