We have talked about a whole slew of logic and interconnect technologies including TTL, CMOS and assorted low voltage versions. All of these technologies have in common the fact that they are single-ended, i.e. the signal is measured as a “high” or “low” level above ground.
This is great for simple uses. But when you start talking about speed, distance, or both, the single ended solutions don’t look so good. To step in and carry the torch we have Differential Signalling. This is the “DS” in LVDS, just one of the common standards throughout industry. Let’s take a look at how differential signaling is different from single ended, and what that means for engineers and for users.
Single Ended
Collectively, standards like TTL, CMOS, and LVTTL are known as Single Ended technologies and they have in common some undesirable attributes, namely that ground noise directly affects the noise margin (the budget for how much noise is tolerable) as well as any induced noise measured to ground directly adds to the overall noise as well.
By making the voltage swing to greater voltages we can make the noise look smaller in proportion but at the expense of speed as it takes more time to make larger voltage swings, especially with the kind of capacitance and inductance we sometimes see.
Differential
Enter Differential Signaling where we use two conductor instead of one. A differential transmitter produces an inverted version of the signal and a non-inverted version and we measure the desired signal strictly between the two instead of to ground. Now ground noise doesn’t count (mostly) and noise induced onto both signal lines gets canceled as we only amplify the difference between the two, we do not amplify anything that is in common such as the noise.
Continue reading “When Difference Matters: Differential Signaling”
It’s been said that hackers are enamored with complex networks. In the 60s and 70s, the telephone network was the biggest around, singing a siren song to an entire generation of blue-boxing phone phreaks. I started a bit closer to the house. As a child I was fascinated by the heating system in the basement of our home: a network of pipes with a giant boiler in the middle. It knew when to come on to provide heat, and when to kick on for hot water. I spent hours charting the piping and electrical inputs and outputs, trying to understand how everything worked. My parents still tell stories of how I would ask to inspect the neighbors heating systems. I even pestered the maintenance staff at my nursery school until they finally took me down to see the monstrous steam boiler which kept the building warm.
