AN1690

DC terminated fail-safe resistor value calculations

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5

DC terminated fail-safe resistor value calculations

The external resistors are selected so that they provide at least a 200 mV bias across the

line, without excessively loading the active driver. In addition, some other conditions should

be met:

●

The pull-up (Ra) and pull-down (Rc) resistors should be of equal value in order to load

the driver outputs symmetrically.

●

Termination resistor (Rd) should match the characteristic impedance (Zo) of the line

cable, in order to avoid signal reflections.

●

At the other end of the cable, the equivalent resistance of Ra, Rb, and Rc should also

match the characteristic impedance of the line. In the following Figure 7, the equivalent

resistance is Rb II (Ra+Rc), which means Rb must be greater than Zo and Rd.

Figure 7.

External fail-safe and line DC termination resistors

The fail-safe bias Vid is the voltage drop across the line. Therefore, the fail-safe bias is

simply a voltage divider between Rb II Rd, Ra and Rc. Note that this formula neglects cable

resistance, and that Rb is parallel to Rd (Rb II Rd).

The choice of resistors must take into account other factors such as power supply voltage

tolerance and resistor tolerance, so that under worst case conditions, Vid is greater than

200 mV.

5.1

Example calculation

For this example, based on Figure 7, we assume that the cable has a characteristic

impedance Zo=120

Ωand that the power supply voltage Vcc is 5 V. We also assume that Rb

and Rd are equal and their value matches Zo (Rb=Rd=Zo=120

Ω).

●

Calculate the equivalent resistance of Rb II Rd. Rt = 120 II 120 = 60

Ω.

●

Calculate Ra and Rc for a Vid = 200 mV.

–

Vid = Vcc (Rt/(Rt+Ra+Rc)). Solving for Ra+Rc

–

Ra+Rc = ((Vcc)Rt/Vid)-Rt. Ra+Rc = ((5 V)60

Ω/0.2 V)-60 Ω = 1440 Ω.

–

Ra = Rc = 720

Ω

●

Recalculate the equivalent termination resistance at the end of the cable.

Req = Rb II (Ra+Rc). Rb = 120 II (720+720) = 110

Ω. This value is close (<10%) to the

characteristic impedance Zo. However Req could be matched to Zo by setting the

following equation: