Re: [volt-nuts] Precision resistors
On 11/08/2015 17:53, Richard Moore wrote: True of any resistor that you want to trust to better than 10ppm, including the Vishays. Of course, but Dr Frank's experience with the oil filled Vishay foils has been good - from another eevblog post: 'Only the hermetically sealed, oil filled types (e.g. VHP202Z) give a big advantage. Their rate is typically 2ppm/6yrs., and therefore will add about 0.02ppm/yr only. In picture 1 you’ll find long-term stability monitoring of 3 EA of my 5 VHP202Z. After 2 years, they really remain within 0.5ppm of their initial value, so that is obviously no fake advertisement. (Remark: The measurement stability was improved also during that time.)' That's pretty close to the SR104's typical drift of .2ppm/year, .5ppm max (1ppm first 2 years), but the TCR of his parts were much worse than the SR104's .1ppm typical ranging from .3ppm to 1ppm. He might well have got lucky but I read on a Chinese volt-nut type blog that Vishay originally specified 2ppm/10 years but reduced it to 6 years - presumably due to complaints/experience. Personally I would be happy to trust that they would remain within 10ppm for many years but you would have to get them measured periodically to know for certain. Depending on how cheaply you could buy one, it might be cheaper to buy new ones for 2 or 3 years rather than getting one calibrated and using them to determine the drift of the earlier parts. And you would have a collection of resistors to improve confidence in the secondary standard. Does anyone know how much it would cost to get a 10k resistor measured to 2ppm in the UK by a calibration company? Does anyone know how much it would cost to buy a 1% Vishay VH102Z/VHP101 or similar with a 2ppm measurement? Edwin Pettis quoted me $7.28 for one 10k resistor ($5.46 for 11 to 24) so they could be a viable way of getting accurately (approx 1ppm) measured resistors. The higher TCR, 3ppm/C would increase the uncertainties though as it would require them to be used within 1C or so of that when measured by Mr Pettis, but he can select for 1ppm TCR or less for an extra $2 which would easily be justified for this purpose. Obviously the uncertainties due to transport shocks/vibration (although Mr Pettis claims they are very rugged) and drift related to the time in transport would need to be considered. On Aug 11, 2015, at 9:00 AM, volt-nuts-requ...@febo.com wrote: Edwin Pettis states his resistors drift is typically better than 2ppm in the first year, so pretty good but you'd still need to have them measured every few years. If you have to get them professionally calibrated it may be cheaper to buy the Vishay parts. Edwin could provide the measured values as could Vishay if you bought directly from them. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 3458A reference boards on ebay
Orin, What do you intend doing with it? I was thinking about getting one of those, with a view to putting it in a box with a couple of terminals to have something to compare with my 6.5 digit 3457A. But what put me off is a lack of knowledge in knowing how to convert a 3458A reference board into a boxed unit with a known output voltage at the terminals. How would I avoid / control thermal EMFs? I'd be interested to hear what your plans are for it, and how you intend tackling those issues. Do you know what the difference in the reference is between a standard 3458A (8 ppm) and the high stability option 002 (4 ppm) model? I'm guessing the chips for the option 002 might be the top performing ones. I wonder if there's any way to tell from your board if it came from a standard 3458A or a 3458A with option 002. Dave Take a look at this site for some good info on using the 3458A reference: http://www.maxmcarter.com/vref/ Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] LM399 Long term drift specification
On 11/09/2014 01:50, Mike S wrote: On 9/10/2014 7:00 PM, Tony wrote: I've just noticed that TI and Linear's specs for 'Long Term Stability' (typical) are different. TI state 20ppm/1000Hr while Linear state 8ppm/SQRT(kHr). That's a big difference - is this likely to be a real difference or just specmanship? I note that Linear (in Note 4) also state that Devices with maximum guaranteed long-term stability of 20ppm/SQRT(kH) are available. Presumably they would be a special order as there doesn't appear to be a unique part no. Would they be likely to be much more expensive? Isn't 8ppm/SQRT(kHr) better than 20ppm/SQRT(kH)? Why would the latter be more expensive? Or is it the difference between typical and guaranteed? I'm guessing that typical in this case means the one sigma value so the three sigma value would be 24ppm. In any case three sigma still only means 93.32% of parts come within that limit, or 6.7% exceed 24ppm, and a few could be considerably worse. That compares to a guarantee that all all parts meet 20ppm. This link: http://www.gellerlabs.com/LM299AH-20_Case_Study.htm provided in Andreas's response is very interesting: Certified Long Term Drift The National Semiconductor LM199AH-20, LM299AH-20, and LM399AH-50 are ultra-stable Zener references specially selected from the production runs of LM199AH, LM299AH, LM399AH and tested to confirm a long-term stability of 20, 20, or 50 ppm per 1000 hours, respectively... So in this case they really do mean a guarantee. And I doubt that individual testing came cheap. I say 'came' because I wonder if they still 100% test the 20ppm parts or if they select them using some lower costs means? Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] LM399 Long term drift specification
Andreas, Thanks for taking the time to respond. Actually I've seen many of your postings on eevblog and here - you've clearly done a great deal of work in this area and would like to thank you for making it available to us all. On 11/09/2014 06:07, Andreas Jahn wrote: Hello, many questions I will keep it short: All ageing specs are typical if you want to have guaranteed values you will have to measure it over a reasonable time. (I recommend min 6 months). Every treatment (soldering, mechanical/temperature shock) of a reference may create a new ageing cycle with different slope. True. I guess that the new ageing cycle from soldering in an LM399 is not going to be as bad as that for a surface mount plastic device. So 100ppm/15 years outside of lab conditions (23 deg , constant humidity) is something that I would not guarantee without re-calibration. I had a feeling that would be the answer - though surely humidity shouldn't be a factor as these are hermetic parts. The questions remains though, what level might you specify - if you were forced to come up with a number (ok a guess!) - for non-selected, non-pre-aged parts after 15years continuous operation without re-calibration? Obviously this is given the context of the presumably limited numbers of samples you've tested and I guess you wouldn't have bothered to further test early rejects. Although typical drift of pre-aged + selected references will be in the 1-2ppm/year range if properly treated. What would you classify as pre-aged? Do they need to be powered up or can they be maintained at a suitable temperature? How many rejects would you expect to get to get one that achieves 1-2ppm? Is it known if the major instrument manufacturers preselect and burn-in LM399s themselves for their middle-range instruments? I'm pretty sure the top end kit will be all use carefully tested and selected parts, but what about a 34401A for example? The basic accuracy spec for that is 20ppm for 90 days, 35ppm for 12 months so even a 20ppm guaranteed part wouldn't be good enough, especially allowing margin for drift in other components. I guess I just answered my own question! Also its meaningless if you want to have LT or National (TI) parts since LT is the only manufacturer which still produces them. With high demands you will also have to sort out the noisy references. Some typical LM399 (all from NS) ageing data can be found on web: http://www.gellerlabs.com/LM299AH-20_Case_Study.htm That's very interesting. I have to agree that the raw data looks suspect. I wonder what the rejection rate is for this 20ppm selection and does it mean that non-selected parts have a high probability of being worse than 20ppm? http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg478496/#msg478496 With best regards Andreas I just came across another part which looks very interesting given its low cost - the automotive qualified REF5050-Q1. Although its only spec'd as 3ppm/C typical, 8ppm/C max, that's using the box method over -40 to +125C. The 'typical' chart however, figure 4, page 5 shows the gradients to be very flat between 25 and 50. Its typical of course, so real parts may be very different aka Vishay foil resistors. The 0 to 85C histogram, fig 1 on page 5, do show the majority of parts being in the range .75ppm/C to 1.75ppm/C which is pretty good, and with luck, in the 25 to 50C range may well be much better so a crude heating arrangement may be worthwhile (made easier by the 5050's temperature output!) I can't reconcile fig 4 with the histograms though; from the chart I reckon the 0-85 typical is approx 65ppm/85C = .76ppm/C and for -40 to 125C is approx 310ppm/165C = 1.88ppm/C. Figs 1 and 2 though show modal values of 1.25 and 2.25/2.5ppm/C. Am I doing something wrong or are these specs inconsistent? Even more surprising is the headline feature on page 1: EXCELLENT LONG-TERM STABILITY: – 5 ppm/1000 hr (typ) after 1000 hours Unfortunately that seems to be an error as the 'typical' spec on page 4 is: 90ppm (0-1000 hours) 10ppm (1000 to 2000 hours). The chart (fig 23, page 8) showing 1000 to 2000 hour drift of 96 