Precision dual tracking PSU, for characterizing low drift op-amps - Page 1
Forum section: electronics / testgear

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 07:27:42 am
content: Hello voltnuts,I have been designing a precision, dual tracking PSU for use in my lab. The goal of the project, is to design and implement a dual tracking PSU for characterizing precision, low drift, op-amps, A/D and voltage reference ICs.The design goal is nV stability (order of magnitude), so that uV precision/drift can be characterized, within opamps (primarily). I have a need for ultra high stability components, but am finding all of my PSU's to be too high in long term drift. The most precise unit I have is a system I built a while back, with a drift of roughly 8-11 ppm per 7 days (168 hour). It's initial drift is roughly 500nV in the first 6 hours of test (after 2 hours stabilization period).The purpose of this question is to determine the usefulness, and therefore further complexity, of building this new supply out past 10V. A +/- 10V precision supply is quite easy to design and implement, however it's not the full range of most opamp rails (etc). The complexity of building the PSU's beyond a total 20V scale/range is quite a bit more challenging. Especially in terms of software (corrective factors and averaging).I am looking for opinions on what the ideal range is for such needs. Obviously in a precision device opamp drift is quite a big factor in long term output (load) stability. However I find it unlikely that swings of more than +/- 10V are required criteria.I am greatly oversimplifying the topic, for the sake of brevity. However I have to wonder, isn't it more useful to have a PSU that meets or exceeds the full useful range of the IC under test? For example most opamps (comparators etc) have a mean +/- 15V range on their rails, and many more have some headroom out to +/-18v. Obviously to characterize the offset at those potential peak ranges, we need a dual tracking PSU that can output slightly more (overhead).So in your opinions, or personal project needs.....what voltage range would you be looking for, for such a task? Again the drift figures can remain the same for each option, but obviously complexity/price would increase as total dynamic range increases. Total drift = 0.05 ppm per 1000hrs, after 168 hour stabilization time. The opamps and other bipolar devices only need to pass stability windows of (roughly) 8 hours. They are used in an automation system and are nulled and reset after each production cycle. They need to maintain stability in a +/-100 ppm range. So the PSU used to power them should be just fine @ 0.05ppm/1kHRS (possibly overkill).Any opinions on the subject would be appreciated, but I am mostly concerned with opinions on the total dynamic range...and have set a poll to keep that topic simple.Thank you for any insights and opinions...

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2015, 09:26:46 am
content: Quote from: TunerSandwich on January 20, 2015, 07:27:42 amOf course.Anything but one of the world's lowest drift references is not going to be good enough.I assume the current requirements are going to be bugger all? Like a few mA? In that case you can use just voltage references and not an actual PSU. You could have multiple ones for multiple rail voltages.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 09:56:36 am
content: Quote from: EEVblog on January 20, 2015, 09:26:46 amactually this is where things get interesting.....the need is for about an amp of headroom....the design is based on Ti/BB REF102 (AA grade)....which needs to have miniscule load on it, to maintain that tolerance. The buffers leading to the current sources need to be extremely stable....hence the need to characterize the total driftthe automation system I am designing has a fluid discrepancy limit of about 0.1mL/500mL (MAX allowable).....the input is a precision 32 bit A/D (31 usable).....before I build multiple inline, production control units....I have to characterize and bin components for the tightest possible tolerance....I hate doing this stuff in the software domain and the automation systems run in climate controlled rooms. I had considered ovenizing the whole control package (Still might do it), but that's a story for another dayMy main concern is maintain a 10:1 measurement chain....and hopefully we wind up with a system stability of around 8ppm +/-.....no one really makes a PSU for this specific task (I want to characterize dozens of components, simultaneously, across the same rails). Buffering the leads to each IC socket is also part of the problem....this is why I need 2 orders of magnitude better stability form the sourcethis BOM is getting expensive....so part of my solution is to run 3 parallel REF102's (filtered) into a MUX for each voltage range.....mech relay switches in the next 10V parallel set for 9.99999 V to 19.9999 V......those two rails need to track each-other, with minimal drift, during potential transient peaks (I.E. when actuators pull current off the mains)

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2015, 10:22:46 am
content: Good luck on this one!What's your time frame? Nailing this in one go isn't going to be realistic, this one has a Murphy around every corner.

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2015, 10:24:45 am
content: Quote from: TunerSandwich on January 20, 2015, 09:56:36 am1AMP? with nV level stability?Bloody interesting stuff of course, but again,

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 10:30:58 am
content: Quote from: EEVblog on January 20, 2015, 10:22:46 amOh believe me I know that. This is most certainly not round one of concept. Just to get to the point of realizing I need to build this PSU, has been many designs and simulations, and failures....been quite a fun journey actually...The inevitability of random, unrepeatable results, is part of my encompassing equation....

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2015, 10:36:04 am
content: Quote from: TunerSandwich on January 20, 2015, 10:30:58 amHaving done similar stuff, albeit not quite to that same precision, I am not the least bit surprised by that.This is 6 months of Jim Williams looking like this while holding his tounge at the right angle level stuff:

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 10:37:26 am
content: Quote from: EEVblog on January 20, 2015, 10:24:45 amthis is a block diagram of the original REF.....the stability is there, but the current isn't.....driving the DUT is the issue.....with stable loads it's no problem....but maintaining the uncertainty across large dynamic events (turn on times etc) is the issue.....who knows if that random 500nV creeping in is related to load transients across the meter or the DUT? My guess is the meters, but guess aren't good enough sometimes....

