AN1690

AC terminated fail-safe resistor

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AC terminated fail-safe resistor

The DC termination (with and without fail-safe biasing) increases power consumption due to

the current flow through the termination resistors. In order to reduce the current absorbtion,

the fail-safe network could be modified as shown in Figure 8.

Figure 8.

AC termination with external fail-safe

The RC termination blocks DC current. The value of Ra and Rc can be increased, but not so

much that noise immunity is made worse.

Although Rb always equals the cable’s characteristic impedance (Zo), the choice of C

requires some judgement. Large C values provide good terminations by allowing any signal

to see an Rb that matches Zo, but large values also increase the driver’s peak output current

and the time constant RC, therefore decreasing signal quality.