How "Rail-to-Rail" are we really talking here???....... - Page 1
Forum section: electronics / projects

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 09:07:58 pm
content: Lets take the TL900x series:https://www.ti.com/lit/ds/symlink/tlv9002.pdf"TLV900x Low-Power,RRIO, 1-MHz Operational Amplifier for Cost-Sensitive Systems""The TLV900x family includes single (TLV9001), dual (TLV9002), and quad-channel (TLV9004) low-voltage (1.8 V to 5.5 V) operational amplifiers (op amps)with rail-to-rail input and output swing capabilities."7.10 Electrical Characteristics:Output:Vo - Voltage Output Swing from Supply Rails:Vs = 5.5V, RL=10K --->Typ 10mV, Max 20mVVs = 5.5V, RL= 2K --->Typ 35mV, Max 55mVSo thisRail-to-Rail-Input-Outputopamp can only actually get to best case output of 10mV of the rails, and worst case 55mv?I understand this is significantly better than a nonRRIO part, but that could totally screw a design if those 10mV around zero were important.Note: 10mV is good, but it's not "Rail-to-Rail".Note2: I propose this:For linear regulators we have "Low Drop Out". They don't call them "Zero drop out*" ....."*still need 0.4V".How about "RRI-LHO" = Rail-Rail-Input,Low-Headroom-Output

author: Shay
author_rank: Regular Contributor
timestamp: March 17, 2023, 09:37:00 pm
content: Seems quite decent. I don't know any RR op-amp that can swing better than that.

author: Marco
author_rank: Super Contributor
timestamp: March 17, 2023, 09:44:55 pm
content: Unfortunately there's no zero ohm FETs.

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 09:50:36 pm
content: I'm less complaining about the 10mV here, which as I said I agree is pretty good.It's more about the advertising as RRIO as a banner spec. There isn't even a * footnote. Yes I know you should be reading the whole datasheet.I feel it's like advertising "Free Pizza Today!". Then when you get there, it's actually $1. That's a great price for a pizza but it's not "Free".

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 17, 2023, 09:59:55 pm
content: Quote from: Smokey on March 17, 2023, 09:07:58 pmYes, this is significantly better than common, non-RRIO opamps with swings limited to hundreds of mV below rails.The zero can't be achieved, at least not for outputs. What is zero anyway in physics? If it was 10µV rather than 10mV, would you still feel ripped off? If not, why, it's still not hitting the rails?For inputs, some opamps actually allow going all the way down to the negative rail or even slighly lower.The way to handle this is well known, just use power rails with a slight headroom. The benefit of using RRIO opamps even in that case is that the required headroom can be pretty small.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2023, 10:04:44 pm
content: IIRC, there are/were complicated op amps that had built-in charge pumps or similar to derive a negative voltage below the negative rail applied to the device, for those who actually need zero volts to the rail.Note that many op amps haveinputsthat are capable of going slightly negative with respect to the negative rail.I would follow SiliconWizard's suggestion to provide a slightly negative rail to your device if you need < 10 mV.In general, a positive voltage cannot go to zero unless it is capable of going negative.

author: thm_w
author_rank: Super Contributor
timestamp: March 17, 2023, 10:07:23 pm
content: Quote from: Smokey on March 17, 2023, 09:50:36 pmIts convention. As Shay says, its about as good as you get.You can probably find a hundred examples of this: ideal diodes, zero drift opamps, etc. There was a thread with a bunch of examples but can't recall offhand.https://www.eevblog.com/forum/beginners/why-are-manufactorers-allowed-to-lie-(misrepresent)-datasheet-values/

author: langwadt
author_rank: Super Contributor
timestamp: March 17, 2023, 10:08:40 pm
content: Quote from: Smokey on March 17, 2023, 09:50:36 pmmore like "Pizza $1" but when you get there it's actually $1.002 with tax ....

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 17, 2023, 10:11:42 pm
content: Yeah it's a naming convention, but how would you call it otherwise? If you have a better idea?ARRIO? "Almost rail-to rail"Now how "almost" are we really taking here...?

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 10:12:04 pm
content: Quote from: SiliconWizard on March 17, 2023, 09:59:55 pmThat's true here (TLV900x).Input Voltage Range:Min = (V-) - 0.1VMax = (V+) + 0.1V

author: Someone
author_rank: Super Contributor
timestamp: March 17, 2023, 10:16:02 pm
content: ... wait until you read about zero-drift.

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 10:17:16 pm
content: Quote from: SiliconWizard on March 17, 2023, 10:11:42 pmOk. How about this?For linear regulators we have "Low Drop Out". They don't call them "Zero drop out*" ....."*still need 0.4V".How about "RRI-LHO" = Rail-Rail-Input, Low-Headroom-Output

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 10:19:06 pm
content: Quote from: langwadt on March 17, 2023, 10:08:40 pmIt's too completely different things to advertise "Cheap Pizza" and then have extra fees, than it is to advertise "Free Pizza" then charge anything.

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2023, 10:24:45 pm
content: Quote from: thm_w on March 17, 2023, 10:07:23 pmNice link. I feel like this is different than the thermal limits case that guys is talking about. Under "ideal conditions" that part can output 1A, but under real world conditions, it's often much less.Ok, but in this case, there are no possible conditions that the output of this opamp can hit the rails.

author: peter-h
author_rank: Super Contributor
timestamp: March 17, 2023, 10:41:51 pm
content: It is possible to have rail to railinput(because you can use "depletion mode" devices or LM358-style PNPs) but it is physically impossible to swing anoutputrail to rail.With very low output load you could get it within microvolts however.This is why TI make that funny chip which generates -200mV. It opens up all kinds of interesting stuff. You can swing down to 0V, but cannot swing too far down to forward bias the protection diodes in whatever you are driving. Personally I achieve -200mV with a schottky diode Vf and some other stuff, but it has a big TC.

author: MasterT
author_rank: Frequent Contributor
timestamp: March 17, 2023, 11:08:45 pm
content: Options.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 17, 2023, 11:12:23 pm
content: Quote from: Smokey on March 17, 2023, 10:17:16 pmWhy not. May sound unnecessarily convoluted, but yeah why not! I doubt you will be able to change an "industry convention" that easily though.In any case, that's yet another case for always reading the datasheets, in particular the parameters that are critical for a given application.No naming will save you from having to do that.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 18, 2023, 12:04:50 am
content: Ever bought lumber in the USA? At my local Home Depot (local crap lumber purveyor) I helped a couple of Japanese guys with a measuring tape who appeared flummoxed by 2x4s. When they understood, they had dissatisfied frowns.The fact is, marketing is almost always either an exaggeration or outright lie. I just think of RtoR as a search term to help me find a better op amp.

author: thm_w
author_rank: Super Contributor
timestamp: March 18, 2023, 12:21:35 am
content: Quote from: Smokey on March 17, 2023, 10:17:16 pmYeah that is a valid counter example.But don't worry, we already have "ultra low dropout regulators":https://www.st.com/en/power-management/ultra-low-dropout-ldo-regulators.htmland Maxim has used it to describe a buck boost: "The active rectifier also acts as a zero-dropout regulator if the input exceeds the regulated output".LDO -> ULDO -> ZDO

author: Terry Bites
author_rank: Super Contributor
timestamp: March 18, 2023, 10:38:48 am
content: I'm moving to your super low 0.2% tax region. Go Denmark!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 18, 2023, 11:17:09 am
content: You have to take basic laws of physics into account.(Well, theoretically an opamp could have an integrated switch mode power supply generating supply voltages higher than external supply pins, but that would be a large and expensive part!)It is possible to haveinputwhich goes exactly from rail to rail, or even beyond the rails, yes.But output? It has to drive significant current into the load (resistive or capacitive), and it only has the rails to work with, using transistors in its output stage. Cheap, room-temperature superconducting zero-ohm transistors are not a thing, so there will be a voltage drop.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 18, 2023, 01:58:43 pm
content: Quote from: Siwastaja on March 18, 2023, 11:17:09 amThat already exists for audio applications, eliminating the need for a negative supply rail.https://www.analog.com/en/products/max97200.html

author: Smokey
author_rank: Super Contributor
timestamp: March 18, 2023, 07:03:58 pm
content: Quote from: NiHaoMike on March 18, 2023, 01:58:43 pmhttps://www.digikey.com/en/products/detail/analog-devices-inc-maxim-integrated/MAX97200AEWC-T/2239115That part is $1.68 in 1s qty. Reasonable.

author: Smokey
author_rank: Super Contributor
timestamp: March 18, 2023, 07:11:45 pm
content: Quote from: Siwastaja on March 18, 2023, 11:17:09 amI'm not trying to break the laws of physics. But for a physics analogy, what would happen if someone came out with a cryocooler that they advertised as "True Absolute-Zero Cryocooler" "Drops temperature to Absolute-Zero!*""*Min temp = 1.75 mK"

author: langwadt
author_rank: Super Contributor
timestamp: March 18, 2023, 08:13:33 pm
content: Quote from: Smokey on March 18, 2023, 07:11:45 pmabout 1/1500 the temperature of space, so pretty damn close to zero ...

