I don't think it is productive to argue about this. Skin effect is more complicated than a resistance changing with frequency but don't want to go further down that road.joaquins said:
My concept for how deep to venture into each chapter would be limited by the reader's immediate need or ability to absorb more. Over the years I have found myself in situations where I needed a quick education in some narrow area of technology. Kids today don't know how good they have it, between google and wikipedia answers to almost anything are seconds away. I had to buy and read books.
ricardo said:
(Now, I hope that's a thoroughly true statement, and not another oversimplification of sorts.) So, impedance describes what happens to energy put into a circuit: how much energy gets burned off as heat, and how much gets temporarily stored, to re-emerge at a later time. Obviously, how long such energy is stored, and where and how it is released, can have complicated effects on AC voltages (for example, in the creation of an EQ's frequency curves).
ricardo said:
It depends on how good your rule of thumb is. ;Dleigh said:
Not so sure, but it does depend somewhat on the load... just like follow the money, follow the current. If a load is driven to ground, PS decoupling to ground is useful, if load is driven differentially, rail to rail caps could be more effective. In mass production we do not know in advance what the external loads will look like so have to cover all bases... while deep inside a larger product we generally understand the specific load path.
They should tell us if the op amps have unusual problems, but they should also engineer out unusual problems.
You made me tired just reading that...
JohnRoberts said:
JohnRoberts said:
A major factor is physical size. What is the largest electrolytic you can get 2 off close to the OPA(s)?leigh said:
The makers have to assume a certain level of those who read their datasheets. Scott Wurcer admitted to me that AD should have made a bigger fuss about decoupling for his AD797, one of the quietest & best OPAs in the known universe.
For stability, the length of the PCB tracks to the PSU have significant inductance. Even with no signal, oscillation can be triggered by inherent noise.
ricardo said:
leigh said:Right before they dive into that heavy math, they state that a capacitor driven by a sine-wave voltage source "behaves like a frequency-dependent resistance R = 1/ωC" – so I guess that's where I left it, conceptually.
Horowitz and Hill also state that:As you figured out, even from the best authors, simplification easily turns into over-simplification.
It is essential, at some time in your learning curve, that you fully grab the notion of current-phase vs. voltage.
But I would say that for understanding decoupling optimization, you don't need it. You just ned to know that capacitors are there to provide a low-impedance path for current variations. Typically, if a stage drives a 1kohm load, the capacitor's impedance must be an order of magnitude or two smaller than the load, AT THE LOWEST FREQUENCY OF INTEREST. That's basic math, and should not give you migraines.
The difficult part is managing the path of the currents so they don't interfere with the clean path of signal - including the "ground"! (Don't forget the "ground" is the return path of signal. As such it is as important as what people commonly refer to the "signal path", i.e. the connection(s) from the output of a stage to the input of the subsequent.)
That means you have to see how current is sucked from the power supply pins, and how the capacitors will provide a preferred path to a common point, also see how the output current goes to the load(s) and returns back to ground.
Ideally this should be a real single-point of minimal resistance. It is generally easy to put the decoupling capacitors close to the opamp but it is much more difficult, and takes some experience, to make the return currents go back to the same place without interfering with surrounding circuitry.
Let's say you have stage that drives a fader and several aux send pots:
You must connect their bottom point back to the ground where the decoupling caps are, via track(s) that is(are) not used by any other circuitry.
PCB softwares ignore completely this constraint and will gladly connect the reference ("ground") point of an EQ to the same ground than the fader send. That's why a good AUDIO designer will route the "grounds" manually according to the PCB topology, and run the autorouter subsequently.
The copper pour (ground plane), done without caution, may well ruin that plan; again, it takes some experience to create the necessary channels and islands in the copper pour that will make sure there's no undue short-circuit between "grounds".
leigh said:
It would count as driven differentially since the AC input current is matched by current supplied by the feedback resistor, which comes from the op amp output and ultimately the PS rails, so no current flows into ground for that example.leigh said:
Context.leigh said:
Before you do that, check that Trident don't already have a good Earthing System/Philosophy in place and that the Decoupling Electrolytics are indeed connected to a Dirty Earth.leigh said:
I've never found extra small caps in parallel with my beloved Electrolytics to improve or decrease performance FOR AUDIO either in theory or practice.
ricardo said:
