Class2_10_11_12_Differential_Signaling

Class2_10_11_12_Differential_Signaling - Diffe ntial S re...

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12/4/2002   Differential Signaling Introduction Reading Chapter 6
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12/4/2002 Differential Signaling 2 Agenda Differential Signaling Definition Voltage Parameters Common mode parameters Differential mode parameters Current mode logic (CML) buffer Relate to parameters Timing parameters Clock recovery Embedded clock AC coupling Common mode response Issues with simulation 8B10B encoding DC balanced codes Duty Cycle distortion Cycle
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12/4/2002 Differential Signaling 3 Single Ended Signaling All electrical signal circuits require a loop or return path. Single ended signal subject several means of distortions and noise. Ground or reference may move due to switching currents (SSO noise). We touched on this in the ground conundrum class. A single ended receiver only cares about a voltage that is referenced to its own ground. Electromagnetic interference can impose voltage on a single ended signal. Signal passing from one board to another are subject to the local ground disturbance. We can counteract many of these effect by adding more ground. As frequencies increase beyond 1GHz, 80% of the signal will be lost.
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12/4/2002 Differential Signaling 4 Review of threshold sensitivity The wave is referenced to either Vcc or Vss. Consequently the effective DC value of the wave will be tied to one of these rails. The wave is attenuated around the effective DC component of the waveform, but the reference does not change accordingly. Hence the clock trigger point between various clock load points is very sensitive to distortion and attenuation. Tx Vss Vref Vss Rx2 Vref Long line Vss Rx1 Vref Short line
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12/4/2002 Differential Signaling 5 Differential Signaling Any signal can be considered a loop is completed by two wires. One of the “wires” in single ended signaling is the “ground plane” Differential signaling uses two conductors The transmitter translates the single input signal into a pair of outputs that are driven 180° out of phase. The receiver, a differential amplifier, recovers the signal as the difference in the voltages on the two lines. Advantages of differential signaling can be summed up as follows Differential Signaling is not sensitive to SSO noise. A differential receiver is tolerant of its ground moving around. If each “wire” of pair is on close proximity of one and other. electromagnetic interference imposes the same voltage on both signals. The difference cancels out the effect. Since the AC currents in the “wires” are equal but opposite and proximal, radiated EMI is reduced. Signals passing from one board to another are not subject to the local ground disturbances. As frequencies increase beyond 1GHz, up to 80% of the signal may be lost, but difference still crosses 0
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This note was uploaded on 08/23/2009 for the course ECLT 865 taught by Professor Yinchaochen during the Spring '03 term at South Carolina.

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Class2_10_11_12_Differential_Signaling - Diffe ntial S re...

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