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 18, 2023, 08:15:57 pm
content: I don't think it's absolutely clear that the absolute zero K actually exists in space anyway.

author: shapirus
author_rank: Super Contributor
timestamp: March 18, 2023, 08:18:27 pm
content: Quote from: SiliconWizard on March 18, 2023, 08:15:57 pmNot even there.

author: Smokey
author_rank: Super Contributor
timestamp: March 18, 2023, 09:04:50 pm
content: Quote from: edavid on March 18, 2023, 08:18:50 pmThey give no minimums in the spec table. I'm also not seeing a single supply plot for output voltage in different operating conditions.This is the text from the Feature Descriptions:QuoteThat's an interesting point about the outputs being ohmic and the distance to 0V therefore depends on the output current. So with zero output current the output voltage goes to the rail. Ok but that makes a much less useful opamp if you can't output any current.I guess you could also make the claim that when it's unpowered it also has 0V on the output, but I'm not sure that makes the RRIO claim any more true.

author: TimFox
author_rank: Super Contributor
timestamp: March 19, 2023, 03:46:50 am
content: Quote from: Smokey on March 18, 2023, 07:11:45 pmAbsolute zero is an asymptote, q.v.In the normal definition, temperature cannot go negative, although there are special physical states that can be assigned negative temperatureshttps://en.wikipedia.org/wiki/Negative_temperature

author: Siwastaja
author_rank: Super Contributor
timestamp: March 19, 2023, 09:45:21 am
content: Quote from: Smokey on March 18, 2023, 07:11:45 pmCryocooler is a higher-level component, a product that is supposed to be used by a physicist (not engineer) as is. Opamp is not comparable. Opamps are designed for professional electric engineers whomusthave at least some simple understanding of how they internally work. Not every detail, but like the facts they are built out of existing transistors, with limitations of available technology.It is a very useful distinction to group the devices into those that need 2000 millivolts of headroom and those that are fine with 50, so they can searched efficiently. Making the terminology more verbose just to clarify that their product is not physically impossible is wasted effort.Any designer worth their pay will understand that if one truly needs to output exactly GND or Vcc, there is no other way but to power up the opamp from supplies beyond GND/Vcc. This is the usual, classic way, used before the RRO opamps.The whole idea of rail-to-rail output parts are that in some situations, you can save a lot of effort and cost using such part. You always have to check the numbers before designing one in.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 19, 2023, 01:17:36 pm
content: And it gets even better with modern op amps: Varying complex output impedancehttps://www.ti.com/video/5785293884001(the whole "modern op amps" series is worth watching)Crossover distortion from the two separate input stages (no single stage could do RR in)Of course AoE 5.9: "RRIO amps: the good, the bad, and the ugly"

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 19, 2023, 09:40:24 pm
content: Just give it a negative resistance load...

author: John B
author_rank: Frequent Contributor
timestamp: March 19, 2023, 10:38:21 pm
content: Quote from: MasterT on March 17, 2023, 11:08:45 pmInteresting workaround, though you might fall into some problems depending on the impedance of the load, whether you're driving an AC, DC, AC+DC signal etc. You also need to generate a negative rail anyways, and route that to the op amp with a trace, so I wonder if it's better than providing the negative supply to the op amp in the first place.Thankfully a charge pump circuit can be made quite small, using entirely MLC caps. The usual drawbacks are noise and output impedance, the latter not being too much of an issue as you just need to create a sufficient negative bias rather than a stable DC voltage. I usually LC filter the input to the IC and the output as well.

author: Smokey
author_rank: Super Contributor
timestamp: March 19, 2023, 10:38:40 pm
content: wait a minute.. I can bring this all together!If I was only to get one of those "True Absolute Zero!" cryocoolers, I could make the opamp superconduct and the output resistance would go to zero and it could hit the rail, even with actual current!! I'm a genius!

