Dumbest "equivalent schematic" from a datasheet - how many errors can you find? - Page 1
Forum section: electronics / general

author: magic
author_rank: Super Contributor
timestamp: March 26, 2022, 07:32:08 pm
content: Datasheet schematics happen (albeit not always) and so do small errors or sometimes deliberate omissions.But "small" apparently isn't enough for TI. Here's their (simplified) schematic of TS321, a single operational amplifier equivalent to half of LM358.One might think it should have been a job easy enough...

author: fourfathom
author_rank: Super Contributor
timestamp: March 26, 2022, 07:36:33 pm
content: (#1) PNP output transistor base-emitter short.(#2 Trivial) Missing Vcc connection dot.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 26, 2022, 09:02:05 pm
content: Ah well, shit happens.The not-so-good part is that it never got fixed (just checked). It's still in the current datasheet.

author: magic
author_rank: Super Contributor
timestamp: March 26, 2022, 09:36:36 pm
content: I did take it from the current datash*tBut you aren't wrong, it goes back all the way to 2005.Still one blatant error to go. Hint:- are polarities of the inputs even right?- would it be stable without a whole bunch more compensation?OTOH, I just noticed that the connection of the current sink to output PNP base is not actually technically wrong, because the base is shorted to the output anyway. So perhaps one less than I thought

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 26, 2022, 09:39:08 pm
content: LM358 equivalent circuit for comparison.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 26, 2022, 09:41:28 pm
content: The ST schematic seems to have it right:https://www.st.com/resource/en/datasheet/ts321.pdfShame on TI.

author: srb1954
author_rank: Super Contributor
timestamp: March 26, 2022, 10:25:53 pm
content: Quote from: magic on March 26, 2022, 09:36:36 pmEven if the base emitter short was removed there is nothing actively driving the output PNP transistor. That transistor would just be permanently turned on by the current sink on its base and short the output to GND.

author: fourfathom
author_rank: Super Contributor
timestamp: March 27, 2022, 12:14:54 am
content: Quote from: magic on March 26, 2022, 09:36:36 pmThe + and - inputs are indeed swapped. (pretty blatant hint!)Compensation? That's above my pay grade.

author: tooki
author_rank: Super Contributor
timestamp: March 27, 2022, 12:15:52 am
content: So report the error to TI and see what they do. When I reported an error in the data on the website, they fixed it within days and wrote back to let me know. It’s eminently possible that nobody has ever reported the error.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2022, 12:48:57 am
content: Are you sure this isn't a comparator?Quote from: tooki on March 27, 2022, 12:15:52 amI've pressed the "report" button in datasheets many times and haven't seen anything change, that I know of... but there's always hope.Tim

author: mc172
author_rank: Frequent Contributor
timestamp: March 27, 2022, 01:13:04 am
content: Quote from: T3sl4co1l on March 27, 2022, 12:48:57 amI thought I was the only one!TI have got to be the worst culprit - I swear they use unpaid interns to write their datasheets. The last one I noticed had a suggested value of 2k in place of what was meant to be 2R in the application circuits section.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 27, 2022, 02:21:33 am
content: Doesn’t anyone senior proofreads a datasheet?

author: magic
author_rank: Super Contributor
timestamp: March 27, 2022, 06:13:19 am
content: Quote from: T3sl4co1l on March 27, 2022, 12:48:57 amOf course, high DC gain and positive feedback at AC, how could I miss itQuote from: schmitt trigger on March 27, 2022, 02:21:33 amThat's why it isn't right to report errors past a certain threshold of severity.The datasheet says as much about the manufacturer as about the product itself.Other customers deserve to knowBTW, check out the schematic in OP07 datasheet from the same guys.Been that way since 2014 apparently and no one cares.