parts show the worst case being +25ppm, with the bulk ending approx between 0 and 15ppm. I wonder if they carry on improving after 2kHrs? That's definitely not the SQRT(1kHr) characteristic and is very different from the standard REF5050 which quotes 100ppm (1st 100hours), 50ppm (1000 to 2000 hours). If you are in a position to pre-age them for 1000 hours that 10ppm spec is almost as good as the LM399 and best of all, TI quote a price of $1.60 @ 1k parts, compared to $4.65 for LM399s @ 1k from Linear. One off prices are rather more at $4.15 from Digikey (part no REF5050AQDRQ1) but again is still a lot cheaper than an LM399 at $9.92. At $1.60 and .8mA supply current, using 4, 8 or even dozens is a realistic proposition to exploit statistical improvements and noise
Re: [volt-nuts] Switches in integrating ADC
On 14/04/2014 10:03, John Devereux wrote: Jan Fredriksson j...@41hz.com writes: What kind of switches are used in integrating ADC, ie to switch between voltage sources (ref and external) and to switch in multisloping resistors? FETs? Yes, but I believe they are integrated ones usually. Either ye olde 4066 style or custom integrated circuits in the case of the HP 3458A. As Jan says, the 3458A switches around the ADC are integrated according to the April 1989 HP journal which describes the 3458A design: *Because the switches are in series with the resistors, they can add to the temperature coefficient of the ADC. A custom chip design was chosen so that each switch could be scaled to the size of the resistor to which it is connected. This allows the ADC to be sensitive to the ratio-tracking temperature coefficient of the switches and not to the absolute temperature coefficient.*** I expect that optimising and balancing charge injection would have been an important design objective too. It would be interesting to know how modern off-the-shelf analogue switches compare - ie. with low enough on resistance so that absolute temp coefficient doesn't matter, without introducing excessive charge injection. I expect that's a bit of a tall order. The 8 digit Solartron 7081 uses discrete Fets, but it uses a voltage to time converter for its ADC. The HP 6 digit 34401A uses a 74HC4053D 2:1 Mux to switch the ADC integrator. For interest, the signal switching in the input path of the 3458A, for selecting high voltage divider / low voltage input, current sources and DC amplifier gains etc. all use Siliconix J2472 J-FETs (N channel depletion mode). I guess there were no packaged switches up to the job at the time. Vishay bought Siliconix since and shut down production some while ago so good luck finding any parts or even a datasheet. I expect they are very low leakage types; no doubt there are suitable alternatives available - perhaps ones recommended for electrometer applications? Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Some questions to zeners (1N823-1N829)
On 28/01/2013 22:47, Andreas Jahn wrote: After a run in phase of nearly 1 year the ageing of ADC #13 stabilized. Currently I compare ADC13 nearly every day with 3 heated references (1 LM399 = LM_2 and 2 LTZ1000A = LTZ_1/2). The last half year the ageing is about 0.5 to 1.5 ppm for 6 months compared to the heated references. See picture ADC13_longterm: X-Axis is day Y-Axis left is drift in ppm with red = LM399#2, green = LTZ1000A #1, blue = LTZ1000A #2 Y-axis right is temperature in degree celsius of the temperature sensor near ADC13 reference. By the way: up to now I could not measure any effect which is related to thermocouples. Ok my temperature step noise is still too high. And probably I am using the wrong connectors in my tests: cheap D-Sub connectors where a metal shield is equalizing the temperature of 2 relative close neighboured contacts. With best regards Andreas Andreas, Very interesting results - thanks for sharing your painstaking work. Hope you don't mind me asking a few questions though: How are you dealing with the issue of drift in the thermocouple measurements (including the cold junction compensation)? Do you calibrate it periodically? Thermocouples aren't noted for high stability - but presumably at room temperature its perhaps not much of an issue. Do you know what temperature the LTZ1000 references are operating at, and how long have they been operated for - ie. have they been aged prior to starting the long term test? (Presumably the answer to that is the fact that you are showing results from day 460 onwards?) Have you any insight into how stable the ADC's reference (AD586LJ) is? I.E. Have you made any occasional or periodic measurements with other calibrated instruments during the long term test or is it the long term test results themselves which leads you to state: After a run in phase of nearly 1 year the ageing of ADC #13 stabilized.? Thanks, Tony H ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
