Physical Layer - CAN System Engineering - page 66

and the core, and distances of coil start and end. These parasitic capacitances influ-
ence the attenuation curve and the position of the resonance maximum of the CMC.
The capacitance Cp represents the capacitances between coil input of coil 1 and
coil input of coil 2 as well as the capacitance between the outputs of both coils. The
capacitance depends on the distance between the ends of coil 1 to the ends of coil 2
and the distance between the coil wires to each other.
Hysteresis losses (~ f ) and eddy current losses (~ f 2 ) are represented by the high
ohmic resistance Rfe in parallel to the inductance L inside the equivalent circuit. In
case of a saturation of the core, this resistance will decrease and bridge the frequen-
cy-dependent resistances, and the attenuation would collapse.
The ohmic loads of the coil are frequency dependent caused by the skin effect.
However, these influences are neglectable for signal integrity analysis in the time
domain and, as a result of this the ohmic load, is implemented as the constant resis-
tance R CU in serial to the inductance L .

2.2.2.4 Cable Termination, Supporting Resistances

As already introduced in the former sections, there are several possibilities of termi-
nating a CAN network with the help of termination resistances. The simple termi-
nation and the split termination were introduced. In this section, further aspects for
network termination focused on the topology layout are considered.
In dependence of the topology, there are different aspects leading to an optimized
termination concept. One important point is that the overall line termination should
have a value of approximately 60 Ω. Another important point is that stubs with a
long length in comparison to the wavelength generate reflections; termination resis-
tances should be located at long stubs as well.
Therefore, the termination resistances should be placed in a star topology in the
two stubs with the longest distance to each other. Alternatively, it is possible in an
example with a star containing four short stubs and three long stubs to terminate
each of the three stubs with 180 Ω (180 Ω || 180 Ω || 180 Ω = 60 Ω) or to terminate
the star point with a single termination with 60 Ω.
It is possible to implement supporting resistances improving the EMC charac-
teristics and interference immunity, even in the case of no usage of a split termina-
tion. These supporting resistances are implemented between CAN_H and 5 V and
between CAN_L and ground (GND) to improve the symmetry characteristics and
to stabilize the average potential to 2.5 V. The resistances have a typical value of
1.3 kΩ.

2.2.2.5 ESD Protection

To protect the bus connecting circuitry against current pulses by ESD, ESD protec-
tion components are implemented. Modern transceivers already have internal ESD
protection measures; thus, the location of the ESD protection components should