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 10:47:41 am
content: Quote from: EEVblog on January 20, 2015, 10:36:04 amSometimes, just for fun, I open up the case and breathe on the REF pcb....just to watch my fancy meters move.....good times

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2015, 11:28:01 am
content: Quote from: EEVblog on January 20, 2015, 10:24:45 amI like the suggestion of using references directly. My rule of thumb is that the amount of effort required to reach a certain precision grows exponentially with the number of stages/elements added.At 1A and nV precision the wires are going to play a huge role as well. 1 milli-Ohm of resistance already results in a voltage drop of 1mV at 1A.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 11:38:25 am
content: Quote from: nctnico on January 20, 2015, 11:28:01 amIndeed it does. The general construction of modular sections, has to be accomplished with point to point construction. This was a problem I ran into a long time ago.....the planar impedance vs. frequency in most PCB substrates causes some resonant issues, and results in having to filter/compensate for that. Which once again diminishes stability, because of more control systems.There are some easy ways around these problems. The main voltage divider (for output control) is a kelvin-varley quad decade pot (ESI dekapot). I have characterized it's drift at sub uV, full scale. It's essentially a "metrology grade" divider....being passive and linear, it requires only one gain stage between it and output to the current source.....obviously finding a current source with high enough stability across an AMP, is another issue, but that is why the design is modular.....the ref interconnect into the amplifier stage is parallel.....so ultimately the stability is up to the precision of the amplifier (located at the actuators)...IF our REF out is stable to an order of magnitude above the POL.Again the real problem in discovering that answer....is being able to characterize the components themselves, and determine which fall within the tolerance of the control signal algorithm. The less total drift between the system components, the less total band-aid software needs to be written.Clocking is a separate subject, but thankfully we can buy nice clock REF for quite little $, in prepackaged enclosures.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 20, 2015, 09:19:31 pm
content: If PSRR is in the vicinity of >100dB (10^5), how the hell do you need any better than 1mV at the power supply to get 100nV at the output?Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: January 20, 2015, 09:25:11 pm
content: Hi TunerSandwich,I do not think u can use this setup.The DMM is not good enough, you need a 3458a in a stable environment.Also you need better connection to your KV divider (AC & Temp shielding), Mr Seeback is a bitchWith a self-designed 10V voltage reference in a oven, i measure about +-0.2PPM short term drift.Most of it is due to Mr Seeback...because you want to achieve something at the border is what is possible.I hoop to see more of your project on this forum :-)Kind regartsBlackdog

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 09:39:25 pm
content: Quote from: blackdog on January 20, 2015, 09:25:11 pmThe DMM is obviously in question, but to determine that we would need an order of magnitude of stability beyond it's capability first.The 3068 is "good enuff" for some preliminary characterizations, as the source rejection/smoothing of the max6350 isn't good enough to eliminate the horrific noise from the AC/DC conversion feeding it.This is a cart before the horse kind of issue. Have to know we can trust the stability of the REF out, before we can even quantify doubts on the rest of the signal chain. Obviously a final cal would be done against the 3458a, as well as against a null standard cell.....The connection to the KV divider is a "worst case scenario" and piss poor in this test. So obviously we can do better with some solid coax and some correction on it's output.The main intent here is to illustrate that in the worst case scenario, the KV divider is at least good enough to represent 4 decades of linearity.....the 20V range would be a x2 indicator....driven by a mech relay, switching in the next bank of series ref102's (again another parallel set into a mux).Ovenizing the package is is really the only way to get to the drift specs I would like, but at this stage determining the total V scale seems paramount. I think the 40V (+/- 20V) seems the most logical, considering the total scale of the OPA227 is 36 V? Would you agree that seems like a reasonable assumption (gives 2V headroom full scale)

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 09:42:40 pm
content: Quote from: T3sl4co1l on January 20, 2015, 09:19:31 pmlong term stability....remember the PSU is simply a device that will be used to characterize and bin each component in the REF and current source for the automation control package. The drift of AREF in the final production units only needs to be uV stable. Obviously to determine that, we need a PSU that is an order of magnitude "better". Not sure you read the full scope/purpose of the PSU....or I possibly didn't go into enough detail on the intended purpose...

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2015, 09:47:57 pm
content: Maybe it could need some more explaining... I guess you are using the PSU also to provide various input voltages for the opamps?

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 20, 2015, 10:03:15 pm
content: Quote from: nctnico on January 20, 2015, 09:47:57 pmyes, the PSU is only needed to determine/characterize the bipolar components and bin them into groups. The stability needs to be determined full scale, at various points in the total range. The components which exhibit the closest range deviations will make up a "matched set".Again the final production units only need to stay stable in the uV range, but to get there we have to characterize the component groups with an order of magnitude more resolution....in which case the PSU needs to drift no more than 500nV full scale.The 1amp requirement is simply there to allow overhead in the dynamics of the potential load......the load might be a few banks of socketed op-amps.....so after the PSU is stabilized, and we introduce the load, the drift isn't wildly skewed by the turn on transient. The junctions will pull some current until they reach saturation and stabilize.....

author: rx8pilot
author_rank: Super Contributor
timestamp: January 20, 2015, 11:31:54 pm
content: My head hurts just reading this thread. It sounds like a circuit spec that would crumble when a cat sneezes 8 miles away.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 12:33:10 am
content: Quote from: rx8pilot on January 20, 2015, 11:31:54 pmWith proper isolation/shielding, and a fully "ovenized" package, it isn't really that difficult of a spec to maintain. Most precision medical and pharmaceutical research equipment reaches these levels of calibration.The issue with ovenizing is cap life....a lot of the components require tantalum....thankfully it's solid tantalum, but ovenizing at 100 deg or -20 deg represents the ideal conditions for the REF102.....the secondary cells (battery) needs to have a separate chamber at either of those temps....but again that isn't all that difficult. Lots of good TEC control modules out there, and limp mass vinyl makes an excellent (thin) insulation layer.Thankfully our production rooms are calibrated to 20 deg C +/- 0.5 deg...under production conditions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 02:38:23 am
content: What insane spec are you trying to bin that is so utterly dependent upon supply? I still don't see how my point about PSRR falls.The only thing I can possibly imagine being so sensitive is output saturation. If you simply measure that relative to the supply rather than to ground, your problem magically goes away...Tim

