pucho812 said:
what is acceptable in terms of gear manufacturing when the design or part of the design is pulled right from the data sheet?
I'm talking what is cool to do, not cool to do, etc, etc, etc
It depends. At one point in my career I managed an engineering group, and the objective manager says don't re-invent any wheels that already roll (and rock). If the app note design is practical and cost effective, don't mess with changing it. The subjective "engineering" manager in me, always thinks any design worth doing is worth improving, while this doesn't mean just adding more expensive parts (I worked at Peavey and that was a sharp pencil crowd).
In some cases it depends on the app note. I seem to recall many app notes from larger manufacturers that had a tendency to load them up with glue parts made by them also, even if they weren't cost effective for the application, so be alert for that.
At a minimum manufacturers will often adapt a app note to use parts already in the manufacturers system to reduce the cost and overhead of bringing in otherwise similar new parts.
I have (almost) never used a design app without changing something (perhaps a personal affectation) but it is foolish to not dig deep enough to understand the design choices. Some things you just don't mess with. If the manufacturer says a so and so needs this much PS decoupling to be stable, you use that or more.
I recall one example where a well know kit designer (who shall remain nameless since he is still alive and well respected) just about copied a design application note verbatim for a kit he did way back when, but he did make one change substituting an opamp for a comparator to simplify his parts list. I can only speculate that he didn't fully appreciate the difference or reason a comparator was used in the original app note (to prevent input bias current from causing a DC error after a blocking capacitor). I guess he could have gotten away with the opamp swap if he used a bifet or JFET input opamp, but this was back in the old days when those were premium parts, so his design did not work as well as the app note.
For lots of simple stuff there just isn't much external glue to change ... For example I am using a slick class D driver chip in one new designs that just has a couple input resistors, and simple output filter (ferrite beads and caps). Not much for even me to mess with.
When in doubt use the app note as written. It should always be a good starting point. Some products like switching power supply controllers can include a preferred PCB layout. Deviate from that at your own risk. OTOH sometimes the app note is cheapened down for mass appeal. One example of that was one or more THAT corp VCA app notes. I recall reading the AES paper the VCA designer wrote with a detailed inspection of all the improvements in the latest generation of VCAs. If you paid attention to the journal paper you would notice that the circuits shown for the performance graphs used higher performance opamps than their application notes. So the app notes may not always be best but should at least deliver the promised performance level.
One last anecdote about using app notes verbatim, and I've told this story before so bear with me, when I did a kit CX record NR decoder, I not only changed CBS's app note to use a quieter gain element, but I discovered a mistake in their side chain time constant, so I made mine agree with the formal CX system specification, and the standard record encoder. After I advised CBS of the error in their application note circuit, I heard nothing back from them, but later learned from an engineer at the company who made the encoder that CBS changed the encoder and CX standard to agree with the mistake since there were already tens of thousands of decoders built by two different consumer product manufacturers who copied the app note without bothering to understand what the circuit was actually doing.
So there is your smoking gun that some manufacturers do indeed copy app notes literally... and some don't.
JR
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