Has my 3458A got a problem? - Page 1
Forum section: electronics / general

author: IanJ
author_rank: Unknown
timestamp: November 07, 2023, 09:54:54 pm
content: Hi all,Check out my 3458A at around 7:26 in on the video below, any ideas?If I set up a voltage source, nice and stable with PTFE/ETFE cable, gold conns and then disconnect cable at one end then reconnect my reading sometimes changes…..by quite a lot in 3458A terms.Have tried all sorts of different cables and voltage sources.https://youtu.be/m2wV7W5zASs?si=WF9AAo9L8aleS3SJIan.

author: Kosmic
author_rank: Super Contributor
timestamp: November 07, 2023, 10:02:25 pm
content: Thermal EMF due to banana connectors ?I guess if you let the temperature equilibrate it goes back down after a while ?

author: IanJ
author_rank: Unknown
timestamp: November 07, 2023, 10:05:26 pm
content: Quote from: Kosmic on November 07, 2023, 10:02:25 pmNo, it stays forever and will exhibit the problem even if I let it sit for hours.Ian

author: Kosmic
author_rank: Super Contributor
timestamp: November 07, 2023, 10:10:58 pm
content: Quote from: IanJ on November 07, 2023, 10:05:26 pmHo ok I see

author: alm
author_rank: Super Contributor
timestamp: November 07, 2023, 11:10:13 pm
content: Have you tried spade lugs, or clean (freshly stripped) copper wires? I don't trust banana plugs for (sub)-ppm level work. I have the feeling that there is something going on with the contacts, like dirt that sometimes gets in the way, and other times not.

author: Kosmic
author_rank: Super Contributor
timestamp: November 07, 2023, 11:19:19 pm
content: Also is the meter in auto ranging mode ? you could try manual to isolate the prob.

author: IanJ
author_rank: Unknown
timestamp: November 07, 2023, 11:27:41 pm
content: OMG…….how did I not notice the meter was jumping between 1V and 10V ranges…….there is NO problem.Reviewed this on my phone, so will check tomorrow.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 08, 2023, 12:46:43 am
content: As already mentioned below your video, the 3458A should read the same within 0.3ppm, when you have done an ACAL DCV just before, and maybe you also should have a look on difference between zero readings in both ranges, when you apply a short.The deviation between both ranges of 3.2ppm is way too high for the 3458A, and also your foregoing calibration is 1/10 of the deviation, i.e 0.68ppm instead of 6.8ppmFrank

author: deepfryed
author_rank: Unknown
timestamp: November 08, 2023, 04:38:08 am
content: Quote from: IanJ on November 07, 2023, 11:27:41 pmhaha happens to the best of us

author: IanJ
author_rank: Unknown
timestamp: November 08, 2023, 12:22:01 pm
content: Quote from: Dr. Frank on November 08, 2023, 12:46:43 amAfter ACAL DCV the ranges now agree, albeit these 3245A's are quite noisy beasts so can't give a figure. They are stable down to about 1uV, after that it's pot luck, hence why I want to get into the DAC etc and see what can be done about it if anything. The PDVS2mini is less noisy.Ian.

author: deepfryed
author_rank: Unknown
timestamp: November 08, 2023, 12:53:07 pm
content: Quote from: IanJ on November 08, 2023, 12:22:01 pmI'd be interested in what you find, I recently purchased one (can run some tests to compare notes, but I only have a 34401a). Given the higher resolution DAC is a discrete PWM version, perhaps the low pass filters will need some tweaking.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 12:57:46 pm
content: How noisy is the 3245 actually, just to get an idea.Is it actual noise, or residual ripple from the PWM DAC.One possibly eays point could be adding a little filtering the reference / ref. amplification. The PWM DAC can reactor the reference noise near the PWM frequency. This may add some 150 nV/sqrt(Hz) or ~1.6 * the reference noise to the DAC's low frequency noise for a near zero output in the 10 V range.A relatively simple first try could be to add a 1 µF capactor between pin 2 and 6 of U111 and U110. This could do at least partial ( ~ 2/3) filtering. Better filtering would be more at the input of U110, but this needs more effort.

