Specifications and Design Tolerances - Page 1
Forum section: electronics / projects

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 07:22:43 pm
content: I was looking at a commercially produced design that is compensating for the limited drive voltage range near the rails of an op amp, by using a different value of resistance elsewhere in the circuit... instead of using an op amp that is rated to drive near the rails.The circuit actually does not call for an op amp. It's wired like a comparator. It is intended to output ±12V and nothing in between. This is supposed to be fed to the interface through a 1 kohm resistor. The circuit on the other side of the interface selects a resistor value to ground for this point to set a voltage at the interface. The voltage indicates state of a charging process and does not change rapidly. The loading resistor is through a silicon diode so that it only impacts the +12V part of the signal.This commercial design uses an LF353 op amp to generate the ±12V signal from ±12V power rails. The LF353 spec indicates an output drive ±12V into a 10 kohm load when the rails are ±15V. The circuit is within spec when tested, but it is relying on behavior of the chip that is not specified in the data sheet.The output voltage is a function of PVT (Process, Voltage and Temperature). You can test for Temperature and Voltage, but not Process. Now that I see how it was designed, I would never buy this product.In reality, using an op amp is not a good way to implement this circuit. I would use an analog switch to alternately connect the two rails to the 1 kohm resistor. That would match the requirements perfectly, without undue reliance on Process.I've been having a conversation with the guy who designed it and he is being a bit defensive. Maybe I was leaning on the design a bit hard, before I knew he designed it. He even played the, "we've made X number of these and they all work" card. We all know you can't prove a design works by testing.Am I being pedantic? Is "it works" an ok justification for using a part other than within its specification?

author: TimFox
author_rank: Super Contributor
timestamp: November 27, 2022, 07:24:57 pm
content: In that design, how critical is the +/- 12 V signal going into the next stage?Is it merely a binary signal (positive or negative), or does it need >+11.9 V to function?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2022, 08:22:43 pm
content: Show circuit, and surrounding stuff (what difference is the "12V" output to anything?).If you're saying it's using an op-amp as comparator, and comparator output as analog DAC, yeah that's probably pretty crummy. But if the voltage only needs to be say 8 to 10V +/-, i.e. the amplitude and symmetry can be out by 10% easily...... who cares?Tim

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 08:35:46 pm
content: Quote from: TimFox on November 27, 2022, 07:24:57 pmThe tolerance in the spec is ±5% for the open circuit voltage. The spec does not specify the thresholds, but there needs to be allowance for tolerance on the other side of the circuit and noise margin, as this is going over a cable of some feet, at least 25 ft.

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 08:42:32 pm
content: Quote from: T3sl4co1l on November 27, 2022, 08:22:43 pmYes, the op amp is being used as a comparator, but that's not so bad in this case. The issue is the output voltage of the op amp is very broadly specified and if you use that spec, there is no way you would expect this to work reliably.The guy has sold a large number of these, so the op amp clearly pulls relatively close to the rails. My concern is that, since it is not part of the spec, there's no reason to think they will never change the process, with a result of no longer working in the circuit, even if it still meets the data sheet spec.I've heard many stories of using a part in a way that isn't part of the spec, only to find, somewhere down the road, that an equivalent part was substituted (we are facing a shortage these days) or that the manufacturer tweaked their process, with the result that the design no longer works.In this case, an op amp is not really the right part, anyway. I would use an analog switch mux to connect the 1 kohm resistor to either the +12V rail or the -12V rail. No problem with the drive spec there!

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2022, 08:45:41 pm
content: Quote from: gnuarm on November 27, 2022, 07:22:43 pmLots of things are used outside the parameters of the data sheet and provided you go about it with a reasonable plan, that's OK. This may involve consulting the manufacturer or characterizing the parts yourself or some other plan depending on the issue.I disagree a bit with the idea that you can't prove a design works by testing, but I understand what you are getting at. A 'good' design might be considered to be one that is tolerant of of in-spec component variance. A 'bad' one is unnecessarily dependent on your 'process' variable. OK, but sometimes that leaves you without a solution, or at least the optimal solution you are looking for. So testing proves a design works with the specific components you are using and if there isn't a razor-edge stability problem, 100% testing may result in a perfectly reliable product. The guy who tries to copy it with generic components may be screwed.QuoteIt depends. If the design 'decision' was made out of sheer ignorance and not any real necessity or advantage or if it poses any real shortcoming in the operation, reliability or replicability with second-source components then yeah, I'd agree. But if there is a valid reason or the performance of the part is not critical, then I see no point in agonizing over a design that just isn't 'nice'. Especially if it works.I like 'nice', but sometimes good enough really is good enough. There, that last bit should be a good inspirational poster if I can just find the right image....

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 08:58:59 pm
content: Quote from: bdunham7 on November 27, 2022, 08:45:41 pmThe LF353 data sheet says the output will reach ±12V with a 10 kohm load and ±15V supplies. There's no way I would extrapolate that to drive +11.4V into a 1 kohm load with ±12V supplies.QuoteIn this case, there are many other solutions. The obvious one is picking an op amp that is rated to drive near the rails. Or, a better solution is to use an analog switch mux, which connects the 1 kohm output resistor to one or the other rails.You can't test against Process, only Voltage and Temperature.QuoteYou seem to have missed the part about testing not proving that it works. It doesn't, no matter what you may say about it. You can't control Process variations, and you can't test them.Quote"Good enough" and "within component specifications" are two different things. That's my point. This is a commercial product that is also provided as an open source project. I don't care for the fact that a crap design is being sold, but when it's built by the user as an open source project, that's caveat emptor. You get what you pay for. I just think the designer is being a bit pompous about the matter. This came up when he was explaining to an open source user, that the 1 kohm resistor, specifically called for in the relevant specification, should be modified by the output resistance of the op amp. Normally op amps are used closed loop to produce a controlled voltage. That's when I realized he was abusing his op amp.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2022, 09:39:21 pm
content: Quote from: gnuarm on November 27, 2022, 08:58:59 pmNo, I specifically mentioned that (TLDR=I understand but don't completely agree...). Perhaps you can't control process variations but you can look a bit deeper into the design of the component to get an idea of which issues are likely to arise.QuoteSo let's have a look. I put three excerpts from the datasheet below. I, like you, would think first of setting up the op-amp as closed loop so as to have a low-impedance output node. But is that necessary or appropriate in this case? He clearly is using as it a comparator, is that 'abusing' it or torturing the specifications? I'm not so sure.First, look at the block diagram. Yes, I know those are not exact models of the actual op-amp internal circuit, but you can see that there is 1 bipolar transistor plus ~200R between any rail and the output. As long as the circuit can saturate the appropriate transistor, I think the whole thing works including incorporating the 200R in the op amp into your 1K output requirement.Next ask if the appropriate transistor can be saturated reliably. The datasheet might indicate that it cannot, since the minimum output peak is 80% of the rail and the typical is 90%. I don't think this is a process-dependent issue at all, I think the answer is found under 7) Parameter Measurement Information. If that is the test circuit, then perhaps you can't saturate the output in a unity-gain configuration but you can in an open-loop circuit. I'll bet that is why it works properly and this would be a good example of a characteristic not found in the datasheet that can nonetheless be used perfectly reliably. I doubt that any reasonable process adjustment would change that characteristic. You could determine this by, as I said previously, consulting with the manufacturer, characterizing your own parts, guessing or perhaps just getting lucky.

author: TimFox
author_rank: Super Contributor
timestamp: November 27, 2022, 09:46:07 pm
content: Quote from: gnuarm on November 27, 2022, 08:35:46 pmI'm confused. Is this approximately +/- 12 V signal a continuous analog voltage, or a binary voltage?Obviously, the LF353 operating with 12 V rails cannot go quite to +/- 12 V, but I don't understand what the required input voltage to the next stage is.Equally obvious is that no normal op amp (or digital switch) operating from 12 V rails can go to (+12 V +5%) output, although the LF353 operating from 15 V rails can do that easily.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2022, 10:06:07 pm
content: I just dawned on me (I'm slow and low on caffeine) that this looks like the pilot signal from an SAE J1772 interface. Is it? If so--c'mon man--that will work just fine and it's no wonder he hasn't had any trouble. Not particularly well protected, perhaps, but perfectly reliable otherwise. Any possible issues with the pilot signal being a bit low are handled back in the EVSE itself anyway--the car isn't going to complain.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 27, 2022, 10:17:30 pm
content: With no more information than provided I vote for pedantic.I totally agree that in a design that is expected to have a large or long production run designing within the guaranteed performance requirements is generally good practice. But there are many reasons not to do it in specific instances. A few have been identified above. Consider also things like a one time buy at a fabulous price of the op amp in question. The purchase is expected to cover all future production requirements. Or a case where an uprated part costs dramatically more. And on and on.And finally consider that even designing within spec doesn't make your design totally safe. First, not all things about a device that may have an effect on operation are covered by data sheets. So future lots may or may not work for you. An example might be anti-static protection. Changes to this can generally be made without changing specified performance, but if your application depended on the older method you will suddenly start seeing failures. There are others. Large users who have long product lives demand and get long lead notice of process changes so they can plan for such discovery. If the cost of the investigation is high enough they do lifetime buys from the current process line.Bottom line, this product may represent an appropriate and thoughtful use of the components in question. Until you find the details of why it was done that way you won't know. And if the engineer currently in charge of the product is not the original designer you may never know.

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 11:31:29 pm
content: bdunham7, I don't agree with regards to your analysis. The values shown are likely typical, and we all know they can't control the actual value of resistances in a chip. So I would expect to see significant variation in the results over process.But that also ignores the issue of needing to be flexible in supply with today's market. Substitute another device and you have to go back to the lab to verify that it will work over voltage and temperature.If this circuit were designed correctly, you could find a replacement part, just by looking at a data sheet. I've done exactly this in the last year. The testing was only to verify that I had not missed something significant. Anyone can make mistakes. Unfortunately, I could not use the substitute in all the same sockets as the original, only half, but it got me through the production run of 8,000 units.I'm a design engineer and a business owner. I'm not in the business of second guessing the chip makers. If they can assure a spec on a part, then they will put it in the data sheet. If it ain't there, I'm not relying on it.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2022, 11:43:56 pm
content: Quote from: gnuarm on November 27, 2022, 11:31:29 pmThe ST version of the LF353 shows 300R output impedance. That might take some adjusting to. So what? That's just how the game is played, IMO. You can take the assured-building-block approach if you like, but that doesn't mean others can't be more imaginative and still make reliable products.Edit: I just saw this:QuoteWe do? How do they make an LM7805 then?

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 11:44:51 pm
content: Quote from: TimFox on November 27, 2022, 09:46:07 pmThe output of the op amp is intended to be either +12V or -12V. I don't know what you mean by a "continuous analog voltage".Initially the output is +12V through the 1 kohm resistor and there is no load on the output, so the pin is +12V. When connected to the car, a ~2.7 kohm resistor pulls (through a diode) the static output to about 9V. This signals to the EVSE that the car is attached. The EVSE then changes the +12V to a 1 kHz PWM signal between +12V and -12V, at a duty cycle which indicates the current available through the EVSE. There are further steps to this dance, but none of this is important to the issue with the op amp. The op amp's only job is to drive + or - 12V to the 1 kohm resistor.The op amp is simply a voltage source of ±12V. A more appropriate circuit would use a simple analog mux, connecting the 1 kohm resistor to the power rails, with virtually no dropping of the ±12V from the power rails. It works without having to qualify unspecified behavior of any parts.In reality, this circuit is a level shifter, that changes a +5V CMOS signal to a bipolar 12V signal. The LF353 op amp is not specified to do this job when powered from ±12V rails.

