Making do with what you got/ LiPo battery power for voltage, current references - Page 1
Forum section: electronics / general

author: View[+]Finder
author_rank: Unknown
timestamp: January 24, 2021, 09:04:14 pm
content: Hi All.ViewFinder here.Following the lead of Dear Leader Dave, I built a 1 microAmp and a 1 mA current reference as one of the first tasks of setting up my lab 10 years ago. I was into RC aircraft and LiPO batteries, so I incorporated that into the build. Still working, just charge every year or so . . .Regards,Donal

author: View[+]Finder
author_rank: Unknown
timestamp: January 24, 2021, 10:19:20 pm
content: I've been involved in the discussionhttps://www.eevblog.com/forum/metrology/buyers-beware-misleading-34470a-reference-modules-(clones)-on-ebay/75/When my investigation turned to assess the quality of available DCV references for testing the Keysight 34470 reference as installed in a 34465, starting a new forum thread seemed more appropriate for reporting.The short story is that I am currently testing an HP 3458 reference in a metal box powered by LiPo. I got the reference several years ago and tried all the AC to DC ways to power it, finally deciding on the lithium battery as having the lowest association with noise as well as an excellent ability to hold a charge compared with other storage possibiities. What I am learning now is that noise is easier to control than temperature--I don't work in a cal lab and the space I use for electronics varies from 24C at night to 29C in the warmest part of the day.

author: TiN
author_rank: Super Contributor
timestamp: January 24, 2021, 11:29:00 pm
content: It is not hard to build a makeshift oven styrofoam box (few resistors, simple temperature controller with relay) and put a ref in there. Total cost could be as low as $5-$10 and suitable to keep temperature stable.Next step is active thermostat box with some ready-to-go controller, such as Arroyo (example) or ILX 5910B (one was soldfor $80). Add some cheap chinese 40W TEC, heatsink, RTD from Digikey, cooler for hot side and getting stability better than 0.01°C become trivial for small device like A9 3458A reference.Did you figure out why A9 module did not work correctly? Temperature coefficient of A9 is below 0.05 ppm/K (I've tested more than seven modules before) so even if you temperature shifts from 20C to 30C you will get just 0.5 ppm difference from reference, if its operating correctly.Noise is not a problem, as readings are averaged out anyway over time (hours-days). Also CMRR of reference is very good and there is no measurable difference innoiseresults from linear power supply versus battery. Ground loops and shifts are different story and that's one of the key reasons commercial references have battery power. Second key reason for battery is transportability, to maintain drift of the reference predictable and reduce risk of hysteresis effects. Using lithium battery kills the benefit of transportability, as you can't ship it easily and need all those extra hops for dangerous goods paperwork, etc. (esp for DIY ref).

author: Kleinstein
author_rank: Super Contributor
timestamp: January 24, 2021, 11:30:58 pm
content: A good reference should not react very much to temperature. From the other thread it seems this one has a non working temperature control for the LTZ ref. So it will be quite temperature sensitive. If the heater / temperature sense part is broken at the chip, one could still consider an external oven for the whole ref board, especially if reading the temperature still works.Regular noise at the supply is usually not a problem with reference or modern amplifiers. The problem is more with interference from higher frequency background, e.g from a SMPS.Battery power may be still useful, as the gives good common mode isolation, kind of the ideal of an isolated supply. The big plus is that one can be reasonably sure to have a clean an well isolated supply. So those cheap 5 V power banks with a SMPS are not a good choice. Even with a good SMPS it is hard to exclude an EMI effect for sure, as the reference circuit may not be full tested for sensitivity either.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 26, 2021, 08:47:52 am
content: Quote from: View[+]Finder on January 24, 2021, 09:04:14 pmThis is a very strange practice for this forum. I read many topics that were written 5 years ago. And imagine what would happen if I now came across an interesting information to me with a postscript put a "like" and I write about it .... This is a vicious practice.There is a wish to write - write. No desire to write - don't create a topic.

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 07:19:37 pm
content: In my list of projects in progress, there are a half-dozen that i want to write about and that I think will of be of general interest. Ok, some--like the Fluke 332D, may interest only a few. So, YES, I will write and have photos and notes on schematics I have found (blah, blah . . .) Okay?First, I wrote in an existing thread and got accused of "hijacking" and now, after following the helpful suggestions of 'super contributors,' I started new threads, open to anyone to contribute. In order to prioritize the posting of content, I asked for an indication of relative interest ('thank') so I don't waste time . . .I never learned to touch-type and am not that fast even two-finger style like the old war correspondents in the movies. I also try to edit before posting and because I have more than the typical USA-person's experience living abroad, I am conscious of the broad reach of Dear Leader Dave's forum.Now, on to posting what I've been working for the past hour . . .