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 04:34:32 am
content: Quote from: T3sl4co1l on January 21, 2015, 02:38:23 amHave a look at the block diagram I posted earlier......I am not talking about opamp rail supplies....I am talking about characterizing drift full scale, and at what point the specific IC's reach a point of "acceptable" stability. If they can't reach a point of stability equal to the REF....then I have chosen the wrong op amps for that purpose (which is why such a PSU is needed to begin with).Even Ti OPA227 (AA grade) IC's are going to have a high and low SDEV, across a few dozen discreet parts. I am more worried about the PSRR of the pre-regulator and REF itself. If none of that mattered, I could just whack a nice zener inline with a CC source and call it all done.....Maybe the opamps themselves need to be driven with a pre-regulated zener? That remains to be seen....but of course that is going to need an equal investment in characterizing the FETs driving current into the line.In any scenario, we have to have a point of reference to make such decisions by. I think the overview is being mis-interperated, or I am poorly articulating the process. First we need the PSU, to make decisions about what is and is NOT acceptable for the final product. It's entirely possible that the goal is unrealistic, but to even answer that I need some situational evaluations of each individual IC.P.S. I highly doubt the PSRR of the op amp rails is going to be an issue.....>100dB is obviously more than acceptable, however that is not what I am trying to quantify....the signal i/o ratio is either going to match the SDEV of the REF out, be higher, or somehow magically lower (which I seriously doubt). Output voltage vs time is rated @ 5ppm/1kHR and far less after 168 HR stabilization time. That is a "typical" rating. However Ti is showing that some IC's exhibit orders of magnitude better stability. So it's luck of the draw. The chips are cheap.....so the devices that beat the nominal rating get binned "premium" and the rest get tossed in the "jellybean" bin for use elsewhere.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 05:02:10 am
content: here is an example.....although it's vastly oversimplified, in comparison to my intended application, it's a relevant scenario....I.E. the SDEV in = x, does the SDEV out also = x?Without isolating the opamp and characterizing it with an order of magnitude more resolution, then who knows. Just whacking it inline and assuming each IC will do the same thing is not really acceptable.So the need to bin each IC is real. Even if the outcome is only the realization that that specific opamp is NOT an acceptable choice, at least that is an answer with some basis in fact, and not an educated guess, or blindly following a datasheet.The more relevant concern (at this stage) is how much overhead is prudent for the dual tracking PSU? The OPA227 full scale input is 36 V. So it would seem that +/- 20V is the ideal rail PSU, as the 2v headroom on top and on the bottom seems like it would produce a more accurate result. If it was +/- 15 V then a full scale transient might clip and that measurement becomes "useless" (aside from knowing it clipped). This is why i posted the poll. I am genuinely curious as to what others thoughts might be on that specific topic. I'm not sure I see the usefulness for 60 V of dynamic range, for this application, but that is why the poll is up. If 70% of folks think they need +/- 30V, then maybe I should assign some more weight to the validity of that dynamic range.example :

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 21, 2015, 09:05:23 am
content: Hello,back to your first question, about the stability of the double PSU, I fully agree to what Tim already stated.The OpAMP mostly have high PSRR of > 100db, at least.The OPA227 has 120dB for DC.That means, that any change on the PSU rails will be rejected by the DUT - OpAmp by a factor of 1E-6.10nV output stability would mean, that he PSU would be allowed to change by 10mV for the OPA227, for example.This requirement is easily met even by medium grade PSUs.Therefore, if you measure noteworthy variations on the output with only 500nV variation on the PSU, this effect definitely does NOT come from the PSU! Even instability effects from CMRR should be easily supressed by this OpAmps, and most others, also.Excuse me, but I think you have an error in reasoning about your circuitry..Instead, you may search for many other obvious effects, as the others already mentioned, namely Seebeck thermal voltages, offset voltage drifts on your OpAmps, DVM, reference voltage (REF102 is very mediocre in these terms), and so on.Even ageing may come into play. For example these REFxy have about 50ppm/khrs, therefore that's 0.05ppm/h, 0.25µV/h for a 5V reference. And that's 2µV over 8 hours, isn't it? Even a LM399H is not much better.A LTZ1000 reference would be much more suitable for your purpose, I think.And if I look on the picture of your breadboard setup, it's very obviously, where all these drift sources come from, especially thermal voltages.It's still not fully clear for me, what you really want to measure, and why.But I have the gut feeling, that it's not possible to realize such stabilities especially with the components you intend to use.Your requirements are really on the absolute edge of analogue measurement techniques @ room temperature environment, where normally only the highest grade of components and instruments can cope with, but definitely not the class of equipment you are using.For example, 10nV DCV is the lowest limit of difference in voltage, you can measure with analogue measuring instruments.(That's the reason, why even from 2018 on, the SI-Volt can not be realized practically better than 1E-9, although it will then be uncertainty to ~ 1e-16, by JJ array.)For that level, you normally use these Keithley nV meters, or the 34420A.Even the 3458A can not cope with that stability, although it resolves 10nV!Btw.: the ESI Dekapot is a nice KV divider, but also has relatively mediocre 2ppm/K drift for the ratio.Really stable is the Fluke 720A with 0.1ppm/K, or you may use selected metal foil resistors (e.g. VHP202Z) for the divider.At last, I would like to understand better what you REALLY need, to think of possible solutions for your problem, or to identify possible over-engineering..If you want to measure fluid flow or - level, and your stability / uncertainty requirement is only 0.1mL / 500mL = 200ppm, then I feel that the sensor system by any kind of volt measurement is not appropriate.In other words, the gain or translation factor of your sensor (from fluid something into volt output) is too low.Voltage measurements can easily made stable on the 1ppm level, but you quest for < 0.1ppm, so the gain of the sensor maybe < 0.001 ?! What's the principle behind that??Frank

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 09:20:18 am
content: Quote from: TunerSandwich on January 21, 2015, 04:34:32 amIt's a bunch of things linked together, but I don't see any purpose to it, or what it applies to in this thread. There's no "DUT", there's no voltages and currents, there's just arrows and boxes... electrons don't flow in those though...QuoteWell then why are you talking power? Because that's just microampere stuff. The only reason you'd use anything mA is simply to keep impedances low and swamp the noise (a valid purpose!).What do you mean by "full scale"? Can you give a working example? Are you literally doing like just inverting / noninverting amps and running them at various operating points? There are better ways to do that, like shifting the power supplies around to substitute for the common mode range, and preloading the output with a resistor to either side to account for current gain effects.It's true in all domains, from fundamental physics to numerical computation: minimize differences wherever possible. Any time you're adding a very small number to a very large number and trying to determine the difference (if any), you're almost absolutely doing it in the very worst way possible!QuoteAnd by "driven", do you mean supplied? That's also confusing...Tim

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 11:12:46 am
content: Quote from: Dr. Frank on January 21, 2015, 09:05:23 amthat is one of the lowest drift 10V REF on the market, when temperature controlled. Correct me if I am mistaken, but LTZ1000A is 7V ref and is not as easy to implement or trim (and triple the price....not including additional supporting components). I had discussed thermal considerations earlier on, and that is a given.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 11:36:11 am
content: Quote from: T3sl4co1l on January 21, 2015, 09:20:18 amDef not limited to that use.....

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 11:48:47 am
content: Quote from: Dr. Frank on January 21, 2015, 09:05:23 amIn your opinion is that an easier way to implement a significant degree of greater stability, when compared to Ti's "low noise" application of the REF102C? Not at all opposed to investigating that product again.Out of curiosity, what is your inclination to the poll question (main thread topic)?

