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] Precision resistors
On 06/08/2015 16:25, David C. Partridge wrote: > Looking for a set of precision resistors for calibration purposes. > > The crucial factor isn't that they be *exactly* the values above, so I don't necessarily need 0.001% parts. Low TCR is important, and I will need to know that actual values to 10ppm or better. > Presumably long term stability is more important than TCR - not many resistors are guaranteed to drift less than 25ppm/year, even unpowered. E.g. Welwyn 4800 metal foil 25ppm/year, Caddock USF ultra stable 200 & 300 series 20ppm/year dropping to 10ppm/year after 10 years. Short of an SR104 or the like I doubt you can do better than the 2ppm 6 year life stability of the hermetically sealed Vishay metal foil resistors including the HZ series you mentioned or the VHP101, VHP203 etc. Frankly I find it hard to differentiate between the Vishay sealed foil resistors - they seem to have almost identical specs. For example is the VHP203's .05ppm typical TCR (0 to 60C) better than the 10ppm max for the VHP101 (15 to 45C)? I've no idea but given that the maximum TCRs are usually at least 10 times worse than the typical figures, maybe not. Is the 'VHP101 < 10 ppm (+ 15 °C to + 45 °C)' even a maximum given that Figure 4 says typical TCR is .3ppm and has a 'typical' curve showing approx 8.5ppm max change from 15 to 45C (and has a TCR of .55ppm from 15 to 20C)? It looks to me that the 10ppm figure isn't specifying a maximum TCR at all which means you could get almost anything! Interestingly a note in the HZ series datasheet says 'For maximum TCR < 1 ppm/°C, see VHP100 and contact application engineering' The VH102Z seems to have the best guaranteed TCR of .6ppm maximum (100 to 100k ohms) compared to 2ppm for the VHP203 and the HZ series. 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. The spec for the Fluke 5450A's 10k is 6.5ppm max drift/year, but an old one may be a good solution as hopefully the resistors will be very stable by now. The only way you could be sure though is to have it calibrated at least twice to see how much it actually drifts which wouldn't be cheap. An alternative is to buy one or more 10k VH102Z as a standard and buy or build a 10:1 and perhaps a 100:1 Hamon divider and use the 10k reference to calibrate 1M, 100k, 1k and 100R resistors in a bridge arrangement using a null meter or suitable DVM. Adding a 2:1 would address the 20K, 200K etc. requirement. This approach might not be suitable for very high or low resistances, but would allow you to have confidence in many of your resistors relative to your 10k standard. The divider could also be useful for voltage calibrations or checking your other calibrators. Dr Frank describes the one he built, achieving uncertainties of 0.2 / 0.5 ppm for 10:1 / 100:1 releative to output, in reply #8 and #10 here: [url]http://www.eevblog.com/forum/testgear/hp34401-measurement-of-linearity/?nowap[/url] I thought he had a more detailed description somewhere but I can't find it just now. You could probably get away with using cheaper resistors at the cost of slightly increased uncertainty. ___ 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
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
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] Matched resistors
Randy, On 24/07/2014 04:22, Randy Evans wrote: Tony, Your improvement factor of SQRT(n) assumes that each resistor in the group has random changes uncorrelated to all others in the group. For similar type resistors, I would think that is not likely to be true. Yes/,/ I'm well aware of that which is why I discussed that point further down in my post. It was a long post though so I don't blame you for getting bored and not getting that far! For shelf life stability it is likely that they all "age" in a similar way. Unless the resistors are in a hermetic package, humidity would impact all the resistors in a similar manner. Randy Exactly. Since they are being used in a 1:1 divider configuration, if they age in a similar way, the tracking ratio stability will be good. The reality however is that there will be some variance between components, and using multiple resisters will reduce that overall variance. Part of the variance between individual resisters will likely follow a Guassian distribution and thus the improvement factor for that element will be SQRT(N). Some of the variance will likely be due to random factors which have a rather different distribution, probably highly skewed with long tails, and thus the improvement probably won't be SQRT(N). My conjecture (ok random speculation) is that factors such as stress differences due to microcracking in the ceramic substrate or at the terminations may cause some of the latter. Nevertheless, even though part of the variance doesn't follow SQRT(N) the variance will still reduce by using multiple identical resisters (if there are enough*). The problem is knowing how much - it probably can only be determined by lengthy experimentation, unless some good empirical data can be obtained from manufacturers or research papers. Another complication is that I believe that thin film resistor stability and TCR characteristics improve as the resistance reduces. This is not usually reflected in the datasheet but using multiple resistors in series allows lower values to be used which may perform better. On the other hand, thermal EMF problems may increase proportionally. TCR tracking is much easier to measure, so it might be interesting to see how it improves with increasing numbers of resisters. However, I understand that ratio stability is likely to be a bigger problem than TCR tracking. The other end of the spectrum, using a single Vishay VHD foil divider is certainly the simplest; however bear in mind that Vishay's stated typical tracking TCR of < .1ppm is just that, and the one that you buy may be anything but typical. And if you can work out the maximum tracking TCR from the VHD144/200 datasheet, you're a better man than I. My guess is that its probably better than .5ppm which is likely good enough for your application. But would it perform better than, say $30 worth of Vishay DFN, 3ppm 4-resister networks, 1 year shelf life ratio stability < 20ppm? I don't know. Of course there's nothing to stop you using multiple VHDs if you can get them at a good price. Ebay maybe? *) If all resisters are identical expcept that 1 in a 100 is markedly different, then any 10 will have a good chance (90%) of being identical; using 100 will have a good chance (64%) that at least one is different and thus the overall error would be at least 1/100 of the difference. 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] volt-nuts Digest, Vol 56, Issue 9
