Signal Integrity TUTORIAL

Differential Signalling

A differential signaling system uses two dedicated wires to transmits two different voltages which are compared at the receiver. The transmitter injects a small current, eg, 3.5 mA, into one wire or the other, depending on the logic level to be sent. The current passes through a resistor of about 100 O (matched to the characteristic impedance of the cable) at the receiving end, then returns in the opposite direction along the other wire.

The differential signalling is different from Single ended signalling in the return current path. In the single ended signalling the current returns through the ground path.

Differential signalling offers many advantages.

Immunity from Ground Noise

The fist advantage is the immunity from the noise in the ground plane. 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.

Immunity to Crosstalk / Electtromagnetic interference

If there is an external electromagnetic noise trying to enter the Differential Bus, it induces the same amount of noise on both the signals ( assuming they are close enough, which they usually are). At the receiving end, the receiver sees the difference of the signal and rejects what we call common mode noise.

Power saving by use of small voltage

The differential signalling uses very small amount of voltage which saves power.

High Data Speed Possible

Very high data rate is possible by use of Differential signalling. Currently over 5 Gbits per second can be transmitted using differential signalling.

FOr PCB Designer

PCB Designer must know which traces are differential signals. Some of the clock nets in today's design are differential signals. You must be aware of the LVDS signals, Hypertransport signals, USB signals, Ethernet signals - which employ differential signalling.

For differential signalling a PCB designer must be aware of few things - the first is that the length of the two lines must match EXACTLY. The difference is the lengths of the two signals are usually required to less that 50 mils. For higher speeds ( > 1Gb/s) the requirement is even more stric.

The second thing a PCB designer must be aware is that the gap between the + and - terminals of the signals must be as small as possible

The differential signals have a differential characteristic impedance. The PCB designer must ensure that its characteristic impedance has the desired value.

In the next page we will see what factor effect the differential impedance and how to calculate it.

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