EEVblog 1697 - What's Up With Different Brand Op-Amps? - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: July 19, 2025, 01:08:17 am
content: Why does this Uni-T UDP6731 PSU use different brand LM358 opamps on the same PCB?Links:http://www.artschip.com/pdf/LM358.pdfhttps://www.st.com/content/ccc/resource/technical/document/datasheet/61/46/87/01/98/ed/44/c5/CD00000464.pdf/files/CD00000464.pdf/jcr:content/translations/en.CD00000464.pdf

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2025, 02:38:48 am
content: I am sure Dave got it right for that Uni-T power supply; the exact manufacturer did not matter so they ordered the same nominal part from different manufacturers. This is a good design feature which make manufacturing easier.There sometimes is a reason to purchase a standard or jelly bean part from a single manufacturer however. Sometimes a designer has to rely on a part's specification which is not guaranteed in its datasheet, like absolute maximum rating, leakage, operating temperature, for Ft. In this case, a part from a specific manufacturer may be qualified for that application.For example if I needed a low leakage diode, I might qualify specific transistors from specific manufacturers as meeting my requirement, where the same part from a different manufacturer would fail. Another example would be the old 2N3055 transistor where the official specifications allow considerable variation in performance. At one point there were at least 3 different versions of the 2N3055 in production with significant differences in performance which could cause problems if they were swapped.

author: EEVblog
author_rank: Unknown
timestamp: July 19, 2025, 04:05:45 am
content: Quote from: David Hess on July 19, 2025, 02:38:48 amSure you can do that, but if you do then you are begging for a problem down the line.Let's say you get in a new purchasing person who knows that a 2N3055 is "jellybean" and should work and you'd BOM didn't specify an exact part in giant neon flashing lights.In this case it's way safer to spec in a specific part that isn't classed as jellybean.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2025, 05:44:07 am
content: The LM358 is a low grade jellybean part. The best feature is the price and initially it was one of the first single supply OP-amps.There are also higher grade jellybean parts, like the OP07 also found on the UNI-T board. It is the jellybean precision OP-amp, but there are still different grades. Often (likely with the UNI-T supply) the exact version does not matter, but in other cases one may want a specific manufacturer / variant with slightly tighter specs.The different versions of the 2N3055 are relatively well known and the difference is quite substancial ("old" RCA version is slower and better SOA). So it is a bit unfortunate that they got the same number. It may still be a possible source of error, either at the design step or at the purchase step.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2025, 12:38:28 pm
content: Quote from: EEVblog on July 19, 2025, 04:05:45 amSometimes there is no other choice. For instance what if I want diodes with a leakage of less than 0.1 picoamps? I know where I can buy 10 picoamp, and even 1 picoamp diodes, but not 0.1 picoamp diodes, even though they do exist.Designs which require selected or graded parts might have the virtue of being difficult for competitors to duplicate, making it a form of IP protection.QuoteUsing a better specified part is preferred, like the 2 MHz 2N3055G or 0.8 MHz 2N3772G from On Semiconductor in place of a random new production 2N3055 which could be as fast as 6 MHz. Besides speed, some of these parts have significant differences in safe operating area.But in the case of the 2N3055, companies like Tektronix did exactly what you describe. They were meticulous in their documentation about exactly where each variation of a part like the 2N3055 was sourced from, and some of their products used two or more variations of the same part. Looking through their parts catalog, I see 8 different sources for 2N3055s.

author: EEVblog
author_rank: Unknown
timestamp: July 20, 2025, 12:09:31 am
content: Quote from: David Hess on July 19, 2025, 12:38:28 pmThen you are no longer in the world of jellybean parts, you are in the hand selected parts business.99.9999% of desings that use the LM358 are going to be absolute jellybean.QuoteYes, that's what I said in the video, if your needs are precise then you have to be precise in your BOM, and also need to have full control over your purchasing and assembly chain.Using a classic jellybean part like the LM358 andnotbeing precise in your BOM indicates to purchasing people and even assembly people that "anything goes", just get an LM358 from whatever source you can.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2025, 05:13:15 am
content: Quote from: EEVblog on July 20, 2025, 12:09:31 amI am sorry if I was not clear. In a production environment I used to source 1 picofarad diodes from National 2N3904s. I qualified every lot for years by sample testing, but every part passed in production and there was never a failure. I am currently using Diotec BC337s from a single lot for 0.1 picoamps diodes, and again, they all pass. In my next parts order, I will get another lot of BC337s and reserve the ones I have now for use as low leakage diodes.I suspect Tektronix was qualifying manufacturers for the 741. They had specific internal part numbers for National and RCA parts, but that is the only criteria they list, so I suspect they qualified something special about those particular ones. I originally checked their documentation for the 324 and 358, but they had nothing like that for them. They did internal grading for a selected version of the 324 for input bias current, but like you say that is hand selection.They also had a selected version of the 741 for +/-20 volt supply voltages, but why didn't they buy the +/-22 volt version of the 741 instead? Maybe they were selecting high voltage parts out of the 36 volt 741C grade to save money? Was the price of the 741 and 741A that much higher in the past?Bob Pease discussed this sort of thing and warned that process or design for a qualified part could change without notification, and recommended keeping a reserve inventory of "good" parts in case an alternate supply had to be qualified. He also mentioned that some parts can be bad while meeting datasheet specifications; he used an example of fast diodes (1N4148s?) that had long forward recovery time which is something that I doubt anybody would expect, or even believe without seeing evidence.It is possible that there is something different about the Artschip and ST 358s, but I agree with you and doubt there is any difference that UNI-T is taking advantage of. Many years ago I qualified 1458s, a dual 741 class operational amplifier, because of a difference in recovery when one or the other operational amplifiers was driven into saturation; not all 1458s from different manufacturers behave the same under this condition. Some 358s, and especially 324s, can behave oddly when one operational amplifier is used as a comparator, and it varies between manufacturers and is rarely documented. I recall that the LT1013/1014 datasheet has a discussion about this, but maybe it was a different Linear Technology quad part.