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 20, 2023, 01:02:35 am
content: Quote from: peter-h on March 17, 2023, 10:41:51 pmDo you happen to remember the IC part number? Or at least how it is called?I assume it is from the charge pump family, but cannot find it with their selection table.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: March 20, 2023, 12:15:20 pm
content: Quote from: schmitt trigger on March 20, 2023, 01:02:35 amI do:https://www.ti.com/product/LM7705

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2023, 01:13:23 pm
content: Yes, within ~50mV sounds quite good to me.If you're trying to amplify very near the supply, well first of all, stop doing that. Second of all, you may still be able to get away with it, if the load pulls towards that rail. Use it as a good old fashioned "single supply" amp, but with your choice of which supply to single out. And with much more load capability than the ~50uA pulldown ye olde LM358 offered! Operating within some mV can be reasonable this way, at least for CMOS types. Mind that bipolar (complementary open collector style output stage) will still have some minimum saturation.Two common examples I can think of for needing near-rail operation, are:- Current sense: the input is single-ended, and the output is common-rail. You just need some gain so as to get away with a smaller shunt resistor.- Limiter circuit: the output is constrained between rails, and maybe you even run the amp from +VREF so its output is precisely (less that annoying VOH/VOL, that is) 0 to REF.For current sense, maybe the low end doesn't actually matter, you just need to compare with a modest or large threshold/cutoff (e.g. peak current mode SMPS, current sensing/fusing, etc.). In that case: great, who cares!Or, maybe the low end does matter, and you need to measure near zero say for measurement, metering, etc. purposes. In that case, simply offset the input by bleeding in a little offset voltage. If it's a noninverting amplifier, just run +IN from a divider from +VREF to +IN to shunt. A few mV will do 'ya.Maybe you even need a slight negative range, say for a power supply with down-programming (bleeder/leakage circuit) so you can measure it accurately over the whole range; or maybe something something battery metering so you can see charge and discharge currents, but the magnitudes aren't symmetrical so you only need a little negative range. Same idea, use an offset or differential measurement, done and done.For the limiter, three options:- Redefine the supplies so they are slightly in excess of what's needed (problematic if you don't have a negative supply handy and need even just some 10s of mV below GND!)- Redefine your signal's range so that the natural saturation levels are just outside of the required range- Don't use saturation for limiting; you need too much accuracy anyway (i.e. when a ~50mV margin isn't good enough), use a proper active (precision rectifier?) circuit to implement the limit instead.Also maybe worth mentioning saturation recovery, but most modern types boast of their prompt recovery, supported by waveforms or settling time. Amps like TLV2372 are well behaved, I have no problem using it as a jellybean. Simply check and confirm, and keep shopping if you don't see adequate data regarding this.Quote from: Smokey on March 17, 2023, 10:17:16 pmAnnoyingly, "LDO" has been taken to mean any linear regulator these days. So 7805s are "LDOs" now. Maybe we should adopt the backronym "linear dropping output" or something..?Regarding op-amps, there are some with missing +V or -V range (RRI but not O, or RRO but not I; or "RI" or "RO" in the sense of, only one rail is met), something to watch out for. I've seen many precision amps of this sort, which is kind of a shame, but also not such a big deal in a lot of applications. What's more annoying is distributors don't track input and output rail compatibility, so you have to flip through a lot of candidates to find the right one.And again, simply limit signal ranges, don't rely on device ranges if you don't have to. Sometimes this means adding an extra couple volts supply, may be annoying but it's far from intractable these days.Tim

author: TimFox
author_rank: Super Contributor
timestamp: March 20, 2023, 01:51:01 pm
content: A straightforward distinction between the devices called "rail-to-rail" and the others:Depending on the output devices, when swinging the output towards a rail, is the swing limit a voltage (such as the saturation voltage on a BJT) or a resistance (such as the ON resistance of a FET)?

