Originally posted by: ahock
Hi Guys can somebody help me explain how diferential signalling works? This is the common interface nowadays. How the "EYE unit interval" carries an information of 1 and 0? For conventional single ended is a lot easier since it just outputs 1 and 0 direct.
Not too difficult a concept actually. I assume you understand that 0's and 1's are just represented by voltage levels.
In a single ended system, there is one signal line (plus a ground), where 0V usually represents a 0, while some voltage (in old systems 5V or 3.3V, in newer systems 2.5V or less) represents a 1.
In differential signalling, there are two signal lines (plus a ground), "hot" and "cold", that carry the complement of each other. For example, say you have a system that uses a "1" value of +5V and a "0" value of -5V. Therefore if you were trying to send a "1", the "hot" line would have a +5V signal on it, while the "cold" line would have the complement, or -5V, on it. If you were trying to send a "0", the situation would be reversed - the "hot" line would have the -5V signal, while the "cold" line would have the +5V signal.
A finer point: frequently it is necessary to go from differential to single-ended signals. In such a case, the receiver generally uses a subtractor to transform the signals: if you subtract cold from hot, in the case of a "1" in the above situation you would end up with a +10V signal, while in the case of a "0", you would end up with a 0V signal. You can then step up/down the signal as necessary...
The big deal about differential signaling is that it is very good at
rejecting noise, because any noise foreign to the system is likely to affect both signal lines the same (assuming the two lines travel closely aligned paths). Therefore, when you subtract the difference, you end up subtracting out most of the noise induced along the signalling lines. This is why microphones were one of the first uses of differential signaling: dynamic mics in particular put out very small signals (a few mV) that need to travel over long distances (as much as 100' or more) before reaching the first amplifier stage. Therefore it was critical to reject as much noise as possible. If you look at a microphone cable even today there are three pins: hot, cold, and ground.
Microphones, of course, are a low-bandwidth analog-domain problem. For high-bandwidth digital domain issues: Ethernet, SATA, USB, Firewire, etc. all use differential signaling to achieve high speeds with low-voltage signals.