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No most of the people tell to provide reference plane. why is important question.. Differential signaling is a method of transmitting information electrically with two complementary signals sent on two paired wires, called a differential pair. Since external interference tend to affect both wires together, and information is sent only by the difference between the wires, the technique improves resistance to electromagnetic noise compared with use of only one wire and an un-paired reference (ground). The technique can be used for both analog signaling, as in balanced audio, and digital signaling, as in RS-422, RS-485, Ethernet over twisted pair, PCI Express, DisplayPort, HDMI and USB. The opposite technique is called single-ended signaling. At the end of the connection, the receiving device reads the difference between the two signals. Since the receiver ignores the wires' voltages with respect to ground, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal.(with the help of Wiki).
Hi barry thanks for the replay, can you please explain me how it effect controlled impedance and i think there will be no return path on reference plane for differential signals and i am working on RS485 and USB interfaces.
For RS-485 you won't need a plane because the speeds are relatively slow. The whole point of impedance matching is to minimize reflections, which can lead to ringing, etc. But at RS-485 speeds, any ringing will have disappeared long before the signal is sampled (assuming it's sampled in the middle of the bit period).
The effect of a ground plane for differential pairs can be analyzed as common mode impedance. Some IO-standards like USB require a specific common mode impedance that can't be implemented without a ground plane or coplanar ground. If you don't rely on a specific common mode impedance, you don't necessarily need a ground plane/coplanar ground, but...
Higher succeptibility to interferences has been already mentioned.
Other disadavantages of omitting a ground are
- stronger coupling with signals in the vicinity
- the range of other objects affecting the differential pair impedance extents considerably because the electrical field is widespread
- higher radiation losses