author: dmills
author_rank: Super Contributor
timestamp: March 27, 2022, 04:43:57 pm
content: Given that TI appear to have forgotten how to make chips, there may well be nobody there who still understands how to read a schematic!I have a TI regulator in a design that only started working after I added two mysterious extra resistors found on the app note circuit (but nowhere on the datasheet!), most unimpressed.Where are the Pease/Williams/Widlar trained ones?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 28, 2022, 10:48:34 am
content: The AD8367 inverts the signal. There is no mention of this in the datasheethttps://www.ti.com.cn/cn/lit/ug/tiduc87a/tiduc87a.pdfFigure 3 not correct. Although the article below is correct. I reported the bug 3 years agohttps://e2e.ti.com/blogs_/b/industrial_strength/posts/a-modern-approach-to-solid-state-relay-design

author: Zipdox
author_rank: Regular Contributor
timestamp: March 28, 2022, 11:12:27 am
content: So.... is anyone gonna email them to have it fixed?

author: tooki
author_rank: Super Contributor
timestamp: March 28, 2022, 11:20:55 am
content: Quote from: MegaVolt on March 28, 2022, 10:48:34 am”Reported” by leaving the comment on the Web version, or reported by using the Submit Documention Feedback link inside the PDF?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 28, 2022, 11:30:59 am
content: Quote from: tooki on March 28, 2022, 11:20:55 amLeaving the comment on the Web version.

author: tooki
author_rank: Super Contributor
timestamp: March 28, 2022, 11:54:56 am
content: Quote from: MegaVolt on March 28, 2022, 11:30:59 amI wouldn’t expect that to be monitored, especially not for an error in the PDF. Use the Submit Documention Feedback link inside the PDF.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 28, 2022, 12:21:34 pm
content: Quote from: tooki on March 28, 2022, 11:54:56 amI took advantage of this option. Thanks, I didn't notice that link in the pdf.I don't know what to do with Analog Devices. Access to their site is closed for my country. Perhaps someone else will contact AD.

author: EPAIII
author_rank: Super Contributor
timestamp: March 29, 2022, 06:24:05 am
content: RE: "#2 Trivial"IMHO, there is nothing trivial about this.I have never liked the convention of drawing schematics without using the overpass style wire crossing where there is no connection. This NON-trivial error is the best argument for that dislike. And the fact that it has apparently survived over a long period speaks against any arguments that will be made against my dislike.Not only was it an easy error to make, not only does that error render the circuit non-functional if followed, but apparently no one sees any real need to correct it even when told about it.This style of schematic should not exist. It should have never existed. All existing such schematics should be properly updated with the overpasses ASAP by the original creators. And the original creators should not be paid a single penny for do this. And copies should be distributed by those creators IN PERSON, taking the time to insert them in place of the original, defective schematics in whatever form those originals may have taken.Rid the world of electronics of this scourge! NOW!Quote from: fourfathom on March 26, 2022, 07:36:33 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2022, 08:18:42 am
content: Meh, overpasses are old fashioned (and still subject to transcription errors all the same). Current best practice is to never use 4-way junctions, except for crossings.Which, the above circuit seems to take advantage of otherwise: the output is dog-legged from thecurrent senseB-E shorting wire, for instance. And then the capacitor and IN+ cross in an orderly fashion as we expect. But then Vcc goes into a damn current source, and all bets are off.(Another reason against overpasses is the implication of orientation, ordering, or z-height, an irrelevant detail -- a minor one to be sure, but it does technically add to the visual noise. Conversely, one might argue the 3-way junction rule, harnesses visual noise instead: such connections tend to be less symmetrical, arguably uglier -- which highlights their importance, in a way.)Tim

author: magic
author_rank: Super Contributor
timestamp: March 29, 2022, 08:30:06 am
content: Which is funny because TI is normally very religious about avoiding four-way junctions.I gather that they really meant to connect VCC to that damn current source rather than everything elseThey are similarly obsessed with never drawing current mirrors with a single line crossing through all bases (like everyone else) but snaking around those transistors and branching off to each base

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2022, 10:35:19 am
content: Quote from: magic on March 26, 2022, 09:36:36 pmI admit, the only error I saw was the inputs reversed. I can't believed I missed the missed the base-emitter short on the output transistor.Quote from: bsfeechannel on March 26, 2022, 09:39:08 pmThe drafting errors are more obvious and indeed understandable, after seeing the original, nut why didn't they simply copy and paste it? It's probably even the same die as the LM358, just with only one half connected.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 29, 2022, 11:17:33 am
content: Quote from: magic on March 26, 2022, 09:36:36 pmFascinating thread.My spidey sence makes me wonder if they might of done that to try and catch counterfeiters.

