Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2000-04-27
2001-12-04
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S331000
Reexamination Certificate
active
06326817
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a circuit for suppressing a common mode component in a signal from a communication bus, comprising:
a first supply terminal and a second supply terminal for the connection of a supply voltage,
a first bus terminal and a second bus terminal for the connection of respective wires of the communication bus,
a voltage divider connected between the first bus terminal and the second bus terminal,
a differential amplifier having inputs connected to taps of the voltage divider,
a first current conducting element for coupling a center tap of the voltage divider to the first supply terminal, and
a second current conducting element for coupling the center tap of the voltage divider to the second supply terminal.
Such a circuit is used in so-called Controller Area Network (CAN) bus systems, which are used inter alia in cars. Such a communication bus has transceivers (transmitter/receiver) connected to its first and its second bus terminal for the transmission and reception of information, which is transferred as a difference signal over the two wires of the communication bus. The two bus wires, which are commonly referred to as CANH and CANL, are often long and susceptible to high-frequency electromagnetic interference. Such interference has the same influence on both wires and consequently manifests itself as a common mode component in the difference signal.
In the customary circuit the voltage divider attenuates the signal voltages on the bus wires CANH and CANL in order to suppress the common mode component. By means of two current conducting elements, usually resistors, the center tap of the voltage divider is set to a fixed voltage, frequently half the supply voltage. The attenuated signal, to be processed by the differential amplifier, is available at the taps. As a result of the attenuation the useful difference signal is attenuated likewise. The choice of the attenuation factor is a compromise which depends on the common mode swing of the differential amplifier and the envisaged maximum common mode voltage. The detection of the useful difference signal becomes more inaccurate as the attenuation is higher.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a circuit for suppressing the common mode component having improved characteristics. To this end, the circuit of the type defined in the opening paragraph is characterized in that
the first current conducting element comprises a first pair of transistors of a first conductivity type arranged in parallel and having their control electrodes connected to further taps of the voltage divider, and
the second current conducting element comprises a second pair of transistors of a second conductivity type arranged in parallel and having their control electrodes connected to the further taps of the voltage divider.
Owing to these measures the common mode voltage variation on the center tap becomes less dependent on the common mode voltage variation on the bus wires CANH and CANL. As a result of this, the differential amplifier needs to handle a smaller common mode swing and can therefore be of simpler design. The attenuation factor chosen for the voltage divider can also be smaller, as a result of which the useful difference signal at the inputs of the differential amplifier is larger. This improves the accuracy and the speed of detection of the difference signal.
It is to be noted that FIG. 2 of the International Application WO 98/28886 shows a circuit for suppressing the common mode component, which also has a differential amplifier and two transistor pairs of opposite conductivity types arranged in parallel. However, this known amplifier does not have a voltage divider and the differential amplifier and the transistor pairs are connected differently.
REFERENCES:
patent: 4280221 (1981-07-01), Chun et al.
patent: 5179577 (1993-01-01), Ilyadis
patent: 5187448 (1993-02-01), Brooks et al.
patent: 5216674 (1993-06-01), Peter et al.
patent: 5357518 (1994-10-01), Peter
patent: 5448200 (1995-09-01), Fernadez et al.
patent: 5475323 (1995-12-01), Harris et al.
patent: 5572658 (1996-11-01), Mohr et al.
patent: 6037798 (2000-03-01), Hedberg
patent: 9828886A2 (1998-07-01), None
Boezen Hendrik
Van Den Heuvel Abraham Klaas
Biren Steven R.
Callahan Timothy P.
Nguyen Hai L.
U.S. Philips Corporation
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