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 07:34:12 pm
content: Testing a Battery as a Reference:I have been using a PDVS2 as a reference for testing the Keysight 34470 voltage reference in my 34465 DMM. It was suggested that the 399-based PDVS2 might be the source of noise incorrectly attributed to the 34465.It was suggested to use a battery as a reference and I had a LiPo close at hand so started a test yesterday. This is not yet a test of the 34465, it is a baseline test of the 'battery reference' using a 3458 and with the 34465 only tasked with collecting temperature measurement from a thermistor in the box containing the DUT (the battery).A photo of the setup and a plot of the results posted below. The 1000 observations were captured on 15 second intervals.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 26, 2021, 08:10:53 pm
content: Something looks quite noisey in the readings of the battery, I bit like there is somerhing going wrong (e.g. a loose cable, a mobile phone way to closem wrong PLC, non AZ or similar). The data look like essentially constant for some time and than jump quite a bit.Even the original 34465 should normally read a much more stable voltage.

author: iMo
author_rank: Super Contributor
timestamp: January 26, 2021, 09:56:58 pm
content: I spent a month messing with measuring batteries (wanted to see how my 34401's 399 is popcorning). Never saw a steady voltage. You would need a battery with pretty small internal discharge and the battery kept at constant temperature. LiPo seems to me the worst choice..

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 10:39:48 pm
content: Quote from: imo on January 26, 2021, 09:56:58 pmFrom your experience testing batteries, what's a better choice? What makes noise in a LiPo battery? My guess would be the chemistry, at least from what I have read about Weston cells in comparison.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 26, 2021, 10:46:42 pm
content: The LiCells can still have some stress / chemical gradients after charging. So they may take some time (e.g. weeks) to stabilize after charging or heavy discharge.In most batteries it only takes a little to disturb internal passivating layers or move liquid around. In that respect a not so liquid electrolyte is good. A low internal resistance also helps, as it means that there is likely no strong passivating layer. AFAIK alkaline cells can be OK, but need some rest hafter handling and a stable temperature as the TC is relatively large.

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 10:53:42 pm
content: Quote from: Kleinstein on January 26, 2021, 08:10:53 pmAttached a plot of voltage vs time. I trimmed the first 200 observations as it appeared that the system had not yet stabilized.No loose cable, NPLC=10. I'm not sure I can find the "constant for some time and than jump quite a bit" you describe. The plot of the internal temperature of the 3458 shows that, however that is just the granularity of the temperature from a query.How tight a plot of a 10VDC 'perfect' reference should I expect from the 3458 under the best setup? My major goal is to identify and minimize negative influence.

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 10:55:58 pm
content: Quote from: Kleinstein on January 26, 2021, 10:46:42 pmWhat bout those 9V 'lithium' long-life batteries made for smoke alarms? I have a few of those available.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 26, 2021, 11:04:55 pm
content: The noise of the LTZ1000 ref in the 3458 should be some 1-2 µV_pp at 7 V or 1.5-3 µV at 10 V for some 100 points. The ADC at 10 PLC may give some 200 nV_RMS or 1.2 µV_pp. So expect some 1.7 -3 µV_pp of noise for a perfect 10 V ref. The Curve shown is more like 50 µV_pp - so there is something really wrong - likely using the 100 V range.The long life batteries tend to have high resistance and a relatively strong internal passivation and may not be that good. I would more like think of 6 new alkalline AA cell.

author: View[+]Finder
author_rank: Unknown
timestamp: January 26, 2021, 11:29:55 pm
content: Quote from: Kleinstein on January 26, 2021, 11:04:55 pm"Using the 100V Range" Well yeah, that's what happens with a 12+ volt measurement. Thanks for catching that; I didn't even think about the impact of different range selections. What about the NPLC=10 and the 15 second interval? Easy to change.While I'm confessing ignorance . . .there is enough equipment on in my lab that the temperature controlled exhaust fan is on. The temp control is a wireless thermometer that controls a power plug for an AC motor fan. The thermometer is 2 meters from the 3458 as is the fan. See any problems there?

author: TiN
author_rank: Super Contributor
timestamp: January 27, 2021, 12:43:05 am
content: QuoteHere's very old plot with KX LTZ1000A ref data. 4 DMMs used measuring xDevs.com KX 7V output. Normal deviation over hours period with stable conditions (temperature, humidity, air drafts) should be 1-2 uV/V typical. So that gives ideal <0.2ppm deviation over days in static conditions with 3458A. Dedicated setup with nanovoltmeter in opposition scan can do <0.1ppm on good day (Here's log with 6C ambient variation for example.)Anything better than 0.1ppm with commercial devices is nearly impossible for DC voltage at room temperature.