author: magic
author_rank: Super Contributor
timestamp: March 29, 2022, 04:08:19 pm
content: Quote from: Zero999 on March 29, 2022, 10:35:19 amWell, wrong transistor type was the first thing I noticed too and VCC the last.But for me it was familiarity with LM358 internals followed by "no way that they really changed it, no one ever got three stages compensated at low cost".Quote from: Zero999 on March 29, 2022, 10:35:19 amPart number sounds like it's second source of STM product. For whatever reasons (aka NIH), schematics of such things are usually redrawn, by all manufacturers. The poor intern tasked with this job got the STM schematic and a firm instruction to bring it to TI standards, including elimination of all 4-way junctions, which he didRegarding the internals, STM actually manufacturesa single channel version. There is also LM321 made by National Instruments and ON Semi, likely a similar thing.

author: fourfathom
author_rank: Super Contributor
timestamp: March 29, 2022, 06:01:30 pm
content: Quote from: magic on March 29, 2022, 08:30:06 amI am violently opposed to four-way schematic junctions, but they show up in so many places I was beginning to feel like I was out of touch. I even wrote a polite and well-reasoned (IMHO) letter to QST (the ham radio magazine) in which I suggested that 4-way junctions should not be used in their schematics. The letter wasn't published, and they still use the 4-way style.Hey, you kids get those 4-way junctions off my lawn!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2022, 12:54:25 am
content: Quote from: Zero999 on March 29, 2022, 10:35:19 amWell, C&P won't do for updating it to whatever the latest documenting style is. Or maybe they want to have a vector or EDA version of it, who knows.I suspect they don't have any standards as far as what schematics are drawn in, as long as they meet some minimum format and appearance standards. Possibly vector format is included in that. I've seen Altium, OrCAD, etc. in schematics, at least in the application sections / notes, so they're not picky about those at least. Probably block diagrams, inline figures, etc. are done in just, InkScape or any other variety of vector-format graphics tools. I've seen so many goofy, avoidable errors with shifted lines and inconsistent and dumb symbols, it can't be drawn with library symbols, at least it's not all the time.Here's a particularly dumb example:They also have a tendency (but, again, no consistency) to use the stupid emitter-arrow "MOS?" symbol, and I've seen it wired in reverse before (some LDO, don't remember offhand what). Which is exactly howI'dthink you'd wire such a bastard of a symbol, but nah, the convention is apparently analogous to an NPN and the "emitter" is supposed to be the source.And as you can see above, sometimes they draw an insulated gate, sometimes not. Hm, don't think I've seen -- wait wait wait hold up I've got another one.https://www.ti.com/lit/ds/symlink/ucc256304.pdfThe HV startup circuit, they consistently(!) refer to, throughout the datasheet (and its relative the -404), as using a "JFET". These are CMOS chips. There's no fukkin' way they put an HV JFET in there. The block diagram shows a MOSFET, as you'd expect. Is the block diagram actually lying then?Anyway, I was going to say, while they've sometimes removed the gate oxide, I don't think I've seen a gatejunction(arrow to/from substrate, i.e. a JFET) before, just, at the simplest, the IC version of symbol (symmetrical D/S, SS left off, gate circle/not indicates P/N channel). Without it actually being a JFET, I mean (and at that, mainly just among the classic op-amps; very little indeed uses JFETs anymore? at least on block level).Tim

author: magic
author_rank: Super Contributor
timestamp: March 30, 2022, 05:51:48 am
content: Of course vector formats are used, for inclusion in PDF datasheets if nothing else.TI has always had a consistent style and symbol set for transistor level schematics, so did others like National. I think I could recognize these two 90% of time if not more. Well, at least for so long as they published schematics at all, so up to 2000~2010 or so. Their contemporary "block diagrams" are becoming less consistent.This botched TS321 is obviously done in their standard style. Another example here; you just know it's TI.Quote from: T3sl4co1l on March 30, 2022, 12:54:25 amIt's a fairly widespread convention in CMOS IC design to draw things like below, by analogy to bipolar.The N-MOSFET symbol in your schematic looks like somebody got creative combining both approaches. Well, at least the body-channel arrow is shown correctly.