author: Smokey
author_rank: Super Contributor
timestamp: March 20, 2023, 05:11:12 pm
content: Quote from: Andree Henkel on March 20, 2023, 12:15:20 pmOh Look... Now they get all honest....From the LM7705 front page description:https://www.ti.com/lit/ds/symlink/lm7705.pdfQuote2.Applications:•TrueZero Amplifier OutputsI figured out their plan. They trick engineers into using "RRIO" opamps with a misleading name, and then sell you the solution to the problems once you realize your thing doesn't work.-----------------------------------That's actually a killer part. The -0.23V keeps you under the 5.5V max voltage of the low voltage opamps.And the most important part... cost...https://www.digikey.com/en/products/detail/texas-instruments/LM7705MMX-NOPB/2020262QTY UNIT PRICE EXT PRICE1 $0.88000 $0.8810 $0.78800 $7.8825 $0.74800 $18.70100 $0.61440 $61.44250 $0.57432 $143.58500 $0.50752 $253.761,000 $0.40068 $400.683,500 $0.37800 $1,323.00That's pretty reasonable. Especially considering board footprint.-----------------------Note: There is no entry in the electrical specification for output current. Other parameters are specified at an Iout of -20mA, but Iout does not get it's own table entry. They do have plots at various input voltages and temperatures. Digikey lists the output current at 26mA.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 20, 2023, 07:21:58 pm
content: Quote from: Smokey on March 18, 2023, 07:11:45 pmYeah that seems pretty good - especially in a world where some advertised specs are way way off.Also, take the humble ancient LM358 - every datasheet I have seem for it mentions "Vout can go to GND" or "Vout includes negative rail" when in fact, it can only reach within 80mV of the negative rail if the sink current is <~30uA. Now granted this isn't advertised as rail-to-rail, but it does indicate its capable of getting much closer to the negative rail rather than say, the TL072. It did catch me out back in the day when I was still green.Of all the specs to get angry about this is one of the least deceptive. Buying a RRO part and expecting the output transistors to have single digit mOhm resistance is pretty unrealistic. You're an engineer - read the specs that are relevant to your application rather than skimming over the first couple of pages of the datasheet.

author: shapirus
author_rank: Super Contributor
timestamp: March 20, 2023, 07:34:42 pm
content: Quote from: Buriedcode on March 20, 2023, 07:21:58 pmNot really. They say thatinputcommon-mode voltage includes ground.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 20, 2023, 07:42:53 pm
content: Quote from: shapirus on March 20, 2023, 07:34:42 pmI think I looked at the wrong datasheet - I checked the front page of the LM358Nfrom ti.https://www.ti.com/lit/gpn/lm358syas on the first pageQuoteSTmicro's datasheet :https://www.st.com/resource/en/datasheet/lm358.pdfsaysQuoteAgain, I am not suggesting this is misleading, just that, not every spec on the first page of the datasheet should be taken literally or without caveats.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 23, 2023, 07:19:42 pm
content: A datasheet’s first flowery paragraphs, which are almost always written in large, bold fonts, is certainly written by marketing guys. As such, it is full of milk and honey. The phrase “caveat emptor” applies here.The brutal honest truth is always in the middle pages, written in a near-invisible font. It is our responsibility as engineers to read and digest these obscure, inconvenient facts and determine, to the best of our abilities, whether the components will perform successfully in our circuit under our operating conditions.I can always differentiate a green from a seasoned engineer, by witnessing how thoroughly they study a component datasheet.

author: Infraviolet
author_rank: Super Contributor
timestamp: March 23, 2023, 09:05:21 pm
content: Surely if things in the 10mV nearest ground were your target you'd find a way to shift them to varying about a virtual ground instead before feeding them to the op amp, if the signal of importance isn't DC a capacitor in series could do it.

author: Smokey
author_rank: Super Contributor
timestamp: March 30, 2023, 03:38:10 am
content: Wow. I can use the temperature sensor in the AD7191 to measure that "True Absolute Value Cryocooler"!https://www.analog.com/media/en/technical-documentation/data-sheets/AD7191.pdfFrom page 16:"When the temperature sensor is selected, the device should return a code of 0x800000when the temperature is 0 K."

author: Smokey
author_rank: Super Contributor
timestamp: April 17, 2023, 06:41:54 am
content: ...because we were talking about it up-thread...Here is an example I just came across of a "Low Dropout" regulator... First red flag was when they didn't list the actual dropout voltage on the first page..https://www.diodes.com/assets/Datasheets/AZ1117I.pdfJUST1.3Vohhhh soooo lowwww

author: KE5FX
author_rank: Super Contributor
timestamp: April 17, 2023, 06:46:57 am
content: Rail-to-rail: "Nothing weird will happen as the output and/or the input approaches a rail. However, something weird might happen somewhere in between."

