Noob volt-ref Qns - Page 1
Forum section: electronics / beginners

author: 3roomlab
author_rank: Frequent Contributor
timestamp: August 03, 2015, 01:37:31 am
content: i have again some noob questions about certain stuff that need more veteran inputs, they stem from re-reading some of the adventures of various "volt-nutting" threads1)http://www.ti.com/lit/ds/symlink/lm399.pdfthe datasheet seem to suggest some advantage that 299 has over the 399, eg : max RMS noise, 299 seems to be half of 399. max voltage fluctuation seems lower WRT change in supplied current. so i should get 299 instead of 399 if the the price gap is small. yes?2) there is a 299H and 299AH, i cant find what is the difference in them on the datasheet, does anyone know of its difference?3) in many posts discussing use of opamps with the zener, while i understand the temp drift spec (AD706 = 0.2uV/oC). i saw a AD8622 (=0.5uV/oC). and AD8622 has a lower 1kHz noise @ 12nV (1kHz). could this be even better? (ref codeboy2k, post#15https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/)(ref this notehttp://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0127.pdf--> it claims AD8622 as lowest noise)4) by arranging the op amp to give a voltage of 10.48575v, if we then introduce this VREF into the low noise 20bit DAC (1048575 steps), each bit step should then give us 10uV resolution. (or was it 1/2 of this Vref?). so by attaching a further gain stage ... this DAC can then be use to digitally VREF / Vselect endless other stuff5) i believe it was andreas (or was it some1 else?) who describe additional drift stability by "over" baking the zener unpowered, i recall another mention of not only overheat but also overcool. will this help the op amp IC?n thanks for any input on the above

author: Vgkid
author_rank: Super Contributor
timestamp: August 03, 2015, 02:20:20 am
content: You do realize that the lm299 is listed as obsolete.The A vs. non A specs, the A spec has a lowr temco than the non A spec.regarding the ozen, it was either in the lm399 or the ltz1000 thread, that by rurning off the heater, the drift rate is decreased, at the expense of a lousy tempco. though what I have noticed is that it doesn't seem to affect your high end multimeters, which don't seem to drift much over the years.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: August 03, 2015, 06:21:20 am
content: no no, not the internal heater. i think it was andreas? who mentioned baking the devices for 1 year @ 150oC to artificially induce accelerated aging, and thus reduce inherent operating drift (to =< 1ppm / year).and i think i missed-placed the question for (4), does anyone calibrate a Vss / Vref to a "power of 2" number so that it is easier for the DAC ic to just simply "digitize" the output into whole units of 10uV or 100uV? because i do see some Vref output are values in the "power of 2" (ie : LTC6655BHLS8-4.096#PBF) with ref to AN86, should the Vref be 5.24288v instead of just 5.0v?

author: rs20
author_rank: Super Contributor
timestamp: August 03, 2015, 07:20:18 am
content: I'm no volt nut, but I suspect 4.096V references and LM399-grade references are for completely, completely different usecases. An LM399 goes into an instrument where its purpose in life is to below-driftandlow tempco, but not absolutely precise! The absolute precision comes from calibration - the LM399 has an absolute tolerance of 2%, also known as 20,000ppm! So having some sort of ultra-critical, carefully trimmed analog stage which converts to 4.096V and has to never ever drift, and then feeds into a DAC, seems dumb. All the drifts in the analog stages will just blow your precision away. If you must have a DAC hooked up to a LM399, you're going to need an extremely good DAC to not utterly waste the performance of the LM399, and you're going to want to avoid having any other drifty electronics in between. Software has no drift or error, aside from the quantisation error that you'll haveanyway. So software calibration is the way to do it, less electronic analog components = less error & less drift.Meanwhile, the 2.048V references: take the one built into the MCP4821 DAC, also has an initial accuracy of 2%. It's a simple, easy to use thing which you write trivial software for that just assumes that 1LSB is 0.5mV, screw calibration, easy maths (*2 or /2 is very cheap in hardware), 2% accuracy out of the box, job done.These are completely separate worlds for different usecases with completely different accuracy and drift requirements. I urge you not to confuse them.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: August 03, 2015, 07:51:27 am
content: 6) in blackdogs' implementation of 4x LT1012-5 (https://www.eevblog.com/forum/projects/10v-reference-%28i-did-it-my-way%29-no1/)i am trying to further understand the effect of Cx/Rx roll off on the input noise of the op amp LT1012as the Ft of the RC is = 0.5Hz (approx). that would mean @ 5Hz, general noise is attenuated by 10dB, and @50Hz by 20dB (at this point did i get anything wrong?)Teslacoil gave a very nice explaination of the function, but i am thinking the buffered "guard" / ground could remove more noise? maybe?for easy correlation to the datasheet, say the RC Ft = 1Hz, it means @ 1kHz noise attenuation is -30dB, does it mean this attenuation could affect the input noise figure stated in the op amp? ie : if we look at say TLC2652 with noisier 1khz input noise of 23nV, would having such a large RC filter just sitting at the input pin reduce this internal noise (by 30dB) ? which is = 0.023nV ? (or in theory should such a noise "pull down" work like that? or there are other equations for this?)