author: deepfryed
author_rank: Unknown
timestamp: November 08, 2023, 01:11:35 pm
content: What about noise in the U110 and U111 power supplies, worth checking ?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 01:26:14 pm
content: The power supplies are worth checking with an old used meter, as a possible defect.Normal noise with U110 / U111 would be less an issue, as they are not that noisy (~16 times less than the reference). A possible thing would be the settling after the switching spikes. However this would be more a INL issue, not so much noise.Before any modification one should measure the noise to know which oder of magnitude one is looking for and what frequencies.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 08, 2023, 01:47:58 pm
content: Quote from: IanJ on November 08, 2023, 12:22:01 pmIan, you might replace the LM399 by an LTZ1000 circuit, and put a plastic cap on top and bottom, around the whole reference circuit.That gives a much less noisy reference, and the possible air drafts inside the instrument also create noise.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 02:45:21 pm
content: Chanigng to a LTZ1000 is quite a big change. A ADR1399 is about half the way and needs only minor changes for the rest.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 08, 2023, 03:32:14 pm
content: Quote from: Kleinstein on November 08, 2023, 02:45:21 pmNope. That ain't no such big change.The 34465A /470A use either LM399 or the usual LTZ1000 circuit as a seamless plug-in.That's been demonstrated, that this is a very simple replacement, provided the 3245A reference circuit is similar to the 34465A, or its predecessors.Also the calibration software must swallow the higher reference voltage, maybe a Firmware modification is required.LTZ1000 has about half the noise of the LM399, so are there already any reliable noise measurements on the ADR1399 available?One could even go one step fruther, to use an ADR1000 instead of the LTZ1000.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 04:58:33 pm
content: AFAIK the LTZ1000 is more like 1/4 the noise of the LM399 and the ADR1399 is about half the noise. There are a few noise measurements around here in the forum - not sure how reliable. About half the noise makes sense with the higher ( ~ 4 x) reference current.The filter part and reference change address 2 different ranges: the filter only helps mainly for the lower DAC setting, no more near the full scale. The better reference mainly help with a higher output, but not that much at near zero output. Ideally one would have both, especially as the filter is not really expensive compared to a lower noise reference.As far as I can see from the limited DAC part plan there are a few other noise / drift sources that are not really at a LTZ level, like a TL072 to buffer the reference to the faster DAC.

author: iMo
author_rank: Super Contributor
timestamp: November 08, 2023, 05:00:29 pm
content: Branadic has shown 2uVpp, I saw 2-3uVpp with 1399. That is aprox half of a typical 399 (I saw 4-5uVpp)..RC filtering is difficult, imho..

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 05:24:32 pm
content: RC fitlering for the really low frequency part is nearly hopeless. However filtering for the PWM frequency range (e.g. some 200 Hz) is relatively easy.The point that is easily missed is that the PWM switching acts like a mixer and brings the reference noise from the higher frequenicies back to near DC when the ouput is at 0 ( 50/50 pwm setting). So it is about getting rid of this higher frequencies. It is less noise than the low frequency reference noise, bit still quite a bit more than the OP-amps and resistors that are other otherwise relevant for a near 0 V output.

author: IanJ
author_rank: Unknown
timestamp: November 08, 2023, 06:32:58 pm
content: Quote from: Dr. Frank on November 08, 2023, 01:47:58 pmI have two 3245A's and they are both about the same in terms of noise. One has an LM399AH, the other an ADR1399.The noise is being generated further down the chain on my first quick look.If you look at the Pcb layout for the Vref circuit: The LM399, the precision resistor array IC and its op-amps, and the first thing that struck me is they are spread out surprisingly quite far across the analogue board, and there is very little if no filtering on the Vref signals at source (+11.6V & -11.6V)........so definitely something can be done there, and I suspect the best approach would be other than improving the supply rails (+/-15Vdc) would be do build a replacement daughter board circuit to generate the precision +11.6V & -11.6V and inject them at the appropriate place. But first I need to scope things and characterize where the noise is mostly being generated from.Here's a very quick snapshot of the noise from the LM399AH 3245A courtesy of WinGPIB.I just need to find time to get into all this.......

author: iMo
author_rank: Super Contributor
timestamp: November 08, 2023, 07:21:40 pm
content: Do you have the 1uF ser 5R1 at the 1399's zener there?PS: 3-4uV jumps at the LM399's zener are quite normal..

author: Kleinstein
author_rank: Super Contributor
timestamp: November 08, 2023, 09:16:35 pm
content: The noise does not look that bad. The jumps still look a bit large (1.2 ppm if I understand the scale right) for the LM399 typical popcorn noise (normally around 0.6 ppm).Ideally one would look at the noise for a few output cases, including 0 V and some +/-10 V, so close the full DAC range.The noise with a large output and zero output can be 2 different issues.Is there an extra 1 V range with an extra gain step, or is this directly some 10% from the DAC ?

author: deepfryed
author_rank: Unknown
timestamp: November 09, 2023, 12:50:25 am
content: Quote from: Kleinstein on November 08, 2023, 09:16:35 pmNeed to confirm in schematics, I think the extra 1V range has a separate gain stage. It's calibrated separately and from what I notice output is not a direct 10% from the 10V DAC output.