author: View[+]Finder
author_rank: Unknown
timestamp: January 27, 2021, 01:34:38 am
content: Hello Illya,Lots of good data to pour over, it will give me a good basis for comparison. I believe you are in Taiwan, right? It looks like your air conditioner might be cycling the relative humidity on the one graph. My temperature range at this time of year can be as much as 21C to 29C (we don't run the heater at night) and I keep the lab door closed. Keeping temperature under control is a major headache for me. I have the ceiling vent blocked to avoid the drafts on the meters and daytime temperaturehere in winter could be anything from sunny and warm to ice on the car.Your PPM does capture the negatives as well--I was wrong in my comment in an earlier post--I must have gotten information on PPM from some of the 6-sigma stuff here in the US. From the data I have seen from my own collection, median and mean should be very close and the median is less affected by a few outliers. So your calculation is "value minus the median, averaged" as a PPM of the median, right? I see nothing wrong with that.Right now I'm going to check some voltages on the sick 3458 reference.Many thanks,Donal

author: TiN
author_rank: Super Contributor
timestamp: January 27, 2021, 02:08:44 am
content: There is no minus operation in relative percent or ppm scales.Only division with formula (REFvalue / TESTvalue) * scale. For easier operation I only remove 100% level for clarity, no point to say it everytime.Often accredited labs and science people show deviations in units/unit scale which is also easy to use. E.g. value of 1 ppm from 1.2345678 V equals 1.2345678microvolts on absolute scale, or can be written also as 1uV/V .If deviation shown asabsolute error from nominal(in above example 1.2345678 is +234567.8 ppm off ideal 1V). Such presentation often listed as such deviationfrom nominalin note/comment, but it's not a requirement for every "ppm" value). I found using ppm scales much easier than trying to do math every time how big was a change from 7.054835 to 7.054885 V arbitrary values.You can refresh these basics from SI units conventions.If you want good stable measurements, you have to maintain conditions constant. There is no way around it, short of putting all equipment into thermal chamber. All instruments drift and depend on ambient conditions, it is very rare to see ones that don't suffer from this.

author: View[+]Finder
author_rank: Unknown
timestamp: January 27, 2021, 05:19:51 am
content: Plot of 'Battery as Reference' 1000 observations 15 second interval NPLC=10 LiPo 2S2P 8.4V nominal tested a 8.34VDC.UPDATE: Same setup; second set of data . . .

author: iMo
author_rank: Super Contributor
timestamp: January 27, 2021, 08:32:21 am
content: Quote from: View[+]Finder on January 26, 2021, 10:39:48 pmThe requirements are obvious - a constant temperature, temperature as low as possible, no electro-mechanical contacts, low internal discharge, low load/charge caused by your DMM. I would be happy to know the best choice too

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2021, 05:11:13 pm
content: The Li battery seems to have a temperature sensitivity of some 200 µV/K from the curve shown. So it need a really stable temperature to use it as a short time reference - so think about 1/100 K stability and better. This would be something like some extra mass (e.g. water bottles) and than inside a well isolated isolated box. If it would be very easy, we would have more battery type references. The old Weston cell was also chosen to have a low TC.There also some small jumps - these could indicate a problem, but with so much drift it is hard to tell (could also be an EMF problem).For the sampling and timing, I would use something like 100 PLC and more continuous sampling (e.g. 4 s spacing).

author: SilverSolder
author_rank: Super Contributor
timestamp: January 27, 2021, 06:50:03 pm
content: Quote from: View[+]Finder on January 26, 2021, 07:19:37 pmYou need a session with Mavis Beacon!http://www.mavisbeaconfree.com

author: View[+]Finder
author_rank: Unknown
timestamp: January 27, 2021, 07:40:06 pm
content: Quote from: TiN on January 27, 2021, 02:08:44 amOn your 'Test' plot you show "Deviation from Median, PPM" how were minus values shown on the Y-axis using the calculation above? What exactly is the "scale" multiplicand in the formula?

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2021, 12:36:59 am
content: Scale is 1.0 for percent representation or 1e-6 for ppm. Minus values mean reading deviation is lower than reference value. Plus values are higher than reference.

author: View[+]Finder
author_rank: Unknown
timestamp: January 28, 2021, 02:45:24 am
content: Quote from: TiN on January 28, 2021, 12:36:59 amI think I understand Plus and Minus.If TEST < REF then REF/TEST will be >1. Does that mean that you set values greater than 1 as negative for plotting?LET:REF = 10TEST = 9Then:REF/Test = 1.1111 Shouldn't that be the other way round? (assuming REF is the desired value and TEST is what was measured)If I measure a 10ohm resistor and get 9ohms, that's a -10% error [ (9-10)/10 * 100] =-10% (the way I would do it) Or in metrology, is the error based on the measured value? [ (9-10)/9 * 100] =-11.1111%10/9= 1.111 * 1 ?? I just can't get my head around what that means. Sorry for being dense . . .

author: View[+]Finder
author_rank: Unknown
timestamp: January 30, 2021, 08:57:15 pm
content: Attached is a plot of differences in measured voltage readings from an LTZ1000 based reference. The readings were differenced by a Lag+1 such that the ratio was:Reading(+1)/Readingthen subtract 1 (to remove 100% for clarity as TIN suggested)and multiply by 100 (for 'PerCent' -not needed) ormultiply by 10^6 and finallyshow as PPMSummary:mean = 0StDev = .25intercept = 0slope = 0R-Squared = 0My conclusion: no serial/auto correlation in the data. Data measured at Lag+10 and Lag+100 show similar values.