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 21, 2015, 12:16:23 pm
content: Quote from: TunerSandwich on January 21, 2015, 11:12:46 amWell, maybe. But in your setup - is it already in an oven?If not, these 2.5ppm/°C .. 10ppm/°C are not good at all, and you're going after drifts in this range!?Which voltage do you use, 2.5, 5 or 10V?Then, 5ppm/khrs were claimed as a typical value, but in the hermetic package only.. The plastic package has 20ppm/khrs already (not 50 ppm/khrs, as I assumed) Which one do you have on your table, actually?Concerning the LTZ1000, there is no free lunch.. if you really need that level of stability, there's no other way than to spend the money.The LTZ1000 voltage of ~ 7.12.. 7.15V can not be trimmed at all.It is fixed and given by the silicon and slightly changing by the fixed temperature of the internal oven and the external components.Same goes for the LM399H, and both need a stable, trimmable amplifier , if you really need an even reference value (5.0000, 10.0000V, whatsoever) .See Fluke 732B or 7001, what is realistic to realize with PWW or other ultra stable resistors, concerning T.C. and time stability, i.e. <0.04 ppm/K and < 1 ppm/yr.At best, you use the 7,1xx V directly as a reference for your A/D, and scale by software.. for calibration of the system, you have a precise DMM available, anyhow.You are putting a lot of effort in complicated oven construction, and other high grade components, just hoping to come in the proximity of the performance of the LTZ1000.., And you REALLY are posing the question, whether the implementation of the LTZ1000 is not so easy??Compared to your approach, the circuitry of the LTZ1000 is relatively simple, and only 3 or 5 precision resistors are needed for <0.2ppm/K , < 1ppm/yr. stability.If you really sum up your BOM for the reference circuitry, I doubt it will be much cheaper than simply using an LTZ1000. Not speaking of the LM399H, which easily delivers < 1ppm/K.Frank

author: dannyf
author_rank: Super Contributor
timestamp: January 21, 2015, 12:26:56 pm
content: QuoteNot sure why you feel that such a beast is needed to characterize an opamp - they aren't that sensitive to unbalanced rails (to a point).If you have to go down that path, one approach would be use a reference for one rail and then an opamp to invert that reference for the other rail.Another, in the digital domain, would be to use an adc+mcu+dac to measure and maintain each rail.Totally unnecessary but does that job you want.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 01:14:49 pm
content: Quote from: Dr. Frank on January 21, 2015, 12:16:23 pmAhhhhhh I am starting to see where we have a disconnect here. The device on the table (on the perfboard) isn't my Ti based REF. It was simply there to push 5V into the dekapot, to get some initial short term drift results out of it for this post. That was by no means the Ti REF102C based module. That is a just a REF example I built to illustrate some resonant tank issues for another thread about SMPS. It's just a MAX6350 with a 350T behind it (and some purposely implemented capacitive issues). I thought I had described that earlier. Which is why I had mentioned lets not even talk about grd connections or temp considerations of that signal path....it's generally only 8-10ppm stable (maybe).The REF102C prototype modules are sealed in an insulted metal oven. The plastic package has the same 5ppm/1khr drift figures as the can, AFTER 168 hours (the can isn't available anymore). It's really quite a nice REF, as it's cheap, easy to implement, and only 3ppm off the LTZ1000A 1kHR stated figures.I hope it's being understood that I was just showing the dekapot in my home lab. The ambient conditions here and lack of quality power don't even warrant attempting to measure the REF102C module here. The lab at work is calibrated to 20degC and has some half way decent power at the wall.Maybe this is where our disconnect is?Back to the LTZ1000 though. It was one of the REF IC's I had played with and considered, but I found it's implementation to not be as straight forward as the REF102C and the extra 3ppm claimed on the datasheet didn't really amount to that in the real world (maybe I just didn't give it enough time or a fair chance to shine). They were generally similar in our lab test environment and the LTZ1000 is more difficult to trim (work with), because of the required external components.I think there is a large disconnect, and maybe I am not articulating fully what this thread is about. Going into extensive detail about the hows and whys around choosing the REF102C was not my intent. The implementation is actually quite a bit easier, for the intended tasks of that building block. I had mentioned earlier in the thread, that I didn't really want to deal with software....so series 102c's switched in with relays was the approach. I am really looking at implementation from both a module and system standpoint.....I would bore you to death if I sat here and described the entire automation system. I digress.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 01:22:05 pm
content: look at note 2 for the P, U packages.....the M package (can/TO) is discontinued.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 01:41:52 pm
content: also.....http://www.ti.com/lit/an/sbva002/sbva002.pdf

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 21, 2015, 02:02:29 pm
content: Yes, the difference in understanding is obvious now..Just a brief correction.. The LTZ1000 and also the similar Fluke LTFLU @ 45°C deliver typically < 1ppm/ YEAR (i.e. in 9000 hours), that's proven by experiments, and by really existent instruments.Whereas all these other plastic references deliver maybe as low 5ppm / 1000hrs, maybe 20-30ppm typically per year...see all these 6 1/2 dig DMM specs.That's not a mere 3ppm difference, that are nearly 2 orders of magnitude better stability for these ovenized, buried zener "Reference Amplifiers".Sorry, but the LTZ1000 really plays in totally different league.The description of your setup, I will have to analyse later.Frank

