Symmetrically and Asymmetrically Operated Circuits

Symmetrically and Asymmetrically Operated Circuits

Symmetrical circuits The outgoing and return wires of the reference ground are separated in symmetrically operated circuits. The electrical circuit is connected with the reference ground with a third wire so that a symmetrical circuit makes up a three wire system. The useful signal flows to the device through the outgoing wire and back down the return. Many interferences can be reduced with a symmetrical connection which is also quite often the reason that they are used. Typical symmetrical circuit:  Connections in measurement systems between sensors and electronics  Connections for symmetrical data connections (RS422 / V.11)  Telephone connections between participants and the central exchange

Asymmetrical circuits In an asymmetrically operated circuit, the circuit is closed with the connection to the earth reference plane. The wanted signal flows to the device through a single wire and back down the earth reference plane. NOTE: All connections run through coax cable are asymmetrical connections.

Differential mode and common mode interference The useful signal is fed into the circuit in differential mode, i.e., the useful current flows in on the feed wire and out on the return wire or the earth reference plane. Interference can also be fed in as a differential mode signal. Interference can however also be fed in as a common mode signal. Common mode interference means that the interfering current flows in the same direction on both branches of the circuit and is returned on the earth reference plane. When the reference ground wire is not connected well, the interfering current caused by the common mode interference can be transmitted to other signal lines that are connected on the same device.