On 14/04/2014 18:46, Jan Fredriksson wrote: It was the April 1989 HP journal that made me post the question. The article makes really good reading about the core of the 3458. It also made me think about how one could implement the AD with the components available today and bench instruments. It should not take that many parts to make a single voltage range, moderate speed, single shot AD using a bench clock / counter / timer. Just for the learning. But about the switches there is not much in that article, just the paragraph quoted by TH, "A custom chip design.." etc. The article is otherwise seems like a very good starting point for learning multislope ADs. It seems like it would almost be possible to set up a spreadsheet with the data given. I noted that they use a 390pF integration cap which made me wonder what kind of switches where used, as any FET capacitance / charge would have to be compensated / cancelled / nulled somehow. I suggest you take a look at this patent from 1993 where HP describe improvements to the ADC switches (I don't know if they ever used it an a saleable product): https://www.google.com/patents/US5321403 "The errors are significantly worse when standard components such as off-the-shelf analog switches are used for the input switching circuits. Prior art investigators have attempted to overcome these problems by implementing the input switching circuits in application-specific integrated circuit form and tightly controlling the manufacturing process, leading to very expensive solutions." Their solution is a different arrangement of switches (see patent for diagram): "The switches which control selection of the positive and negative reference currents are implemented in such a way that current surges are minimized. That is, each switch is a series-parallel pair of switches in which the series switch of the pair provides a path to the integrator summing node while the parallel switch of the pair provides a path to ground, and one of the switches in the switch pair is closed while the other of the pair is closed. State machine diagrams are used to express the algorithms used by the controller in operating the switches throughout the integrate and de-integrate cycles. The order and sequence in which the switches are operated eliminates the effects of charge injection due to operation of the switches as well as signals that are cross-coupled from the control lines of adjacent switches." 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] Switches in integrating ADC
On 14/04/2014 10:03, John Devereux wrote: Jan Fredriksson 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.
Re: [volt-nuts] Some questions to zeners (thermoelectric effects)
On 27/01/2013 20:31, John Beale wrote: On 1/27/2013 9:36 AM, Ed Breya wrote: I think your expectations are not realistic - even if you could make such a reference, you could not transport its voltage to the ADC without thermoelectric effects causing error that would swamp the performance. To keep everything below the 1 ppm/deg C range you would have to put the entire circuit in controlled temperature - the reference, the ADC, and the signal connection to the outside world. Presumably, if the voltage reference uses an amplifier then a four wire connection can be used to eliminate all the EMFs between the reference and ADC other than those at the Kelvin connections at the ADC and reference/amp so that the reference and the ADC don't have to be in thermal equilibrium with each other. Could the sense wires be welded to the ADC pins between the solder connection to the PCB and the package to avoid the thermal EMFs of a solder joint? I assume the hardest connections to keep thermally equalized would be the terminals connecting your reference/ADC to an external device. If your voltmeter is limited to low voltages, optimizing this suggests the smallest and most closely-spaced connections possible, embedded in an insulating but thermally conductive matrix (ceramic?). Standard banana jacks with 3/4 inch spacing and surrounded by plastic, seem far from "small" or "closely spaced" or "well thermally coupled" For a couple of data points, here's one manufacturer's approach to dealing with thermal EMFs: http://www.hpd-online.com/reversing_switch.php Their low thermal reversing switch uses plenty of copper and aluminium to minimise thermal differences, claiming typical thermal offset of only 3nV, 10nV max! Its not clear (to me) though exactly where that 3nV is being measured and how effective the 1.5mm thick copper lugs connecting the reference source/DUT's terminals are at minimising temperature differences between the terminals in the presence of normal air currents in a typical Lab. And this scanner: http://www.dataproof.com/scanner.pdf claims thermal EMFs of less than 15nV typical (30nV max) using lots of aluminium to keep relay contacts in thermal equilibrium. 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] HP 3458A DC current accuracy
Frank, Thanks for taking to trouble to respond. Its interesting that the Datron 1281 has exactly the same issue - best 24hr uncertainty: DC V: .5ppm + .3, Resistance: 1+ .3, DC A: 10 + 2 So its not a HP specific design trade-off. Perhaps there's something more fundamental such as the difficulty arranging the self-calibrating circuitry to include the shunt resistors. Perhaps your suggestion that current measurements are seen to be the poor relations to voltage and resistance has some merit, but I find it hard to believe the designers of these high-end instruments would compromise the current measurement accuracy unless it was very hard and/or expensive to avoid it. Having said that, the voltage burden when measuring current is extremely poor for almost any multimeter you care to look at, making them useless for current measurements in many low voltage situations. Eg. measuring the short-circuit current of a .55V solar cell. I've never understood why relatively expensive and sophisticated instruments don't have significantly lower resistance shunts in conjunction with appropriate amplification (at least as an option). The resulting loss of accuracy would be more than compensated by the reduced impact of the shunt resistor on the circuit under test. I can't count the number of times I've had to use a 10 or 20A range to measure a few tens or hundreds of milliamps to prevent the shunt resistor badly affecting the measurement or even stopping the circuit working altogether. If you've only got a 3 1/2 digit meter you're not left with much resolution! Tony ___ 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.