author: macboy
author_rank: Super Contributor
timestamp: January 21, 2015, 02:11:40 pm
content: Youmustget rid of those aligator (croc) clips, nickel plated bananas, and test clips. I'm serious. The thermoelectric voltages generated in those will be throwing everything off. Just 1 degree C can result in several microvolt of difference. The slightest draft ... all bets are off.Strive to use only pure copper. Copper oxide is horrible, so try to use a good contact cleaner that will leave a slight film behind to protect against future oxidation.I like to use twisted pairs removed from good quality plenum-rated ethernet cable. The twisted pair helps reject noise, the conductors are solid copper, and the insulation (for plenum rated cable) is Teflon/PTFE.I have three 7.5 digit meters, and when I wire them all in parallel with banana leads to a common voltage source, they drift around within +/- 7 ppm or so of each other over time. When I use leads made from copper twisted pairs, they track within < 1 ppm of each other. This is with ~20 V input. At ~2 V input, the difference is greater (those microvolts due to thermals are relatively bigger).Read and understandKeithley's Low Level Measurement Handbook. The wisdom in it will help with both your measurements and your design.p.s. Some older high precision equipment like KVDs, voltage references, calibrators, etc. may be soldered with Cadmium bearing solder, due to its almost nil thermoelectric potential vs. copper. Just beware if you go poking around inside these.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 02:24:53 pm
content: Quote from: Dr. Frank on January 21, 2015, 02:02:29 pmSee the chart again. There is one test unit that exhibited 1.3ppm between 1 hr and 14205 hrs. Obviously every other unit tested did worse, and some by orders of magnitude.....but those tests ambient conditions weren't fully described.I have found that the low noise configuration/application (posted that basic schematic earlier)....when temp controlled and components selected/binned does far better than those "MAX" warranted claims.Now you may well have a good point about the total cost nulling out vs. the LTZ1000A, but the implementation still remains ideal. It's a 10v REF with some broad applications, and is easily trimmable. So 10 V ref/supply is easy, and so is 20 V and 30 V and so on. Just switch them in with a nice/stable relay.Also, by nature, it's a fundamentally different IC, and has broader implementation....at least form my experience. I like the part, but am totally open to revisiting the LTZ1000A, if I can see some data showing sub ppm stability over 1 year, and an implementation that suits my needs.....that seems worth re-visiting.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 02:30:20 pm
content: Quote from: macboy on January 21, 2015, 02:11:40 pmI think you missed what that picture/test is showing....read back a couple posts.Should I just delete that aside?(EDIT: removed to avoid confusion). As it seems to be confusing everyone, that it is somehow the Ti REF102C module....again read back and you will see that you aren't understanding what that signal path is there to illustrate.Quote from: TunerSandwich on January 21, 2015, 01:14:49 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 08:29:11 pm
content: Quote from: TunerSandwich on January 21, 2015, 10:52:19 amWhat is this? Is this what you are intent on measuring?Do you know how those measurements are performed...?!Quote from: TunerSandwich on January 21, 2015, 11:36:11 amYou're not giving us much information here.Tim

author: nctnico
author_rank: Super Contributor
timestamp: January 21, 2015, 09:04:12 pm
content: I'm quite lost too regarding the purpose of a highly stable PSU.When used to power an opamp the PSRR will eliminate the variations in the power supply voltage. So unless the PSU output is somehow coupled into the opamp's inputs I don't see the need for an ultra stable power supply. Low noise definitely yes because the PSRR goes down rapidly at higher frequencies.OTOH If the opamp inputs are connected to the power supply I don't see the need for a 1A current rating.@Tunersandwich: please enlighten us with what you are trying to achieve hereedit: typo

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 11:49:32 am
content: I'm a bit curious why some folks are not seeing the correlation between noise and stability.....they are one in the same....when related to measurements. Obviously they aren't technically the same phenomenon, but stability is impossible to determine, through noise. That is a limitation of measurements and the physics governing them.....PSRR in the opa227 is 2uV/V +/- 2.5-18V.....that is an incomplete specification....but even that specification given determines that characterizing the stability is warranted, in relation to even a uV stable source....before we even go down the road of PSRR and it's governing parameters (PSRR is stated across what frequency range? Ti shows steep rolloff in PSRR and I will link the chart) let's talk about noise vs stability and how they are one in the same (interrelated to measurement)There are dozens of issues being explored in these replies.....and sorry for being vague and not addressing each one, in the same reply.....but let's take them one at a time....since most replies are focusing on LT branded REF's and there is a heavy emphasis on Mr. Jim Williams here (don't know why, he's not the only one who has written on these subjects).....I thought it prudent to post links to two articles....both from LT and one written by Mr. Williams himself.....I see a trend here into grouping discreet discussions into one encompassing "answer me this" type of statement.....I.E. Mr. TunerSandwich explain to my WHY you need XYZ....ok, no problems there....but let's go one topic at a time....thanksI am still further dismayed, by the unwillingness to answer the general topic question.....what is the ideal dual tracking PSU voltage range....for characterizing and experimentation of bi-polar devices....I digress.....here are the articles....http://cds.linear.com/docs/en/application-note/an124f.pdfhttp://cds.linear.com/docs/en/design-note/dn229f.pdfAs noted by LT, Ti is VERY open about HOW the linearity of the REF102C is determined (in regards to temp/hysteresis) and that gives me confidence to choose their product. Which has worked out quite well in reality.So again before we point a finger at Mr. TunerSandwich and say things like "you don't fundamentally understand xyz".....please re-read the intended use of the PSU concept.....Our administrator Mr. Jones seemed to get it's intent right away. I want to drive banks of socket IC's.....to quickly determine the performance characteristics of each IC, and then bin the units appropriate for my needs....therefore we need some headroom in the PSU.....so that we can have confidence that it retains stability during power on transient events.....I pretty plainly said in the beginning here, that I have a goal of nV stability, and that might in fact be overkill, or an unattainable goal....ok fine....that might be factual or not....maybe all that is necessary is 100's of nV stability....fine.....but getting to a point of determining such things isn't as simple as looking at some spec sheet and saying things like "PSRR = x, so yer nuts". That is vastly oversimplifying things.....PSRR at what freq, at what scale etc.....please let's be a little more objective about such statements, and not throw me under the bus so quickly. I may in fact have some insight that others do not, and others may have some I do not....granted......but I have as much experience as the next guy in this forum, and don't want to go down the road of answering patronizing question or statements.....If we aren't peers or equals here....then I don't really want to play along, and would rather devote the time to actually building these devices....if they fail, they fail......but trying at least leads to understanding and insight, for those with an open mind to such things.I am not sure why folks don't want to discuss the ideal voltage range question but that is obviously out of my control

