If you're new to analog circuit design, you are going to come across terms in technical articles or from coworkers that may hit you somewhere between “I think I've heard of that” and “WTF is this guy talking about?” For the newbies this can be daunting. Note that newbies could mean that you're right out of college or that you've just been moved into new job responsibilities where the boss says you'll be doing some analog circuit design.
I want to make sure that you're not left feeling the way you did when you started a new engineering class and realized you were already two semesters behind. I won't go into a ton of detail on this blog, partially because this has been covered in detail before and partially because we will likely cover it in detail again later.
So let's just take a quick look at some items:
You'll see lots of discussion on ground, grounds, and grounding. “Ground” is a term used to refer to a common circuit connection or bus that runs to many circuit nodes (connections to multiple components). It properly means a connection tied to the earth; but in usage, only occasionally means that. It primarily means a circuit-common node at the lowest potential or closest to zero volts DC and AC. By extension, it means a node to which circulating currents flow on their way back to a voltage or current source. (See (Under)standing Your Ground.)
What makes this one tough to understand is that there are often multiple grounds or circuit commons in a system. There may be analog ground; so the voltage or current source mentioned in the paragraph above is probably an analog signal source or possibly the ±15 Vdc rails. There may be a digital ground and the associated logic signal sources. Parsing more finely, there could be a ground for the +5 Vdc rail, the 3.3 Vdc rail, the 1.8 Vdc, 0.9 Vdc, and so on. There are likely many bypass capacitors from many supply rails tied to these many grounds. There are, as a consequence, many paths for the resulting circulating currents. And the grounds all get tied together somewhere. The devil is in the details with that last bit.
Signals (both digital and analog) going from point A to point B (perhaps within the confines of a circuit board or perhaps traveling several feet or many miles) could be “single-ended.” Or they could be differential (balanced, balanced line, differential signaling; twisted pair). Differential signaling on a twisted pair of wires has the advantage of giving the circuitry at the receiving end the ability to ignore or reject electrical noise induced onto the pair.
The bypass capacitors mentioned above are another source of trouble waiting to happen. Besides the ground connection issues, there is the capacitor's none-ideal behavior. The dielectric material (the stuff between the two capacitor plates) is not perfect. It's supposed to insulate and help hold the charge, but in actually has some leakage resistance, and its dielectric characteristics change with the frequency of the applied voltage. And the plates themselves are not perfect conductors. Instead, they have some parasitic resistance and inductance. But otherwise, they work perfectly.
OK, so no easy answers on these issues just now — I just want to get folks thinking more about the problems. Then I'll tackle some of these issues in small, easy-to-handle bites in future blogs. Post comments or questions to help keep this discussion going.