Differemtial Signaling
8 Differential SignalingA single ended signaling is one in which the signal flows in one wire from the driver to the receiver and return through the ground path.
A differential signaling on the other hand involves two wires between the transmitter and the receiver. The signal travels down one wire and returns through the other wire. There is no ground path for the return of the signal.
One obvious disadvantage of differential signaling is that it needs two wires in place of one wire. This requires increased board area. For low signal speeds, therefore, the differential signaling offers no particular advantage over the single ended signaling. The differential signaling is however, capable of transmitting signals at many times the rate of single ended signals and therefore, the number of traces required to transfer a given high data rate is actually lower in case of differential signaling.
8.1 Advantages of Differential Signaling
8.2 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.
8.3 Immunity to Crosstalk / Electromagnetic 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. We assume that the aggressor is at a substantially far distance from the differential bus, as compared to the separation between the differential pair.
8.4 Power saving by use of small voltage
The differential signaling uses very small amount of voltage which saves power. If +v is the voltage impressed upon one wire and –v is the voltage impressed on the other wire, the difference in the voltage seen by the receiving input stage of the amplifier is given by
(+v) – (- v) = 2v
Small voltage level is possible because the receiver only needs to see the difference in the voltage level between the + and – lines.
8.5 High Data Speed Possible
Very high data rate is possible by use of differential signaling. Currently over 5 Gbits per second can be transmitted using differential signaling.
8.6 Rules for Differential Signal Routing
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, PCI Express signals- which employ differential signaling.
There are few simple rules that must be followed in the differential trace routing.
1. The length of the two lines of a pair must match. 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 strict.
2. If your differential signal uses DC blocking capacitor, us as small capacitor as possible and keep the capacitor pad small.
3. If the density of the board is high use tightly coupled signals. Calculate the impedance of the tightly coupled signals.
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