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2015, 12:13:25 pm
content: Quote from: TunerSandwich on January 22, 2015, 11:49:32 amAbsolutely, noise must be low to achieve high accuracy. After all, what is drift, but the tail of the inevitable 1/f region...QuoteThat was the easy question, uncontroversial. What did you expect?Which since you did a poll, it looks like folks are quite literally evenly divided on the subject, as long as it's a useful range (enough to test amplifiers made for the 5-10V domain, it would seem). Personally, I voted 30V, because, if you're going to plate everything with gold, then plate that gold with scandium... as long as you're going all-out, you might as well not just stop there...(Note for those unaware: scandium is a metal somewhat similar to magnesium or titanium, not exceedingly rare in natural occurance, but very expensive due to its difficulty to concentrate and isolate. If it were easier to extract, it might be useful for aerospace alloys and stuff, but as it stands, it's little more than a trace element, occasionally used as, to put it one way, a dopant in aluminum and other alloys. It's very easily worth more than gold, but because of its small market, it wouldn't make as good of an investment. Plus, if you wanted to go out and buy a kg of the stuff, you'll be waiting a long time, because 1 kg is a sizable percentage of annual production!)QuoteI would've just as well assumed that noting PSRR would suffice to 1. bring up some discussion on the actual requirements here (as a sufficiently experienced designer would recognize this concern and be able to state the reasoning behind it), and 2. the rather strikingfive order of magnitude differencewould drive rather a bit of weight behind that assertion, even with such a simple statement.I guess neither has really taken root, so, I guess I have to ask: are you not sufficiently experienced to recognize this concern? 'Cuz see, at a glance, you're just some poster (and so am I): who knows what kind of experience either of us have. If that means asking patronizing questions, shouldn't you be prepared to tolerate a couple of those? I mean, this IS the Internet after all.....I personally take pride in being able to make some rational statement or defense concerning every single point of every circuit I make: down to every last resistor! The way it should be. Those reasons may not always be the best (almost always, there's some element of "well, that's just how I did it", but hopefully that's only applicable to the least important elements of the design), but I have them, and am ready and happy to talk about them.Tim

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 12:59:16 pm
content: Quote from: T3sl4co1l on January 22, 2015, 12:13:25 pmIndeed. I did an experiment, in another thread, showing that the SDEV of a simple BNC to banana adapter was enough to completely nullify any usable uncertainty results (in true precision measurements). That was only in application to uV stability (I built a fancy chart showing the total deviation across time and across mean of mean).One aspect I do like about the Rigol dm3068, is its truly excellent statistical functions. It has quickly become my favorite bench DMM, because of it's functions. Granted it doesn't have the greatest performance in small signal offset, but that is easy enough to quantify and null....Consequently that same experiment also showed that a 1/4 watt "jellybean" 50 ohm resistor (in the BNC terminator) will drift beyond that same uncertainty level, when it fully polarizes/saturates. The total SDEV of the measurement went down, as the resistor "settled", however the average offset went up.....which obviously correlates to everything we know about such devices and the physical properties of substrate materials.Determining the actual noise and then relating it to "stability" is quite an exercise in itself....but I suppose the simple proof of that concept is watching your least significant digit bounce everywhere (during fast integration times). That is a blindingly oversimplified "mistake" I have found a majority of people make.We can nullify or at least determine/quantify "noise" and attempt to remove it from our SDEV, but at a certain point that ideal collapses, due to not having a "known standard" which is many orders of magnitude beyond, on hand, and nulling against it for each measurement cycle (and of course factoring in it's own non linearity). I always try to make that a topic of discussion, as I see an overwhelming number of people ignore that concept. Also i suppose it's wholly impractical to employ a PJVS along side your bench meter

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 01:25:55 pm
content: Quote from: T3sl4co1l on January 22, 2015, 12:13:25 pmActually that topic should be more controversial IMO. I.e. what do bipolar devices do when they slam the rails, for miniscule amounts of time. What non-linearities can be predicted/repeated etc etc etc....Also what levels of headroom are really prudent to maintain a determined "spec".....as we approach the absolute peaks of the rails, what kind of artifacts should we be looking for.....also, how long can we hold a signal close to those rail limits and not see a significant de-rating of the theoretical idealsI am not sure that is such a simple answer, but you do make a good point of...."screw it, just go for 60V".In response to lapping conductor substrates with various alloys. We actually did some experiments with plating and vapor deposition a decade ago. We were experimenting with coatings on pins in audio connectors (XLR/cannon) and had some very interesting findings indeed. We tried the same coatings on AC lines and plug sockets and found some equally interesting results.I would love to discuss that topic at length, but will cut myself off here, to avoid mucking up the other concerns.....but the long and short of it, is that the benefits were outweighed by ambient conditions.....especially oxidation and other effects. The big one was clamping force across multiple insertion cycles......gold may beat out nickel in a theoretical ideal, but hardened electroless nickel actually yielded "better" performance over time....gold is soft =) Rhodium and other exotics is a topic for another day though

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 02:04:10 pm
content: Quote from: T3sl4co1l on January 22, 2015, 12:13:25 pmPSRR is a bit too complex to sum up in one measurement. I.E. what frequency....what scale....what kind of feedback configuration etc etc etc.....obviously anyone who deals with the vastness of bi-polar amplifier applications has to be a bit more objective and derive some of their own characteristics, based on intended application.I did, however post some basic specs.....2uV/V +/- 2.5-18V. I think that is enough to at least begin to see why I would want orders of magnitude more precision....to gain insight on what happens outside of those narrow specs. Maybe that is a bad assumption on my part.The PSRR roloff on the OPA227 is pretty steep and really does require some support to get it's 100dB claim, broadband...maybe it's not the best device for the task?It's entirely possible that I am missing a fundamental point you can lend, but across my experiences and statements made by others here.....I just don't see it.Again 2uV/V +/- 2.5-18V is Ti's stated spec.....for offset. However that is a wholly incomplete spec to make determinations I need to make.I am not bypassing the point.....I am simply attempting to move past it, as the PSRR is not in fact 100dB "broadband", and the offset is obviously the figures stated above PLUS the figures outside of the stated scale.....I understand that most opamps are near unity gain devices....however NEAR is the key there. No opamp scales 1:1 signal vs supply....I want more insight on that topic, and want to remove artifacts from the supply from the equation.....so that I can have some personal confidence that the additional discoveries I make, are fact and not due to some unrepeatable/random blip in the supply.....is that overkill? possibly.....but tell me why with MORE than a statement like PSRR = >100dB. That's just too vague and doesn't encompass the whole picture or possible applications. I.E. can you think of a scenario where the PSRR is in fact NOT >100dB? I can.....but again I think there are larger issues...I.E. is it even possible to build a nV stable PSU with an amp of headroom.....who knows how useful it is.....one would have to build it first and then attempt to make a discovery using it.....we can't say that it's not, simply because it's uncommon or the datasheet is the holly gospel on every possible performance aspect of a device.....I think that is a bit blind, but maybe that's just me and maybe you can lend some insight on where I am "wrong".The way I have always viewed electronics is that there are worlds of discoveries to be made, and we don't wholly understand how or why things do what they do. After all any idiot can flip a light switch, and a smaller portion of idiots can describe why something happens, and an even smaller portion can describe the physics behind that relationship.....but not one of us can fully describe the behavior of electromagnetic forces. I like to think I am in a very small group of idiots who want to know more and am willing to take some risks to make determinations. After all a determination of something not working the way you thought it would, is just as useful as if it did work.....at nV levels I am sure that there are some interactions that aren't fully described in anyones datasheet.....I could list a dozen experiments i would like to run to make some determinations, and why I would like a precision/stable PSU.....is that what you would like to discuss? If so I am game and my mind and ears are open....