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 11:53:23 pm
content: Quote from: CatalinaWOW on November 27, 2022, 10:17:30 pmI think I've already indicated this is not a limited run item.QuoteZero evidence of any of this occurring. Even then I don't agree. If you are designing something to sell, it needs to work and without interfering with the production line. This is the sort of short cut (not even a short cut really, it's just a bad application of an op amp) that can cause trouble down the line for some other engineer to clean up when they have to change the part.QuoteNo one ever said this would prevent damage from asteroids or lightning strikes. But I control the things I can control and the things I'm expected to control as part of my job, such as the parts selected.QuoteIf a company fails to provide the ESD protection as stated in the data sheet, that's a quality control issue and the parts go back to them. That's not my problem.QuoteI've discussed this with the designer and he thinks I'm being pedantic, without justifying the design, other than saying the equivalent of, "It seems to work so far!"I have no patience for lazy engineers. Or stupid ones either. Some people don't understand that if you ignore problems long enough, they will bite you in the rear.

author: gnuarm
author_rank: Super Contributor
timestamp: November 27, 2022, 11:56:30 pm
content: Quote from: bdunham7 on November 27, 2022, 11:43:56 pmI don't know what specific issue you are talking about with the LM7805, but I assume it is the voltage divider for the feedback. That depends not on the value of resistances but on the ratio of two resistors. Not the same thing at all.Still, I should not have said they "can't" control resistance value. I should have said it's hard to do with much accuracy. That's why some devices get laser trimmed.

author: bdunham7
author_rank: Super Contributor
timestamp: November 28, 2022, 12:15:29 am
content: Quote from: gnuarm on November 27, 2022, 11:56:30 pm+/- 20% would seem to be fine to me and that is quite commonly achievable without trying too hard. And they aren't likely trying too hard since I don't think it is a critical design factor. If they needed it to be tighter, I'm confident they'd manage.

author: TimFox
author_rank: Super Contributor
timestamp: November 28, 2022, 12:40:27 am
content: Quote from: gnuarm on November 27, 2022, 11:44:51 pmFrom that description, the actual required tolerance on the nominal +/- 12 V into the resistor/diode network to obtain about 9 V is loose.For the PWM circuit, the PWM signal will be proportional to the actual output voltage of the LF353, which, as you say, cannot attain 12 V from 12 V rails.However, my question remains: what is the actual required tolerance on the nominal 12 V for that PWM circuit?That is what must be compared with the actual capability of the LF353 or analog mux switch to evaluate the design.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 28, 2022, 12:44:33 am
content: So future lots may or may not work for you. An example might be anti-static protection. Changes to this can generally be made without changing specified performance, but if your application depended on the older method you will suddenly start seeing failures. There are others. Large users who have long product lives demand and get long lead notice of process changes so they can plan for such discovery. If the cost of the investigation is high enough they do lifetime buys from the current process line.If a company fails to provide the ESD protection as stated in the data sheet, that's a quality control issue and the parts go back to them. That's not my problem.So you are absolutely sure that nowhere in your production process or in your customers application is there an ESD exposure above the parts specified capability? And that the parts you use don't exceed spec capability by a significant margin meaning this overexposure is not causing problems currently, but leaving you vulnerable to a future change that only meets the assured performance, doesn't exceed it by a significant margin. One might wonder how you keep your design process affordable.

author: bdunham7
author_rank: Super Contributor
timestamp: November 28, 2022, 12:58:06 am
content: Quote from: TimFox on November 28, 2022, 12:40:27 amThere's the thing--in actual operation there's no point at which 12V needs to be accessible, it just needs to be equivalent to 12V with a 1K output impedance. So if it is ~12V at the node inside the op-amp before the output resistor in the op-amp and you add just enough external resistance to bring the total to ~1K, the circuit will respond to the car as it should.IIRC the J1772 spec for the pilot signal open circuit is 12 +/- 0.6V. So connected to the car, before the car has reacted or requested charging, you should see from -12.6 to -11.4V on the low side and between 8 and 10 volts on the high side.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 02:42:40 am
content: Quote from: TimFox on November 28, 2022, 12:40:27 amThe open circuit output spec is ±5%. They don't specify any other voltages, because they specify all the components. They "nominally" call the other levels 9V, 6V, 3V, 0V and -12V. The resistors are given specific tolerances, min and max.QuoteNo, the PWM has nothing to do with the voltage. Remember, the voltage at the interface depends on the loading from the car, indicating state. There is a sequence of operation, from state A to state B, ect. The PWM specifies the max charging current allowed to the car. The pulse width times are a stepped function I believe. Or at least it's not such a simple relationship with at least two different ramps and some minimum pulse width.QuoteI thought I gave that previously. The voltage on the interface in state A (no load) is +12V ±5%. Since there's no current, that would be the output voltage at the driver, without regard to the 1 kohm resistor.The LM353 data sheet says, it will drive ±12V into a 10 kohm load with ±15V rails. There are various "typical" graphs, but none of them can be relied upon, because they are only "typical". Again, you can't test for Process variations.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 02:50:49 am
content: Quote from: CatalinaWOW on November 28, 2022, 12:44:33 amI don't know what to make of this line of discussion. Anti-static control is well established in the industry. There are numerous actions to take as a part of this. I'm sure I don't need to go into the details for you.I did point out once, that a stool in a work area was not grounded (no chain touching the floor) and the worker would put both feet on the rung. The manager I spoke to about it said everything else is grounded, so there's no way to build up a voltage. I wasn't going to argue with him, but I did leave that CM and found one that had, and enforced proper ESD procedures.So what is all this noise about "overexposure"? How is this in any way similar to spotting a design flaw that is depending on a component detail that is not stated in the data sheet? If there was a flaw in the ESD measures, I would deal with it as soon as it was spotted. Are you suggesting that it be ignored because "it hasn't caused a problem yet?" You aren't really making any sense.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 03:01:03 am
content: Quote from: bdunham7 on November 28, 2022, 12:58:06 amTherein lies the flaw. There is no reason to think the LF353 circuit would behave the same as a +12V source with a 1 kohm resistor. As anyone should know, testing is not a way to prove a design is right. YOU CAN'T CONTROL PROCESS and I'm willing to bet this design has never been explicitly tested for temperature and voltage at the same time. Even if it has, the difference between the actual spec and the assumed spec from bench testing, is so divergent, there's no way it should ever receive a pass.Actual spec, ±12V into 10 kohms, at ±15V power supplies, 3V from the rails.Assumed spec, ±11.4V into 1 kohms, at ±12V power supplies, 0.6V from the rails!QuoteHow did you get 8V to 10V on the high side? Are you factoring in the tolerance of the resistors? There's no reason to do that. Everything else flows from the output voltage of the op amp. With a spec at a much lower current of ±3V from the rails, all of the other voltages are going to be pretty far out of whack, if not missing the threshold completely, it would degrade the noise tolerance significantly.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 03:03:38 am
content: Quote from: bdunham7 on November 28, 2022, 12:58:06 amThat's not actually correct. The open circuit voltage must be distinguishable from the 9V level. So even this voltage is important. Applying the spec from the data sheet, would imply the output voltage may be no higher than 9V. Clearly that is an error.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 28, 2022, 04:03:04 am
content: Quote from: gnuarm on November 28, 2022, 02:50:49 amI apologize. I am not a linear thinker, you are, and my explanations are not clear enough. I am describing situations I have observed.1. Data sheets and manufacturers recommend ESD procedures. And I agree, they should be followed and any people observed not following them should be corrected.2. Data sheets specify robustness or resistance to ESD, typically using the human body equivalent method of a standard that I won't bother to quote since I am sure you are familiar with it.3. In spite of 1, few manufacturers know the actual exposure of their parts to ESD. Usually because of 1. they are well below the human body equivalent test. Note that "usually" and "well below" are not a quantifiable engineering terms.4. While component exposure to ESD is reduced when built into properly designed products it is not eliminated, and most products are not used in well controlled ESD environments. Thus actual exposure in the field is largely unknown.5. In many components ESD protection is provided by bypass diodes to the substrate.6. In the course of failure analysis of devices my organization many times noted parts which were still functioning correctly, but which had one or more of these diodes non-functional due to prior ESD damage. The implication of this is that these parts have a limit to the number of exposures to ESD. When the last diode is gone it is over.7. These parts had been used in what was believed to be a properly controlled ESD environment. This belief was based both on observance of recommended procedures and on absence of failures attributed to ESD. The failure diagnosis that discovered these compromised parts was prompted by a variety of problems.8. All of this leads me to believe that if current capacity of the leads leading to these diodes, or the capacitance or inductance of these leads, or the number of these diodes was reduced in a process change the result could be a less ESD robust part which could still be compliant to its data sheet (of course it could also be more robust). This might in turn reveal a process flaw in manufacture or an unexpectedly harsh operational environment.And finally, I have observed the design process of several organizations and I have not observed one that would be robust to this type of problem. I would infer from your comments that yours is not either.Which then leads to my pedantry conclusion. You have drawn a rather arbitrary line defining the kinds of design risk that are acceptable in a "proper" design. The ESD example is just one of several that I could offer which is outside of most of the standard design to spec approaches I have seen. All of which are outside of this somewhat arbitrary line.Put another way, a proper design spec would have a clear definition of the acceptable probability of failure to perform in the field. Compliance with such a specification is usually almost impossible to accomplish in the design phase. Space Shuttle failure rates being a huge example, but CF light bulbs are another obvious one. So how much effort is good enough to be "substantially compliant" with such a specification is largely an exercise in hand waving and chest pounding. Which is essentially what I am accusing you of. You are saying you know exactly how much effort is required. No more. No less.