author: 3roomlab
author_rank: Frequent Contributor
timestamp: August 03, 2015, 07:59:01 am
content: Quote from: rs20 on August 03, 2015, 07:20:18 amyea i know... the have one but not the other thing ... the boat that cant land at 2 shores at the same timebut since AN86 shows a rather accurate 1ppm DAC ... im thinking ... that 5v vref feeding into that DAC should be around 1ppm tolerance ...and hence the question ... when the DAC spits out its steps, i am assuming it divides the 5v into 1048576 steps, so should'nt it be fed a "number" divisible by 2 (ie 5.24288v) ...which would make each step 5uV exactly instead of 4.7684uV ... or there is some kinda further resistive divider inside the DAC which is already doing the "5uV" thingy ... i am guessing it has not, am i right?(i think some of u know the crazy chinese prototype did try to put the 2 together, but i am guessing the tempco is high, SMDs being used in all stages incl VREF)

author: rs20
author_rank: Super Contributor
timestamp: August 03, 2015, 08:05:37 am
content: Quote from: 3roomlab on August 03, 2015, 07:59:01 amWhat's wrong with 4.7684uV steps? Your microcontroller can divide by 4.7684 just fine. I'm not being facetious here, I'm genuinely curious why you think 4.7684uV steps are a problem.Also, if you're thinking "but 1V is exactly 200,000 * 5uV but if I get 1V using 4.7684V steps that's 209714 steps which is actually 1.0000002 V", then read AN86 more closely: the feedback DAC is 24 bits, not 20, and the composite DAC being used has 24 bits as well. So the actual quantization error (/half "step size") in the digital feedback loop for a 5V reference is 0.15 uV, just absolutely negligible. If you're actually implementing AN86, implementing the code comparator is going to be trivial compared to incorporating a divide-by-4.7684 rather than divide-by-5...Also, that AN is 15 years old. Might not be state-of-the-art perhaps, although it's a perfectly reasonable approach. Except the "code comparator" part is just a microcontroller these days, no need to help the PIC along with 3 74HC chips!!!!

author: 3roomlab
author_rank: Frequent Contributor
timestamp: August 03, 2015, 08:43:49 am
content: Quote from: rs20 on August 03, 2015, 08:05:37 ami think you misunderstand ... it is Vref = 5.24288v.24bit would mean 32 steps to create 10uV.i just wanted to know if anybody has tried to do it this way, or has thought of doing so to get to rounded whole numbers (or voltages)

author: rs20
author_rank: Super Contributor
timestamp: August 03, 2015, 10:59:34 am
content: Quote from: 3roomlab on August 03, 2015, 08:43:49 amA) Specific numbers are irrelevant, and I wasn't far off anyway: 0.3125uV per step means 0.15625uV quantisation error [which is why i said "quantization error /half"step size"]. 0.15625uV, 0.15uV... tiny either way.B) You didn't answer my question: what's wrong with 4.7684uV steps, or any other non-round number? Why exactly are nice round steps like 5uV or 0.3125uV better? This is what computers are for!This goal of getting nicely rounded whole numbers is utterly pointless and misguided. You can see in appendix I of AN86 that none of the suggested references have voltages that resemble powers of 2.It doesn't matter what the reference is, as long as you know what it is -- this is what calibration software is for.Software is the answer. Software.