author: EEVblog
author_rank: Unknown
timestamp: July 21, 2025, 12:40:01 am
content: Quote from: David Hess on July 20, 2025, 05:13:15 amYes, I've done this myself, but it's not really got anything to do with answering the jellybean question in this case.QuoteAgain, that's my point. In a PSU design like this, if you are datasheet selecting a specific brand jellybean LM358 in a basic PSU design like this, then you are doing it wrong. Especially when you have another brand LM358 on the board.You are just begging for trouble down the line.This absolutely be considered bad design practice for a consumer tech items like this.If you have to do this then just pick another opamp that matches your specs and then you have zero chance of anyone goofing it up.

author: temperance
author_rank: Super Contributor
timestamp: July 21, 2025, 02:28:24 am
content: For some LM358's, you can adjust the off set by applying some very light mechanical stress

author: magic
author_rank: Super Contributor
timestamp: July 21, 2025, 04:25:12 am
content: Artschip datasheet contains no hint whatsoever that this is anything other than a generic LM358, but FYI there are reports of LM358-branded chips from China which are faster than actual LM358 or made on CMOS process and completely different (rail to rail output for one thing, also negligible input bias as you would expect).Hence it is not entirely inconceivable that a Chinese board could work with one particular Chinese brand of seemingly LM358 and fail if that is replaced with a real LM358. Would somebody specify a CMOS LM358 rather than any other CMOS opamp, I don't know. But what's there to stop them?

author: EEVblog
author_rank: Unknown
timestamp: July 21, 2025, 04:47:29 am
content: Quote from: magic on July 21, 2025, 04:25:12 amYou could say that about almost any chip that's multiple branded really.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2025, 03:43:55 pm
content: Another time where I had to qualify parts was a military project, where not all of the parts were available as military versions with a military temperature range. During testing, I accidentally flooded the test chamber with liquid nitrogen and the board still worked just fine, which surprised me for a precision analog design.Quote from: temperance on July 21, 2025, 02:28:24 amI remember a story where that happened to National Semiconductor.Some manager questioned why their analog ICs had their own separate, and more expensive, packaging line. So he directed future analog chips to use their digital package line. About 6 months later, the manufacturing pipeline was that long, yields dropped at the testing stage so much that something had to be wrong. Analog packaging includes a bubble of encapsulation around the chip which reduces strain, which digital packing does not require, and that is why it was more expensive. The added strain from digital packaging ruined matching between transistors, which ruined analog yields.I guess National had performance contracts, so for almost a year they had to buy analog ICs from their competitors and remark or package them to fulfill their own contracts, until their analog IC pipeline could refill and start delivering parts again.

author: temperance
author_rank: Super Contributor
timestamp: July 22, 2025, 11:14:40 am
content: QuoteThe story. A small PCB's containing a circuit with an LM358 with a gain around 30 of which many failed during test. After some probing around, I first thought the scope probe was making bad contact because the offset of the op amp was all over the place and shifting up and down. Another probe, same problem... Then it must be something mechanical on the PCB.With the probes attached by means of some wires soldered onto the board, you could "bend" the offset out of spec by applying very light pressure onto the PCB. Those were TI parts. On Semi and ST parts worked fine.I'm still thinking to make a scale by placing an op amp in the centre of a long PCB... As a joke, "dual op amp scale".

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 22, 2025, 03:33:43 pm
content: Could be for mitigating a marginal design, as equivalent parts from different vendors can have slightly different characteristics.But the idea that it's only a matter of sourcing can be considered. Yes, "usually" reels are made of a single part from a single vendor, but that's not necessarily the case everywhere and especially in large chinese assembly houses with very high volumes. They often make their own reels to optimize sourcing and assembly lines throughput, so the possibility that a given part but from multiple vendors may end up on the same reel is quite real, unless the company ordering the assembly explicitely requests otherwise.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 23, 2025, 02:00:17 am
content: a while back I had seen a test on some NE5532 clones, phase margin, unity gain , common mode voltage input .... even noise .. all over the place for Chinese clones ... new TI was better then 80-x NS\TI , NJM was good ...i think not all apples are apples ...

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 23, 2025, 07:39:49 am
content: So Dave is yapping for 18 minutes to tell that they load multiple reels into the PnP to prevent it from running out of parts halfway a production run. Sounds plausible.Did I miss anything in the video, I just skimped through it.Offset changes due to to mechanical stress is a nice comment though. It's one unexpected way in which fake parts could be recognized...I also remember a story from long ago. It was about boards with TTL IC's which had some "analog" parts added, such as RC delays (I can't remember the exact circuit). Sometimes such circuits worked with IC from one manufacturer, but not with IC's from another manufacturer. But the root cause is that the IC's are used out of spec, and thus it's simply bad circuit design.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 30, 2025, 07:57:00 am
content: In:EEVblog #1057 - µCurrent Murphythere are some troubles due to a slightly different opamp in combination with a marginally stable design.

author: EEVblog
author_rank: Unknown
timestamp: July 30, 2025, 11:02:03 am
content: Quote from: Doctorandus_P on July 30, 2025, 07:57:00 amYes, I'm sure I mentioned that in the video.