author: bdunham7
author_rank: Super Contributor
timestamp: November 28, 2022, 04:11:37 am
content: Quote from: gnuarm on November 28, 2022, 03:01:03 amOf course there is, it's called looking at the design. This isn't like using a 1N4007 diode for 1200V because they will usually work fine that way, until someone sells you 1N4007s that pop at 1100V. This is using a characteristicof the designthat is not reflected in the datasheet, maybe like using the 1N4007 as a temperature sensor. Process variations shouldn't change the design and if they do change the design, then I suppose maybe the circuit won't work--although I'll bet that's unlikely. I've explained all that once already so I won't do it again if you haven't read it carefully.QuoteYes, but why wouldn't it be? What I meant by the 12V being 'accessible', I meant as a low-impedance node. Obviously for an open or very high impedance circuit, ~200R plus a saturated transistor isn't going to change that voltage much, so it is accessible enough to anything in the EVSE circuit that needs to see it. Put another way, it doesn't have to supply any appreciable current at +/- 12V. You've been saying something about 11.4V, where do you get that? I'd bet that if I'm right about how it works, you'll see more like 11.8V with a 12V rail and very high impedance load. But I don't have any LF353 floating around unless maybe I go digging in some old circuit boards or something.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 05:32:26 am
content: CatalinaWOW,I can make this very simple. You seem to think that to discuss the issue at hand, we need to discuss the entire design process and address every possible problem, or none at all. Whether or not there are unaddressed problems does not make any problem being addressed, pedantic.So you have drawn a false conclusion.