author: Siwastaja
author_rank: Super Contributor
timestamp: April 17, 2023, 10:04:57 am
content: Quote from: Smokey on April 17, 2023, 06:41:54 am1117 is ancient crap. It's from the time when the low droupout term was originally coined. Linear regulator ICs of the era (like 78xx) had dropout voltage around 2.5V, so it was a significant improvement - but it came with cost of control loop stability.1117 has special snowflake requirements for the output capacitor, and worse even, different manufacturers who make clones behave differently, so you really can't second-source it without being super careful, which is ironical because being able to second source it is literally the only reason to use an 1117 in modern days (good availability from many manufacturers, a cheap part).Drop-out voltage of 1.3V enabled using 1117 for 3.3V buses out of 5V supply, which was (still is) a big thing; not possible with 78xx or LM317.As always, qualitative terms like "low", "high", "good" require looking up the absolute numbers. Modern-day linear regulators are available down to some 200-300mV of dropout voltage and stable with zero ESR output capacitor.Today, thanks to some idiots who started to call linear regulators "LDOs" and other idiots who followed and never checked what bullshit they spew out, the complete class of "low drop out" linear regulators lost its significance. Which is probably a good end result though, since it's way better to use modern-day parametric search systems to pick regulators with the exact specification you need, instead of some arbitrary classification which is useless for your particular case.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 18, 2023, 08:30:43 am
content: FET output based OPAMPs can be quite close to the rails as long as your drive current isn't very high. All thanks to the resistive nature of the channel. BJT based ones cannot achieve this

author: Smokey
author_rank: Super Contributor
timestamp: January 25, 2024, 02:14:52 am
content: Here is another gem:LM2904Q2Thttps://www.st.com/content/ccc/resource/technical/document/datasheet/c8/d5/4d/58/88/9d/44/9f/CD00000535.pdf/files/CD00000535.pdf/jcr:content/translations/en.CD00000535.pdfFeatures:"Large output voltage swing 0 V ..."Description:"... the output voltage can also swing to ground, even though operated from a single power supply."Electrical Characteristics:"V OL = 5mV typ, 20mV Max"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2024, 02:29:17 am
content: Quote from: Smokey on January 25, 2024, 02:14:52 amFYI, LM2904 is just the high-temperature version of the LM358.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: January 25, 2024, 09:43:43 am
content: 20mV worst case is "to ground", no problem with that. Fat-free milk contains 0.5% or so fat, too. Any designer worth their job know very well it is physically impossible to have fat-free milk, or opamp that pulls to exactly 0V with 0-Ohm resistance. They know that "to rail" means "orders of magnitude closer than some old designs".Your fixation on this matter is harming yourself by slowing you down from learning more. You can keep listing every rail-to-rail opamp on this thread, there are thousands of them, all use similar language in datasheets.

author: magic
author_rank: Super Contributor
timestamp: January 25, 2024, 10:34:06 am
content: LM358 is not even R2R and it's worse than most R2R.However, it works OK when loaded to ground (with single supply) and only sourcing.

author: Smokey
author_rank: Super Contributor
timestamp: January 26, 2024, 01:04:20 am
content: Quote from: Siwastaja on January 25, 2024, 09:43:43 amI'm fine. I understand exactly why you can't actually hit the rail. It's not like every time I see someone advertise an opamp that includes "0V output" that I somehow hold out hope they have a new trick that defies physics.The issue as I see it, as I've clearly stated, is that if there is one place where precision of language actually matters it's engineering, where those details actually matter. (Along with loose bolts and crappy software....) Airplanes can fall out of the sky because 0.02V != 0V. It sucks that we have to keep a list of places where datasheets, which are supposed to state facts about the parts, consistently lie about the facts of those parts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2024, 03:13:18 am
content: Quote from: Smokey on January 26, 2024, 01:04:20 amKeeping a list would be quite tiresome. Are you kidding?!Everyone and everything is lying to you, all the time, to some extent or another. Datasheets, the FAA, your faith leaders, science; all to some extent or another.Science has the unique distinction that it tells you it's lying. At least about the stuff it tells you. There's still as much lie by omission of course; a fact that is well-studied by science itself, and arguably comes down to human nature, alas we only have humans to science, so, here we are.That is, all scientific knowledge is experimental; or if proven from some starting point, then that starting point is assumed correct for purposes of further reasoning. There is nothing weknowthat does not have an error bar attached to it.Error bars are fantastically useful. If you want to bet I jump off the ground and land on my ass half a second later, that would be a pretty goddamn good bet to make. Gravity may be just a theory, but it's a damned good one. Other things are less well known, whether because of lack of nearby information (think of all the rules-of-thumb you've seen, that turned out to be utterly false conclusions from a theoretical standpoint, but which nonetheless are correct more often than not -- "missing the point" but coming out lucky anyway), or because of explicit error in known variables. The "known unknowns" are a lot easier to deal with than the "unknown unknowns", of course. But so it is with the totality of existence.I'd say my greatest development in understanding -- not to say it's an epiphany or anything, more that it's not only proven itself time and again, but proven very useful at that -- is that "truth" is not something you just find, it's not something you are given; no.Truth is a verb.To obtain "truth", is to work actively and continuously to maintain it. The moment you leave it alone, it starts gathering dust, an aura of lies, mistaken ideas gather like cobwebs; it is a willful act to dust them off, shine up the facts, turn them over in your mind, and maybe from time to time, discover some new feature, a kink in the surface, a thread pulling from somewhere else, and you can follow it and spread out your knowledge bit by bit.It is only through this process -- at least, in my mind -- I can't tell, of course, how much the methodology, the imagined structure, the practice, is unique to me, or shared by others -- but I do recognize similarities elsewhere, so it seems important enough to describe here -- that one can grow and maintain "truth".If everything is lies, then we must sort out the stronger lies from the weaker, find what kernels of truth remain in them, and cast off irrelevant noise -- or at least hold it in quarantine, until we develop some ideas about it, find patterns, and match pieces of it into a greater web of knowledge.I think a lot of people, make the mistake, or have the habit of, or maybe it's more basic psychology than that, I don't know -- but when they find some factoid, and, it feels reasonable enough, passes the vibe check, then internalize it then and there. Technical knowledge, to them, is often hard-won; the result of months of grind work in school, or beating their heads against it until finally getting the technique right. It's often put on a pedestal, to be admired; taken down and used from time to time, but never connected with other things, or only if by accident; compartmentalized, put on a shelf. If one understands truth as an ongoing process, then one can see how fragile, vulnerable, this method is; to see exactly where the mistake lies.Not asserting anything about you, by the way -- I do mean this generally. Maybe it applies, maybe it doesn't; but if one finds themselves identifying with (or bristling against, as the case may be..!) these ideas; maybe some introspection could deepen ones' understanding?So, rather than sneering at datasheets and appnotes -- or, well, perhaps in addition to -- after all they deserve every bit of shame they get, and shame is a powerful social force when it's able to act -- it's hard to shame anonymous authors or monolithic corporations, of course, and well, it's easy to see, in part, why they exist in such form -- but to say, okay clearly they're misrepresenting this here, but then again, was I even meant to take it at face value, is it a literal versus definitional problem? Many things in commerce are types, not literal values; no one says "250VAC" and means exactly that, they say "250VAC CAT III" (or whatever, also, they say so if they're being generous..) and the fluctuations and transients of real mains circuits is implicit. Or, no one says "1 inch pipe" and means 2.54 cm ID/OD, it's just understood that, in the US, "1 inch" is a designation not a description, and one must look it up in a table to find what it actually measures.Here, "rail to rail", "single supply", etc. are in contrast to, well, amps that do neither, that operate at most some volts away from the rails, or even narrower than that (going way back in history). So we have the historical type definition case. Clearly they aren't perfectly literal, but they can get pretty close, and with some external bias (or even some special ones have a bias generator internal!) they can indeed achieve it.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: January 26, 2024, 09:48:48 am
content: Quote from: Smokey on January 26, 2024, 01:04:20 amBut airplanes are not designed based on front page marketing material, which the datasheet is. Instead, they look at the actual numbers, in the tables, and graphs. If you see an errorthere, then you have every reason to be pissed, and complain to the manufacturer.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 27, 2024, 10:28:19 am
content: The point is the output can get arbitrarily close to the rails with high enough impedance load, this would always be true as the output pin has some resistance even if it were truly rail to rail. If the load is another FET input rail-to-rail opamp, they have input impedances of 1000Gohms and a few pF...