Characterising thermal drift - Page 1
Forum section: electronics / general

author: BradC
author_rank: Super Contributor
timestamp: June 24, 2018, 04:19:19 pm
content: G'day Nutters.I have a 3457a and a couple of 3478a. They live in a room that is not particularly thermally stable. I participated in the "Aussie cal club" rounds, but during the final round a pile of my measurements appeared fairly temperature sensitive. For that reason I haven't published results other than some raw values.I built up a reference from an AD587 and I'm seeing drift in the order of 90uV over the course of a day. I put a heat gun on the reference this morning and got some drift, but it wasn't in the direction I expected.Basically I'm trying to get a handle on the thermals, so I'm after some advice on a reference I can build/buy that will give me some thermal and medium term 48-96 hours stability, accuracy unimportant as long as I can get stability against time and temperature.Any ideas without spending a couple of hundred bucks on an LTZ1000 based ref?

author: IconicPCB
author_rank: Super Contributor
timestamp: June 24, 2018, 08:46:23 pm
content: If absolute value is not important in this experiment check out Rockby's offering . They claim o have a few LM299 at 27 bucks a pop.That ought to give You an ovenised reference comparable to DMM's own.

author: Andreas
author_rank: Super Contributor
timestamp: June 24, 2018, 08:59:51 pm
content: Quote from: BradC on June 24, 2018, 04:19:19 pmbuild a little oven around your AD587?http://www.techlib.com/electronics/ovenckts.htmOr use a LM399 instead.with best regardsAndreas

author: cellularmitosis
author_rank: Unknown
timestamp: June 25, 2018, 04:02:04 am
content: BradC, I think Andreas is too modest to brag -- have you seen the excellent results he has achieved with the AD587? And without an oven at that:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/Even if your part has a different tempco profile, you might be able to adapt his circuit with different component values.Of course, an oven might be the easier route. Have you seen blackdog's recent LM723-based ovens?https://www.eevblog.com/forum/metrology/small-oven-controler-for-voltage-reference/

author: BradC
author_rank: Super Contributor
timestamp: June 25, 2018, 04:43:20 am
content: Quote from: cellularmitosis on June 25, 2018, 04:02:04 amYes, I have been keeping an intermittent eye on that thread. I've taken a look at the ovens also. That is some pretty speccy hardware.I suppose the first thing I need to do is hook up my GPIB interface, put a thermal probe on the meter and the reference and start graphing things. At the moment I'm flying blind and just pontificating.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 25, 2018, 08:09:35 am
content: I'm not sure how the the circuit from the 3478A looks like. In the 34401A it is possible to measure its own reference. In such a setup you will see only drift which isn't related to the reference in the meter. Perhaps that is also possible in the 3478A.The 3478A contains already a heated LM399, but that doesn't even matter. In such a setup you would only see the drift of the meter itself and you would be independend of any reference.

author: BradC
author_rank: Super Contributor
timestamp: June 28, 2018, 02:48:32 am
content: Right. Finally got the bugs worked out of my GPIB logger (was trying to read data from the 3457a too quickly after it asserted SRQ/RDY leading to intermittent failures).Only a short run overnight, but demonstrates the issue.I have 2 Dallas DS18B20 probes. One is taped to the ref (which is in an aluminium box) with PVC tape .The sensor is in firm contact with the shell of the box. The other is inserted into the 3457a behind the accessory slot about in the center of the A/D part of the meter.The first is the measured voltage vs the measured reference temperature. The second is the measured voltage vs the internal meter temperature.I'll get the room nice and warm today. It's going to get down to 3C tonight, so I should get a nice temperature swing.The next task will be to put the reference in some kind of controlled environment to see which curve fits best.(edit: After much fiddling with gnuplot, add a combined graph)

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 28, 2018, 07:10:02 am
content: Hi Brad,that looks really bad. I did similiar tests last month with a couple of meters at home. Due to the hot days the temperature in my room went really high. I used my ovenized Fluke 5440B as 10V reference. I attached a few measurements as a comparison.For the LTZ1000 measurement I used my HP 3456A as Nullmeter (because of the guard connector). The 2.5µVpp is more drift and noise than real TC.Edit: I added the LTZ1000 vs. time graph to clarify what is noise and what is TC.