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 06:01:04 am
content: Quote from: bdunham7 on November 28, 2022, 04:11:37 amYou have explained your false analogy. I've provided the data from the data sheet that clearly shows the part is being used outside of its defined performance specification.If I were trying to get a space capsule back from the moon, I would be willing to accept all manner of such seat of the pants engineering. This is a product that will be sold in the thousands, containing what is not just a potentially faulty design feature, but is an inappropriate design feature. It isveryeasy to design this part of the circuit with fewer parts, that is assured to work correctly, under all conditions according to the spec sheet.There is absolutely no reason to misuse a part in this way.QuoteI've already explained that the data sheet indicates the output of the op amp from ±12V supplies, may be as low as ±9V.Quote11.4V is the required voltage at the pin for that state (A I think they call it) according to the EVSE spec. Even if state A (no load) the op amp output was correct enough, what happens when the interface pin is loaded to produce 9V? According to the LF353 data sheet, the output can be as low as ±9V. How low do you need to reduce the 1 kohm resistor to maintain an appropriate voltage at the interface pin? There is the ~1 kohm resistor between the op amp and the pin, then a diode and a 2.74 kohm resistor to ground. So about a quarter of the voltage will be dropped across the 1 kohm resistor. The value of this resistor would need to be lowered a lot to raise the voltage at the interface pin to an acceptable level.Then look at achieving 6V with 882 ohms and the diode to ground. So 9V at the op amp, subtract the 0.7V for the diode to give 8.3V. Assume 820 ohms instead of 1 kohm (what the EVSE device is actually using). This gives 4.3V on the 882 ohm resistor, plus the diode voltage gives 5V. Rather lower than 6V.If the threshold is 4.5V... I think you will find the noise margin is greatly reduced. Interestingly, the EVSE spec does not set actual voltages for either the states or the thresholds between the states. So I suppose you might be able to make this work better by not assuming the 6V state C is anywhere near 6V. Heck, you could even calibrate each unit for the particular op amp.Or... you can design an even simpler circuit that just works! Replace the op amp and two resistors with... an analog switch mux.Your comment about not having an LF353 on hand shows exactly the faulty thinking I'm talking about.Whatever. We are clearly never going to reach any agreement on this. I think it's time to throw in the towel.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 28, 2022, 08:42:28 am
content: I don't get it."You can't test process", yet, there is precisely one output characteristic in play here, that corresponds to one real function, which can be sampled (characterized, measured at points), and which is continuous to the extent that such sampling is representative of the full continuous function.We can measure the V(I) characteristic of the chip, over temperature if we like, andguarantee, through testing,it will always meet our requirements.And in this case, we might only ever bother to sample a couple of operating points -- corresponding to the load in question -- at the two voltage states.If as few as two points fully describe the relevant characteristic, and measuring those say at assembly test gives 100% test coverage, then what's the fucking problem?Indeed, numerous off-label users, with high certainty demands, have employed such testing to maintain the quality of their products.Apparently they are wrong to do so? Wrong how? You must now give an economic justification for their wrongness, given that their customers demand a degree of quality corresponding to the practices you believe in.Now, whether the party in question is doing a batch / receiving test, or statistical sampling, or anything like that -- that's another matter. It seems, informally at least, they have a sampling method of a sort.Indeed we can flesh out the entire quality system here. Let's see:Receiving: a statistical sample was done at least once (initial design testing). We can interpret this as a frequency. Therefore they are doing some frequency of testing. (Whether or not theyintendto ever do more sampling, is another matter. This observed testing frequency may simply forever decrease... And, the number of units tested, might be as little as one, not a statistical sampling, but we'll conveniently ignore that for now.)Production test: perhaps an assembly test is done to verify the operating point within tolerance.Customer receiving test: if it works, it works.Remediation: if a customer's unit malfunctions, send out a replacement. The customer serves as production test, if nothing else.Thus, that testing gets done one way or another, but it might not be done under a range of conditions until some time later, or at all (those units used in Florida, or indoors, will never experience the same temperature swings that an outdoor unit in Montana will, thus test coverage over temperature is spotty at best).Recall: if a mass of customers report malfunctions, maybe take a look at what parts went into them and check if there was a process change. Switch to an alternate part, and restart from the top.That spans basically all the steps of quality control, so there's something there, in a fashion. The main downside is it puts much of the burden of testing on the customer. And, granted, they might not be smart enough to do that last step in an orderly fashion, and some amount of testing earlier in the process can avoid the troublesome mass-replacement scenario. But also, they might get lucky and simply never have that happen. Who knows.Is it cheap insurance to test more often, or use a better designed circuit? Maybe you overestimate how much profit margin the product has, or underestimate how expensive engineering or testing time is.Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 28, 2022, 11:16:09 am
content: Is this from the OpenEVSE CP line driver? I know that one used a similar scheme, and I always thought it was dodgy.It's pretty clear to me in this case, if the output voltage levels and rise time fall outside of the tolerance band given by the standard, then it is a bad design. If there was a clear benefit in terms of cost and complexity to do it in this way, it might be justified, but in this case they are using the wrong component for the job. When solving a similar problem, I used a simple circuit based around four discrete transistors and got output voltage levels within a few tens of millivolts of the rails.The op-amp solution still likely works well because of the wide margins on the receiver side, but at the cost of reducing the robustness of the link. The fact that they power the op-amp with an unregulated DC/DC also helps, since these provide nominal voltage at full load current, and significantly more at the light loading seen in this circuit. If this design by chance manages to fit within the limits of the standard, and that is verified over all the expected operating conditions then it can be argued that it's acceptable. But then it seems easier to just do it with a more suitable circuit topology in the first place.

author: jonpaul
author_rank: Super Contributor
timestamp: November 28, 2022, 03:02:57 pm
content: bonjour à Gunarm, et tous....Just seeing this long topic, very fine discussion.A moment please......Is the application commercial, consumer, industrial, avionics, medical ?What is the concequence of a failure in the field?Different in a bad $50 consumer game, vs a $10k broadcast processor, or a medical electronic in a surgery suite...In decades of consulting and manufacturing, I used worst case design as a philosophy, and fail- safe in mission critical applications, eg military, avionics, medical.Relying on typical specs was risky. So, a commercial IC has worse tolerance and specifications, better in industrial -25-+85C or best in mil spec -40(?)...125 C.Just the ramblings of an old retired EE (68')bon courage et bon chanceJon

author: gnuarm
author_rank: Super Contributor
timestamp: November 28, 2022, 07:04:18 pm
content: Quote from: Wolfram on November 28, 2022, 11:16:09 amThis is the conversation I always have with people regarding design verification. The golden rule is that you can't prove a design works, by testing it. I've said it before, and I'll say it again. You can test over temperature. You can test over voltage. You can test over voltage and temperature together. But you can't test over process, because you can't control process.I worked on a design once where the FPGA tools were not analyzing timing properly. It would tell us the design timing was good, but it would fail on the bench. We could make it work by cooling. We had a few chips where the design would work on some of the randomly seeded layouts. They could be made to fail by heating. Clearly a timing issue.We had no way of testing over process other than the handful of chips we had. We could buy and test chips until the cows came home, and we would still have no idea how much of the process range we had covered. We ended up spending days and weeks randomly generating FPGA layouts, and testing them using a chip heater. What a PITA!That's why we use data sheets. The manufacturer tests to the data sheet and guarantees those data. Designing to that data assures your design will not fail because you didn't fit the right part into the right socket. In this case, the spec of the LF353 shows far too much range of the output voltage drop to be suitable for this circuit.It's a bad design. The person who designed it should feel bad. No, just kidding. That's what Zoidburg would say. But the person who designed it should learn, rather than waving his arms wildly and claiming it's fine, since he measured a device and it's all good.

author: bdunham7
author_rank: Super Contributor
timestamp: November 28, 2022, 11:56:23 pm
content: Quote from: jonpaul on November 28, 2022, 03:02:57 pmIt is the pilot signal for a J1772-spec EVSE (EV charging station). If it malfunctions, the vehicle will not charge.QuotePerhaps, but many remarkable things have been built with 'select' part, not only relying on 'typical' specs but on best-case specs. However, IMO this is not a case of typical vs worst-case or safety margins, it is about a design characteristic not specified in the datasheet--specifically an op-amp that saturates when open loop and has a resulting large differential voltage on the inputs.

