DC terminated fail-safe resistor value calculations

AN1690

8/15

Equation 1

Then

Equation 2

●

The calculated values for Ra and Rc could be slightly decreased to provide a

Vid >200 mV, and to meet the worst case power supply and resistor tolerance

conditions. Then Ra and Rc could be 500

Ω. However the value of Ra and Rc should

not be reduced too low in order to minimize the driver loading when the driver is active.

An active driver is required to create a minimum of 1.5 V across the cable termination.

The use of low resistance pull resistors makes this voltage more difficult to meet.

Zo

Rb

Ra

Rc

+

()

||

=

Rb

131

Ω

=