author: BradC
author_rank: Super Contributor
timestamp: June 28, 2018, 11:00:50 am
content: Quote from: e61_phil on June 28, 2018, 07:10:02 amYeah, it's a fairly large swing which is why I'm not entirely confident in any measurements I might make with it. Now having said that, the ref is in an aluminium box, on vero board with a trimming network flapping in the breeze. It's not exactly a precision construction.I have 4 fleabay AD588BQ references on the way, with the intention of hooking 3 of them up in series to give me a "stable" 30V ref I can feed to a caddock divider to get 3, 0.3 & 0.03V. It'll be interesting when they arrive to run them in the same configuration and see what is what.You can see now why I'm loathe to publish tested values for other peoples references on this meter when I'm not even sure it is stable itself.One thought I that just occurred was since I have a 3457a and a pair of 3478a's, I could pop the cover on one of the 3478s and measure the reference in there. I believe they use the same or similar LM339 based reference (HP 1826-1249). I would expect it's more stable than the AD587 built on duct-tape and chewing gum. I might look at that once I get the kids in bed.

author: e61_phil
author_rank: Frequent Contributor
timestamp: June 28, 2018, 11:10:31 am
content: I have also two 3478A which I can test after my vacation. They have a LM399, but the resolution of the meter is only 5.5 digits. I would expect seeing nothing during such a test.You could also try to measure the LM399 reference voltage from the 3478A with your 3457A.

author: BradC
author_rank: Super Contributor
timestamp: June 28, 2018, 11:34:35 am
content: Quote from: e61_phil on June 28, 2018, 11:10:31 amYes, that is what I had in mind. I have the 3478a already measuring the same ref as the 3457a, and you are right, there is no noticeable change. They fluctuate 1 count with noise (one alternates between 10.0000 & 10.0001 and the other between 10.0004 & 10.0005). I intended to pop the lid on one of those and have the 3457a measure the reference in the 3478a directly (supposedly somewhere between 6.8-7.1V).

author: CopperCone
author_rank: Super Contributor
timestamp: June 29, 2018, 01:24:22 am
content: you can do a FFT on it like anything else to see if various processes have various time constants. Probably will be messy and you have to figure out the correct windows and shit. No idea what works well for low frequencies. In my limited attempts at this I found difficulties.

author: BradC
author_rank: Super Contributor
timestamp: June 29, 2018, 04:38:36 am
content: This is the last one based on this setup, I'll pop one of the 3478a today and hook up to the ref on that, but check out this swing over temperature.

author: BradC
author_rank: Super Contributor
timestamp: July 01, 2018, 01:18:12 am
content: So I hooked the 3457a up to the ~10V ref output on one of the 3478a.I'm logging the internal temp on both meters and also the ambient, and yes I'm still seeing a significant thermal swing and the curve appears to correlate fairly closely with the internal temp of the 3457a (green curve).Having said that, I checked the service manual for the specifications. The 30V thermal spec for the 3457a is %0.0005 + 1 count per degree C. I'm seeing somewhere around two orders of magnitude better than that, so perhaps I'm chasing ghosts in expecting the meter to behave significantly better than it's rated specifications.When a 5 degree (ambient) swing results in a 10 count change (which if my maths is any good is 100uV in 10V or 10PPM), resulting in ~2PPM/C I suppose I can't be that picky.The accuracy spec calls for TCal +/- 5C. TCal was 23C, so between 18 & 28C. Right now I'm at the bottom end of that.I'm going to let this log run go for another day or so and see what falls out, but while what I'm seeing looks pretty awful it all appears to be significantly within the meter specifications. It also appears to be quite repeatable. The voltage curve follows the internal temp of the meter pretty closely. I wonder if I can log enough data to produce a correction factor that might apply in a stable environment (ie when I'm not deliberately ramping the temperature all over the shop)?

author: CopperCone
author_rank: Super Contributor
timestamp: July 01, 2018, 04:48:19 pm
content: has anyone ever done really low frequency FFT on this type of data?