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 12:24:29 am
content: Quote from: bdunham7 on November 28, 2022, 11:56:23 pmThere's no excuse to comply with J1772, which specify the 1k resistance tolerance, and the oscillator voltage ranges, with recommendation to make it tighter to ease up tolerance stackups and improve inter-operability. It's a simple circuit with simple requirements, and with a zillion non-compliant EVs on the market with their own set of edge cases, so it's definitely imprudent to rely on average cases.

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 12:27:30 am
content: Looks like this is the circuit. Apart from being shit-tastic for no good reason, there are schematic mistakes (e.g. it wouldn't work with that TVS diode, unless it was bidirectional; sure enough, the part is bidirectional which doesn't match schematic). None of this could possibly be UL2231-1 compliant either.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2022, 12:40:40 am
content: Quote from: gnuarm on November 28, 2022, 06:01:04 amOK, but can we agree on what we disagree about? Ironically, I understand and typically would agree with your main thesis about not relying on typical specs or special parts, but I just don't think it applies to this case the way you think and since you've come here to publicly pillory some hapless EVSE designer who isn't here to defend himself, I thought I'd take up his cause.QuoteSo one thing we disagree about is what the data sheet tells us and my comment supports that, not the other way around. You claim that 'according to the data sheet' the output of the op-amp could be "as low as +/-9V". As mentioned, I don't have an LF353 handy to test, sorelying solely on the information contained in the data sheet along with basic knowledge of op amps in general, I am saying that the op-amp will reliably put out ~11.8V +/- 0.1V with +/-12.0V supply rails and a 0-5V 1kHz square wave + input and 2.5V on the - input, which I'm assuming is how the EVSE in question is set up. I'll go a bit further and say that I would expect that to apply to any LF353 from any reliable manufacturer and additionally, if the EVSE designer has used a 750R resistor on the output, the J1772 requirements will be met well within tolerances. Did I guess right on the resistor or did he put an 820R in there? Now you're waving the data sheet at me and saying "No! It says here you can't rely on it to be more then 9V!". I've explained why I think this is true in detail above and don't think it will help to do it again.Just note that I am not depending in any way on testing or on a typical spec.And there we disagree, plain and simple. Fair enough?Another thing we disagree on is the requirement to only use components in ways anticipated and specified by the data sheet. In this case, the data sheet doesn't contemplate operation with the output transistors saturated because that's not really an op-amp thing to do. In theory, they could implement a change that incorporates output saturation protection or something and those parts might not work in this application. However, that's more than a 'process' change and IMO manufacturers are extremely unlikely to do that because their customers will probably not like it. So as long as there is sufficient information to draw a reasonable (and yes, testable) conclusion about the component's internal design, any characteristic that is pretty much assured by that design is probably one you can rely on. In this case, I'm assuming this relatively generic-except-for-the-J-FET-inputs op amp will saturate the outputs under the conditions of the specific circuit in question. If I'm wrong about that, well then the EVSE won't work. But I hear that they do, so...

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 12:49:05 am
content: Quote from: bdunham7 on November 29, 2022, 12:40:40 amJ1772 specifies some of the stuff, e.g. in table 3:Generator voltage high between 11.4V and 12.6VGenerator voltage low between -11.4V and -12.6VSource resistance between 970 and 1030 OhmsErgo, the design is not compliant for some of these, as well as other more subtle stuff like slew rate. E.g.: "This  circuitry shall have rise/fall times faster than 2usec". The opamp seems to barely meet that, so it's at the very least marginal.Honestly to do it right is less effort than trying to prove that something weird like this is standards-compliant, so I have no issue calling out the designer of this in saying that it's not great.If they called it something like "Magic OpenEVSE standard compliant charger", I have no problem with it, but the J1772 label is right there on the schematic..

