how to perform the SN18 test? - Page 1
Forum section: electronics / general

author: niner_007
author_rank: Frequent Contributor
timestamp: April 09, 2019, 05:36:08 am
content: Do I need to short the inputs on the 3458A? Common sense dictates that when running CAL? 72 the displayed value is computed from the internal reference, and inputs don't come into play, but checking anyway.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 09, 2019, 06:32:41 am
content: SN 18 tests the A/D converter chip (CAL? 72), but also the internal reference. For latter, you need a stable external reference.For the A/D test, performing ACAL DCV is required, which is independent from any input, so you can leave your stable external reference connected. Shorting makes no difference.What's not mentioned in the SN 18, that the gain constant CAL? 72 also depends on the environmental temperature, max.or typically0.5ppm/K, according to the DCV T.C. specification. This is in the same ballpark as the drift criterion. Therefore, you need to perform this test at a constant environmental temperature.My A/D has a T.C. of about +0.25ppm/K, so by determining this T.C., one can calculate the real drift of CAL? 72 by measuring the internal temperature.Frank

author: TiN
author_rank: Super Contributor
timestamp: April 10, 2019, 05:03:01 am
content: As Dr.Frank mention, SN18 does not care about what you connected at the input. However SN18 test take very important assumption that A9 reference is stable better than 0.2ppm, which is not always the case. To test A9 stability you must have more stable 10V source (Fluke 732A/B/C or Datron 491x or other proven 7-10V reference).Also SN18's specification (0.43ppm/day) is very wrong. Good stable ADC must be at least than0.01 ppm/dayfor unit to be able meet annual specifications. This is what I test against in my units.My recent results are herethis here

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 10, 2019, 01:15:08 pm
content: Just to render TiNs statement more precisely, a stable voltage reference is NOT required for the A/D hybrid test (the so called 'short-term drift').That really stupid 0.43ppm/day criterion is simply derived from the ageing law of the LTZ1000A reference.All such temperature induced ageing processes (Arrhenius) follow roughly a square-root-law:dU/dt ~ 8.2ppm/year * sqrt(t/365 days), with 8.2 ppm/year being the standard annual drift rate.From that, you can calculate 1 day, 90 days, 1 yr and 2 year drift specification of the LTZ1000A alone, 1 day would be 8.2ppm * sqrt(1/365) = 0.43ppmA good A/D hybrid has to be much more stable, as such high a drift would be a linear effect, i.e. w/o ACAL DCV it would drift 0.43ppm/day * t, giving about 39 ppm after 90 days.The specification btw. contains NO explicit specification w/o using ACAL DCV.There's an indirect hint only, that specification limits are valid within 24h of last ACAL only... very poor specification method.And in fact, a fully working instrument will mostly return to or stay at its original reading, at the same operating temperature.Frank

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 10, 2019, 01:56:35 pm
content: Quote from: Dr. Frank on April 10, 2019, 01:15:08 pmEven if the A3 Board will drift nearly 8ppm/day the meter will pass the calibration. Or is there a calibration procedure at Keysight which will uncover such a drift? Our local cal lab runs ACAL and after that they run through the calibration points. Sounds awful, but the readings will be in spec. If you only looking for absolute values it could be even ok for youThere is no need for the ADC to be stable to 0.01ppm/day in the 3458A (but it needs to be stable enough to fullfil the 24h spec together with the reference, but that isn't tested by most of the cal labs).

author: MiDi
author_rank: Frequent Contributor
timestamp: May 22, 2019, 07:01:44 pm
content: Quote from: Dr. Frank on April 10, 2019, 01:15:08 pmThanks for this insight, now it is clear where this 0.43ppm/day is derived from.Following this, the 0.43ppm/day is not applicable for option 02 (4ppm/year).I do not understand where this same 0.43ppm/day for A9/cal 72 is derived from, as the reference drift should not affect cal 72.Best 24h spec is 0.55ppm for 10V, where 0.5ppm are most due to TC spec for 1°C span and 0.05ppm from ADC linearity.Not much left for ADC-Drift

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2019, 07:35:21 pm
content: The SN18 note is because there are some defective ADC boards that show a lot of drift. On the longer term this tends to cause self test errors, but the meter could be outside the specs before. One point is the drift itself, the other point is an expected negative effect on linearity. A drifty ADC that fails the SN18 limit has good chances to also have a linearity problem.The 0.43 ppm/day limit is kind of arbitrary. Good units should be much better and bad units tend to be way worse.There is also a possible temperature and humidity effect on the ADC gain - so if not used for some time, there can be some drift, that is normal. The steep initial slope in the last plots could be such an effect.

author: niner_007
author_rank: Frequent Contributor
timestamp: October 09, 2019, 07:36:33 am
content: I'm testing one of my recent eBay purchases, the data so far seems good, but it's only been 3 days. The unit is an old HP, with a trash looking enclosure, and old VFD, some of the internals are HP and some are Agilent, but the internals are unusually clean, and in a good state. The unit also has red round/circle stickers on it, as it was marked or tagged, not sure what it means.

author: niner_007
author_rank: Frequent Contributor
timestamp: October 29, 2019, 05:19:21 am
content: here are some prettier graphs, it looks pretty good

author: notfaded1
author_rank: Frequent Contributor
timestamp: October 29, 2019, 01:27:39 pm
content: There's another one with issues that just went up:https://www.ebay.com/itm/AGILENT-3458A-8-1-2-DIGIT-MULTIMETER-FOR-PARTS-OR-REPAIR/274071953856?epid=24019382319&hash=item3fcff621c0:g:nJYAAOSwWq9dt238It says you have to be a US Citizen to buy it.Bill

author: MiDi
author_rank: Frequent Contributor
timestamp: October 29, 2019, 09:25:57 pm
content: Welcome to the club niner_007Is your graph made according to SN18 - are the dppm referred to first measurement?The temperature is stored in CAL 175 for last autocal and gives more digits than TEMP? - might be useful to determine TC of ADC.Did you check if the input Filter is from Schaffner and if the extra Y-caps are RIFA - exchange highly recommended due to high failure rate at this age

author: niner_007
author_rank: Frequent Contributor
timestamp: October 30, 2019, 01:42:34 am
content: Quote from: MiDi on October 29, 2019, 09:25:57 pmThe dppm is strictly relative to last measurement, not SN18.I have not collected CAL 175, I have collectedTEMP?at the end of ACAL, which is nearly the same

author: niner_007
author_rank: Frequent Contributor
timestamp: October 30, 2019, 01:58:01 am
content: Quote from: notfaded1 on October 29, 2019, 01:27:39 pmThat is probably because it is sold by or on behalf of a government agency, and there is concern there might be secrets that should not be exported, but it's just regulation really. I have a bunch of fun 3458As, one was from DoD, another from Boeing and another from Flir Systems who makes the thermal cameras, fun

author: niner_007
author_rank: Frequent Contributor
timestamp: October 30, 2019, 07:04:21 am
content: included is SN18 relative ppm as well: