Vendor provided schematics without reference designators

Question

Sometimes, we get questions about schematics from vendor application notes, datasheets, and such. You'd think that simply clipping the schematic from the appnote and inserting it as an image'd do, right?

Well, some vendors (older LTC and NatSemi application schematics are notorious for this, but I suspect a few others do it as well) don't use a full system of reference designators on their appnote schematics -- I suspect they were drafted using regular CAD tooling or such instead of an EDA tool, and that this was also intentional to avoid confusing people who refdes differently. However, they sometimes leave a few reference designators in and/or use nonstandard reference designators as well (Linear likes to call a comparator IC 'C1' sometimes, for instance, but will call a capacitor 'C1' in other schematics of theirs, and will even designate a combination of parts with a single reference designator. See below for an example (from LTC AN43) of the latter two).

Should we insist on making people either redraw a vendor circuit in CircuitLab (with a rather high potential for lossage due to CircuitLab being missing funny symbols and the likes) or scribble on the clipped image with an image editor to "pencil in" their own system of reference designators (with potential for confusion due to having 1.5 reference designator systems on 1 schematic)? Or should we try to "talk around" the infelicity of not having reference designators instead of trying to make the OP clean up vendor divots (which can be a PITA if you're on mobile)?

Answer 1

It isn't really surprising that appnotes don't put refdes on every component; they really only need them on the ones they're going to talk about in the text.

There's nothing wrong with adding them using an image editor, but "scribbling" them with the freehand tool isn't the way to do it. Most image editors (and PDF readers) can just as easily place text elements, with a choice of fonts, colors and sizes. Obviously, any added refdes need to be checked so that they don't collide with any existing ones.

Answer 2

You are getting hung up on the source of the schematic. That is irrelevant to us. We care that schematics be properly done, not how that is achieved. Since we are here to talk about the circuit, schematics in questions here must have component designators.

In those cases where you are asking about a circuit in a app note, and that circuit doesn't have component designators, the you have to do more work than just copying the schematic from the app note. In this case, the easiest way to proceed is probably to add your own component designators over the existing graphics. This question is a good example of that. Since that question has three close votes, here is a copy of the schematic with designators added by the OP:

The red designators were added by the OP, and made it possible to talk about the circuit. This example is acceptable, but just copying the original schematic would not have been.

In this case I thought using a different color was also useful. Note that the left opamp was designated C1 in the original schematic, but nothing else had designators. The OP used C1 for a capacitor, then designated the opamp IC1. That worked in this case since we could just go with the red designators for all components.

A added nice touch would have been to blot out the original "C1", or even just to cross it out. In other cases leaving original designators might cause more confusion than it did in this case.

Again, though, we don't care how you accomplish the end result, only that you do.