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2022, 01:02:52 am
content: Quote from: uer166 on November 29, 2022, 12:49:05 amHmm, OK those are going to be difficult to achieve with this setup, so scratch the part about meeting J1772. I have no idea why J1772 would require 2us rise times.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2022, 01:21:48 am
content: Quote from: bdunham7 on November 29, 2022, 12:40:40 amRight? Like, you can't compare an op-amp to an FPGA... the FPGA is exponentially more complex. Obviously you're going to have problems testing an FPGA, you can't test every path under every voltage and temperature, in every single unit, under every possible combination of inputs and internal states.The op-amp has exactly one  internal state*.They aren't the same damn thing!A more direct comparison would bea single gate,or gate path or LUT/LE/block in an FPGA. Maybe a couple inverters (74HC3G04 let's say?) to be more concrete. Can you characterize the delay of one specific path? YES. Can you do it over voltage, temperature, etc.? YES. Do you want to in production? Probably not. Can you do it statistically? Probably not (e.g. sampling parts from a lot/batch, or even just sampling random LEs among the fabric -- there will be some distribution over the die itself), but it depends how much margin is required by the application. I mean, in any case, it reduces to a statistics problem, and if you're looking for a 99th percentile say, that's easily enough done by sampling; but if you need the 20th percentile lucky hotrods, good luck and have fun grinding out all that testing. But notice these latter questions are allpracticalones. Which can have different answers for different circumstances. Discarding these possibilities out of hand would be silly.It sounds like OP is just very familiar with the high-complexity side of things, where it is practical to discount such possibilities out of hand, but forgets when they can again be reasonable to consider.*The dominant pole; well, several if you include the higher order poles above fT but they are also largely irrelevant in practice, or beyond just the lowest few.Mind, I don't have any complaints at all, in principle. I wholeheartedly support, understand,and use, such principles in my own work. But apparently some would think I'm some horned devil because I allow practical exceptions to those same principles...Or--maybe I've grossly misinterpreted or misunderstood the intent here, or the concepts at work. I haven't worked with, say, Six Sigma in a very long time (and that occasion was pretty superficial, heh). I'm sure my statistics are rusty. I may be grossly mis-understanding/representing the complexity or cost of these controls or inspection or testing procedures.But if no one steps up to critique my points, I'm left to assume I have asserted them correctly and rightly, that most people here generally agree with them...or at least don't disagree strongly enough to complain about it. I would love for my arrogance to be knocked down a peg, as it needs to from time to time, but, y'know, I'm only going to do so much research on my own here. Someone, please, be the change you want to see! ( ??)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2022, 01:35:44 am
content: Quote from: uer166 on November 29, 2022, 12:49:05 amAs for explaining why it apparently works anyway -- I wonder how strict most (all?) receivers are? What's the penalty for violating the above specs? Does every receiver just fail outright? Do they in fact specify a much looser/narrower threshold, ala RS-232 for example, making the extraordinarily tight transmitter threshold absurd and useless? Do they even measure source resistance (perhaps with a dithering load current or resistance -- and if so, at what frequency, waveform, pattern, etc.?)? How can they tell between a slightly lower Thevenin Vth and Rth, versus a higher (of both)?Framing it in terms of design by contract: there are always three aspects to a contract. There's what the client party thinks they should be getting; there's what the supplier party thinks they should be making; and there's verification -- each party being able to prove to each other that the steps and clauses of that contract have been met, to mutually agreed satisfaction. Clauses without verification are as good as struck, and clauses that are overly specific for their testing methods are only as good as the test itself. It comes down to design-for-test (in analogy to "teach to the test" schooling). That doesn't have to be a bad thing (as that analogy often connotates), but it means we have a responsibility to design our contracts, or standards, or whatever, to a high level. And if we fail to do so, it is our own fault, whether through lack of foresight, laziness, or lack of responsibility/wherewithal to update the standard at a later date to correct those shortcomings (which, yes, can have many issues about it, but it almost always comes down to "political will", being able to convince enough people of the value of the change, and then to push that change to all stakeholders).Tim

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 01:53:37 am
content: Quote from: bdunham7 on November 28, 2022, 11:56:23 pmDepending on how it malfunctions, it can connect the power line to the car when it should not.QuoteI worked on the test floor of a company who designed a super computer (second to the Cray) using sloppy design methods. Timing is not a place where you want to use anything other than max numbers. We had a FIFO chip we would automatically replace when we had a certain failure. Turns out it wasn't the chip, the designer was using logic gates to define a pulse width to operate the FIFO!That was where I learned about Level-Sensitive Scan Design. That is old hat now, but there were many things like this which were just coming into wide adoption. I guess design based on maximum timing specs was one of them.Oh yeah, that company is not in business anymore.

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 02:20:23 am
content: Quote from: uer166 on November 29, 2022, 12:27:30 amI think that is an old version. I never found a current one. The author claims his design has UL and other certs.I only noticed this because someone was asking why his output voltage was low. The author (unknown to me at the time) responded about lowering the 1 kohm resistor to 820 ohms. That's when I smelled a rat.

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 02:31:56 am
content: Quote from: bdunham7 on November 29, 2022, 12:40:40 amNo one is being "pilloried". I wanted to discuss the issues. We've done that. I never even mentioned the guy's name and the project was only sussed out after some mentions of details. In fact, I tried to not discuss the details since most of it is not relevant to the real discussion.QuoteYour assertion has always been clear. The basis for it is not. I reject the idea that you can analyze the circuit as if you had the actual circuit and could make measurements or run the simulation. This is exactly the sort of stuff that is outside of what I consider to be design practice. I've never worked at a company where someone in the design review would not just say, "So why don't you just change out the damn part?" Typically a boss who is tired of everyone beating the dead horse.Actually, the designer has said he is going to consider my switch approach, if I understood him correctly.QuoteWhat's your point???QuoteExcept that there's absolutely NO reason to use the part, when many alternatives are available. Why cobble something together from the junk yard across town, when you would pass five autoparts stores, each of which carry just the part you need.Feel free to respond. i no longer feel a need to discuss this with you. We are just covering the same ground and it's getting very... muddy.

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 02:32:45 am
content: Quote from: T3sl4co1l on November 29, 2022, 01:35:44 amIt's very much EV-dependent. Some are very loose and some are a bit stricter, but generally as a EVSE provider you are trying to design it as tight as possible such that the EVs get the most of the tolerance stack allocation. In testing for example you can have GM cars charge fine and dandy, but a Model 3 error out. The single-ended, high impedance pilot is also well coupled to the mains and the OBC input, so any noise on that can easily swamp out the entire tolerance allocation and kick it out of state C. That is especially true on high duty cycle pilot 80A chargers, where you don't have much time to even look at the "high-going" edges of the waveform.I know UL measures states A1 and A2 (i.e. the +-12V output unloaded), so if that's not within that region they'd likely fail it.

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 02:36:18 am
content: Quote from: bdunham7 on November 29, 2022, 01:02:52 amIt's really important in high duty (read:80A) chargers that output 96% duty. That's only 40us to see and measure the high-side of the pilot, and you want the cable capacitance and the 1k to dominate and account for the entire rise/fall times. Otherwise you'd be robbing maximum cable length off of the spec for no good reason. This reason is explained in J1772 standard

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 02:37:35 am
content: Quote from: uer166 on November 29, 2022, 12:49:05 amI'd have to read the spec again, but I think those voltages are not on the op amp, since that is an internal thing and not part of the spec. Those are voltages at the interface,afterthe 1 kohm resistor. I would say that could be a perfectly valid approach, if the voltage at the op amp output was specified in the data sheet. It's not, at least not very well.QuoteIt's a simple fix, just replace the op amp with a part that is specified to drive near the rails. Or if he wants to change the PCB, use an analog switch mux.

author: uer166
author_rank: Super Contributor
timestamp: November 29, 2022, 02:41:35 am
content: Quote from: gnuarm on November 29, 2022, 02:37:35 amI don't think that's right, they do specify theinternalgenerator voltages, whichever way the generator is implemented. It may be descriptive instead of prescriptive however, the devil is in the details.