author: nctnico
author_rank: Super Contributor
timestamp: January 22, 2015, 02:05:02 pm
content: @Tunersandwich:Regarding your poll: it is hard to answer which voltage range is the best. Every now and then I design using +/-15V but the majority of my analog circuits use 3.3V or 5V at most. I voted for +/-15V but a power supply going from +/-5V could be just as suitable.Reading your last post reminds me of analog semiconductor design class where we had to use translinear loops to describe the behaviour of an opamp. Suddenly the transfer function turned into a tangent based function which most certainly depended on the power supply voltages.If you know the internal structure of an opamp you could create an accurate semiconductor model from transistors and simulate how it behaves when the power supply varies. That should help to determine what causes an effect you might see.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 02:24:47 pm
content: Quote from: nctnico on January 22, 2015, 02:05:02 pmThanks for that experience.I am pondering the non-linearities that occur as dynamics reach the rail limits in a bipolar amplifier. Especially when those artifacts are faster than the amplifiers potential slew rate. How much attenuation occurs? What kind of properties are exhibited at the amplifier output, in relation to signal input etc etc etc.....there is obviously a ton of information out there and the datasheets tell us what happens in ideal scenarios, but what happens outside of those scenarios....THD, linearity, recovery etc...I suppose the real question is how much headroom is "enough". I.E. on a dynamic scale of 36 volts....is a 40 volt supply really enough to avoid such artifacts? Is there a general ratio or percentage which should be adhered to? If so, it has escaped my eye for many many years. Or at least the wealth of information I have seen on those topics is conflicting.....

author: nctnico
author_rank: Super Contributor
timestamp: January 22, 2015, 03:04:33 pm
content: I think you should look into translinear loop based analyses of opamps to get a feel for what kind of influences to expect. In the AC world they would talk about distortion and in the DC world about gain error. A lot has to do with the open loop response of an opamp. If you add a feedback loop things improve a lot but even then the open loop amplification has an impact on the gain.You should get interesting results if you try to determine the open loop amplification for various input voltages. AFAIK the open loop amplification should become lower when you get close to the supply voltages.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 05:02:15 pm
content: Quote from: nctnico on January 22, 2015, 02:05:02 pmNow we are getting somewhere. That is a most excellent suggestion/observation. At small signal levels (proportional to DC offset), especially getting close to rail voltages, we might get small deviations....for miniscule amounts of time....that actually bounce off the rail. Those can possibly load the supply ? There is also the issue of capacitive effects (see also quantum tunneling through silicon) dude to silicon impurities. At what point do these seemingly "useless" parameters really start to influence the least significant digit?Let's say the opamps near unity gain ratio is something like 1:0.999999995 (that 5 is the big if).....then we do in fact have a problem with determining some things @ our nV scale.If anyone knows of an opamp that is warranted to have a perfect unity gain ratio of supply to input, and an equal gain structure from signal input to output....then that would save me a hell of a lot of trouble, and I could get on with it, and stop posting such ridiculous topics.Scenario: we have a stable DC signal from our REF (lets just say it's 1ppm drift at some given time scale).....we then feed that signal into our amplifier.....lets say that signal is close to the rail voltage, and that SDEV (1ppm) slams it against the rail....what happens? even if it's faster than the amplifier slew rate, a certain portion of that energy is going to make it through....how much? how much is due to tunneling effects, how much is due to loading against the PSU? in this case PSRR isn't really relevant is it? what if that energy is from a capacitive issue in the silicon? If it is that is going to be energy in a frequency range that completely nullifies the PSRR....then the issue becomes, what is the recovery time of the PSU/amplifier and how much more SDEV can we expect during that recovery time? I have seen amplifiers do some astonishingly "whacky" things when clipped.....I.E. drop out, attempt to recover, slam the rails freak out and oscillate.....it happens. I would really like to see how relevant and probable that is at nV (small signal) levels.....Am I making wildly inaccurate assumptions, based on my knowledge and observations? I can accept that if I am.....as long as there is a basis for that, that exceeds my scope of information.....everyone has to learn by trying and failing....accepting what is written blindly is not acquiring insight or knowledge, it's just memorizing someone else mistakes

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 05:20:40 pm
content: here are a couple of bigscenarios....if the THD follows that curve on up the scale at only 3.5V RMS....what happens near the rail limits?.....what happens when we saturate the junction? will we have a perfect, clipped, straight line? I don't think we will....I think we will have a small signal oscillation (pulse/square.....more probably a sawtooth)....doesn't that potential oscillation/artifact add to our total SDEV? Am I totally wrong in assuming this? I have tried to set-up experiments to gain insight on this exact topic, however I could never determine if the artifacts produced at the output we from the PSU itself, or the amplifier freaking out.....compound those problems with V sense and I sense compensations thrown into the mix.....maybe there scenarios are where totally unregulated supplies make a lot of sense....think orders of magnitude more headroom and if the supply clips at LF, who cares....PSRR....but I somehow doubt things would play out that way.....I think the topic of amplifier headroom is VERY VERY VERY important here....as well as filters with gain structures that don't decimate our SDEV....maybe this is where converting the REF to a digital signal, and employing phase linear filters in the digital domain would make a lot of sense....in that case no-one needs to concern themselves with such things.....oh BUT wait...we need makeup gain into the ADC....see we just can't get away from this problem.....