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 03:07:21 am
content: Quote from: bdunham7 on November 29, 2022, 01:02:52 amIn states other than A, the signal is a PWM with the duty cycle specifying the available current. So the edge timing is important. 1 kHz, but they slice the duty cycle more finely at the high end. I think the higher charge rates were an add on, but just guessing.I find it an interesting interface actually. They get a lot done with just two wires. One to tell the car the cable is attached and control the lock/shut down charging, and another to tell the EVSE the car is attached and negotiate the actual charging details.If they had a section of the spec that simply said this, the whole thing would become a bit easier to read and understand. Instead you are thrown into the details of voltages and duty cycles without really understanding of what is happening.I think there's also some aspect of the proximity detection pin, for the cable being connected to the EVSE. Seems they've allowed for that too. I think it's the same pin on the other end of the cable, but not connected to the vehicle end, i.e. separate circuits. Not even a wire in the cable.

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 03:17:54 am
content: Quote from: T3sl4co1l on November 29, 2022, 01:21:48 amNO. "ect" is process (the only thing left) and can't be adjusted for your testing.QuoteOr not. I found it a bit long and have not read all the detail. In general, comparing an FPGA to an op amp or even a logic gate to an op amp is not very valid in most considerations. They are very different things. Digital is designed so you can *mostly* ignore the analog aspects. The op amp very much an analog device and the real problem here is that it is being used as a digital device. It's really a 0-5V to ±12V level shifter. That's the problem. There'sno reasonat all to be testing anything about the op amp's drive. Pick an op amp that is quantified, or use a different, and better,appropriateeven, device!QuoteOk, consider it knocked down. But I will return and finish reading later. Not sure how much later...

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2022, 03:20:04 am
content: Quote from: T3sl4co1l on November 29, 2022, 01:35:44 amWell, that's what got me started in the first place--realizing that this was J1772. This is a very old, crude 'signaling' setup intended to be capable of implementation with very simple electronics. Essentially a +/-12V 1kHz square wave with a duty cycle that indicates the EVSE capacity is sent to the car and the car responds by loading the + side of the square wave with one of three resistances to indicate that 1) it is there 2) it is ready to charge 3) it needs ventilation. The last one is pretty obsolete, it came from the days when EVs used lead-acid batteries.The car only has to detect that it is plugged in and then determine the duty cycle of the square wave, in addition to switching in the appropriate resistor when it is ready to charge. The EVSE has to read the pilot signal to make sure that the -12V signal is not attenuated and then from the + signal determine which of the 3 modes it might be in (corresponding to about 9V, 6V or in the now obsolete ventilation mode, 3V) So as you can imagine, you don't need a great deal of precision to accomplish this--which is appropriate given the conditions (electrical noise, weather, etc) that it operates in.

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 03:24:18 am
content: Quote from: T3sl4co1l on November 29, 2022, 01:35:44 amIt works "anyway", because the parts he has used, appear as a voltage source within the required spec and a series resistance of about 200 ohms. He changes the series resistor in the circuit from 1 kohms to 820 ohms to compensate. The problem is, this characterization of the part is not assured, since it's not in the data sheet. Tomorrow, the next batch of parts may only drive to 11.0V or 10.5V or even 9V which is all you can assure from the data sheet!No, not at all likely, but the room for error here is not terribly large, compared to what the data sheet indicates.QuoteAre you a lawyer... being paid by the word? Can you say this part in 20 words or less?

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2022, 03:26:16 am
content: Quote from: gnuarm on November 29, 2022, 02:31:56 amNow there I cannot possibly disagree, a J-FET op-amp would be the last thing I'd think of. My guess would be that the design uses them elsewhere and there was a spare one in a dual or quad package available. Thrifting a design is an art unto itself and I've gained some respect for those that do it.

author: bdunham7
author_rank: Super Contributor
timestamp: November 29, 2022, 03:51:23 am
content: Quote from: uer166 on November 29, 2022, 02:36:18 amOK, that makes sense, especially if you don't know exactly how the vehicle is going to read the duty cycle. Do you know if the standard specifies what section of the signal the vehicle is required to use to read the duty cycle?

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 04:56:20 am
content: Quote from: bdunham7 on November 29, 2022, 03:51:23 amIs this a multiple choice question? What are the options?The signal amplitude varies with the state. The PWM is enabled in state B, after the car has pulled the +12V to +9V. The next state, PWM +9V/-12V, is the when the car drops the +9V to +6V (state C) to say it is ready for power.I suppose the car could measure the pulse width in State B or C, either one. Since it is up to the car to draw the power, it's in no hurry to take the reading. I don't know if the car is supposed to continue to measure the PWM for changes. Ok, I see in the spec they say the car has 5 seconds to modify the current draw when the pilot PWM changes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2022, 03:26:10 pm
content: Quote from: gnuarm on November 29, 2022, 03:24:18 amOk... but those variances can be tested. They are observable. A sampling will show them. I don't know what's so mysterious about your as-yet unexplained "process" that's so untestable here? Surely you must be using a different meaning of "process" than what is coming to my mind?QuoteBeats me. You're insisting upon "standards" and "process"... If you're that committed to standards, I'd have thought the verbosity would be appreciated? Are you not actually, then..?Tim

author: gnuarm
author_rank: Super Contributor
timestamp: November 29, 2022, 05:26:45 pm
content: Quote from: T3sl4co1l on November 29, 2022, 03:26:10 pmWhy would I want to test parts before I use them? There's incoming inspection, but who actually mounts their parts in a chip tester to repeat the testing that was done at the factory? For many devices, such as op amps, the testing is the lion's share of the costs.This is what I'm talking about. People are coming up with all manner of excuses for this design, instead of justfixing it!QuoteI believe I've referred to the J1772 standard, which is the reason for this design existing. I've referred to the specification of the LF353, which is the document that should guide the part's use. The "process" I'm referring to is the semiconductor "recipe" used to make the parts, that can not be perfectly controlled and results in variations, or is intentionally modified for some reason, which results in changes in the device, which are not reported as long as the device continues to meet the data sheet. Your assertion about verbosity is absurd. Clearly, you have nothing further to contribute to this conversation.