author: Rupunzell
author_rank: Frequent Contributor
timestamp: January 22, 2015, 05:29:20 pm
content: If 36 volts go dynamic range is needed, a rail voltage of 40 will NOT do. Yes, there will be output, but the internal devices will not be operating in their ideal range. All those active loads are not going to be in their ideal range, behavior of the bipolars modulate with voltage swing and more effects that absolutely affect their behavior.As for PSSR, look at the frequency -vs- db PSSR, it goes down with frequency and output voltage. Often this is a specification manufactures of op-amps do not talk much about.The other got-ya with op-amps and powering, they can draw a lot of peak current under load while their unloaded current is low, often very low. This absolutely YANKS on the power supply upsetting the entire system. Think adding nice big bypass capacitors will help. that just causes the ground currents in the system to pulse causing other problems.In the ideal case, design out the power supply effects of the device being powered. Often, this is not possible due to using monolithic o-amps as they are more gain-block-black-box that one cannot alter it's internals to influence their behavior. Most one can do is to pick their poison and live with it. The alternative is to design the required amplifier with all discreet components to meet the actual requirements... like in those old days.If more is known about the actual circuit requirements, better suggestions could be made to help.BerniceQuote from: TunerSandwich on January 22, 2015, 02:24:47 pm

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 05:36:20 pm
content: Quote from: nctnico on January 22, 2015, 03:04:33 pmDC is a myth

author: Rupunzell
author_rank: Frequent Contributor
timestamp: January 22, 2015, 05:38:42 pm
content: DC, over enough time, DC can and does change.BerniceQuote from: TunerSandwich on January 22, 2015, 05:36:20 pm

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 06:17:02 pm
content: Quote from: Rupunzell on January 22, 2015, 05:29:20 pmAbsolutely my point, since page one....I am glad someone is latching on to the primary concerns I had here. The amplifier is going to pull down the PSU as we move up the scale.....and that in itself is going to cause nonlinearity in the system.....regardless of PSRR. Especially since there will be silicon artifacts well above the frequency of 0dB PSRR.Now let's compound the problem, with the requirement I posed early on....which is that we would be driving banks of socket opamp IC's....full scale....so that we can quickly characterize and bin the ideal components. I imagine the load on the PSU will be great enough at the edge of the amplifier rail voltages to really effect or SDEV....and without enough headroom we will start to see small signal oscillations from the PSU itself, in the amplifier output.....Have a look at the post just above the one you made (which I am replying to). I have included the CMRR and PSRR curves, as well as the THD vs freq curves.......they are wholly incomplete, and not useful for the levels of precision I would like to achieve. I am obviously on my own in defining those characteristics, out past the limitations of the datasheet......and for the levels of stability I would like to achieve I need to know for damn sure the rail supply isn't the culprit of my woes

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2015, 01:25:06 am
content: Quote from: TunerSandwich on January 22, 2015, 02:24:47 pmIt took four pages to get a straight answer out of you! But at last, you have given a clear and concise problem statement!The best answer is, avoid such regions of operation. You want precision and linear? Stay away from the nonlinearities. If you absolutely must go as close to the rails as possible while maintaining whatever specs, then you must spend the time to quantify that operation. Which is expensive, so it's always better to simply increase the supply voltage instead of dicking with this.But suppose you forge ahead regardless; the concern is only when input or output are operating near the supply rails, so you can perform these measurements with very low supply voltages, perhaps 0-3V, with input and output near ground level. The supplies will be unbalanced, which is fine. That way, your supply needn't be much, you gain an extra decimal place in scale, and everything can be referenced to the supply to null out its error further.PSRR and Vos are DC by definition (assumed as constant for eternity, whether or not they drift,none of that "there is no DC" hemming and hawing), so the standard drop in PSRR vs. frequency, or in open loop gain (and therefore AC PSRR and error) does not apply.For real systems with nonzero bandwidth, you do have to make some consideration of these quantities. But a filtered supply is easy to make, so that's not as big a deal, either; and it only needs to be as filtered as necessary for the purpose. Nothing at all hard about that.Tim

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 23, 2015, 02:16:13 am
content: Quote from: T3sl4co1l on January 23, 2015, 01:25:06 amI said that on page one.....in the first post......"Any opinions on the subject would be appreciated, but I am mostly concerned with opinions on the total dynamic range...and have set a poll to keep that topic simple."I.E. how much headroom should there be in the supply rails.....

author: nctnico
author_rank: Super Contributor
timestamp: January 23, 2015, 07:50:16 am
content: I think it is interesting to run a simulation on an opamp model made out of transistors (i.e. not a functional equivalent model you often find nowadays). The test would be to apply a slow ramp to the input of the opamp and plot the gain (Vout/Vin) in a logarithmic graph. I think the result will be a graph which drops off gradually at first and sharp when the output voltage gets close to the supply rails. This should give a clear picture on the non-linearities you can expect from an opamp.Or just read this paper:http://class.ece.iastate.edu/vlsi2/docs/Papers%20Done/2002-05-ISCAS-YT%20RG.pdf

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 24, 2015, 02:46:18 am
content: Quote from: nctnico on January 23, 2015, 07:50:16 amI do believe Ti has a schematic of the OPA227 internals.....this is actually a pretty interesting concept.....however I don't see how we could really model the characteristics of the semiconductor itself, with discreet components. I mean at that point, if I had an ideal discreet amplifier, that I had full linearity characteristics on, wouldn't it make more sense to take that down to an SMD topology/module and just use it in place of the OPA227?I have no doubt I could easily build a discreet amplifier/hybrid that would put the IC to shame.....but I would still need a very nice supply to provide a current source to the output......my intended use for the amplifier, after the REF (in my design) is to feed the base of an even greater current source....I think building a discreet amplifier module, might not actually be a bad idea.....at least that way I could more easily characterize the nonlinearities on a component level.....that's a bit more work than I wanted to do (maybe someone makes small discreet amplifier hybrids?....)The problem here is I still need a very stable rail supply, to test and characterize the amplifier (regardless of it's topology/packaging).The document you linked is interesting....I suppose it's a way to extrapolate a well educated guess, as to what will happen to linearity as you hit the edge of the rails.....and allows for the 2nd and 3rd harmonics to also be extrapolated.....but it's still not a hardline measurement....I mean how can you be sure those predictions will track reality 100% of the time? Maybe I am missing something in the article and need to read it again?P.S. when I have characterized amplifiers in the past, they do exhibit the sigmoid curve, but with an oscillation (small signal) at the rail.....like I was saying before, they do not turn the supply into a perfect (linear) DC current source (although good enough to weld with).....they "bounce" off the rail and there is a small recovery period (sawtooth).....AND this is an interaction that is going to include other, more sinusoidal oscillations at the rail...from signals that exceed the slew rate of the amplifier.....I mean I would rather have sine nonlinearity than sawtooth, but still....it's quite complex....