author: BradC
author_rank: Super Contributor
timestamp: July 02, 2018, 04:10:20 am
content: Quote from: CopperCone on July 01, 2018, 04:48:19 pmGenuine question (isn't it sad when I have to say that to try and not come off as sarcastic?). What would you hope to see fall out of that?Maybe if I explain what I expect to be seeing from the data, you can help me understand what else I might be able to extract.I have 5 elements.The voltage source is the LM339 + scaling to bring it up to 10V as found in a HP3478a. I've popped the top off, hooked a 1:1 scope probe to C430 & C431 to get 10V and have that probe connected to the input of the 3457a. The top is sitting back on the meter to restrict air circulation (although this one has perforated sides in any case).The logging setup is logging 2 x 3478a, 1 x 3457a & 3 Dallas DS18B20 thermometers. For the purposes of this exercise the logging on the 3478a(s) is irrelevant, but it's still happening so I'll detail it as part of the algorithm.The meters are all set up at max resolution and NPLC (100 for the 3457a & 10 for the 3478a) and the thermometers are in 12bit mode (0.0625C) resolution.The logging routine triggers all 3 meters with a GET, triggers the conversion on the Dallas sensors and then enters the data collection routine.The data collection routine waits on an SRQ, performs a serial poll of the 3 meters and then retrieves waiting data. If a 3478a is complete and the 3457a isn't, the 3478a is re-triggered. This loop runs until data is available on the 3457a. The loop then waits for either 3478a that has an outstanding conversion to complete and returns.This is a hangover from an earlier experiment I had where I was making as many conversions on the 3478a as I could between 3457a conversions (~8 or 9) and averaging those conversions.When the 3457a is ready, it's output buffer is read, SRQ mask set to 0 and the HIRES register read. Provided a valid output buffer and valid HIRES read completes, the returned values are converted to 32 bit integers and the HIRES applied. My float routines are singles, and although the meter returns floats natively I was running out of resolution using float math, so I put some fixed scaling in place based on a 10V scale to get a more accurate numeric representation (with the understanding that those numbers are effectively meaningless but I wasn't corrupting the last meaningful digit from the meter with bad math).This whole routine averages a cycle time of 5.xx seconds. I'm recording the raw output register, hires register, float converted value & fixed point converted value. On the graphs posted here I'm using the fixed point converted value, which is 2 decimal points greater in resolution than the meter is actually capable of (meter is 7.5 digits and resulting calculations are over 9 digits), resulting in some of the noise on the plots.The whole point of the exercise was to try an ascertain whether it was the voltage source, or the measuring device that was drifting thermally.Looking at the graphs, the voltage drift visually correlates with the 3457a internal temperature, and last night I put a 50W halogen over the 3478a to add 10 degrees to its internal temperature and the voltage didn't move (it has been raining here so the ambient temperature has stabilised over the last 24 hours). So I *think* I'm confirming my theory that the 3457a is drifting more than the source voltage when exposed to significant thermal excursions. That is not the result I was hoping for, but the one I was expecting.Although initially my naive and emotional thought was that the meter is faulty. Having a look at the measurement path, and the specs in the service manual. Even if the internal reference is extremely well compensated, there is enough other hardware there which would be extremely difficult to compensate/match that what I'm seeing is probably quite reasonable under the circumstances. If I've read the manual correctly and done my calcs right (never a given) then it's well within specification anyway.Given the voltage swing curve fairly well follows the internal temperature curve I'm now starting to wonder what external compensation I might be able to apply to those numbers and whether that might be a valid thing to attempt.I'm now wondering if I set a tent up in the office with a low-inertia thermostatically controlled oil column heater and put the whole measurement stuff in there when I'm actually doing measurements. That would work in Winter as I could easily stabilise it at 23C (TCal for the meter). Summer might be more of a challenge.This has been an interesting process. Once I figure out what I'm actually doing, and trying to achieve it'll probably be of more use.I'm not chasing accuracy or sub-ppm stability, but I *am* trying to get a better understanding of what my gear is doing and ultimately capable of. What might I see from an FFT over these data sets given most of the swing seems to be ambient related. Can you do an FFT of the weather and get usable data?

author: martinr33
author_rank: Frequent Contributor
timestamp: July 02, 2018, 05:15:46 am
content: Seems that you should be getting enough data, such that you can estimate thermal drift for each unit. Once you have that figured out, you can adjust all the readings for temp and everything should close in together.The errors you have will come from:- long-term drift in the meters- small temperature variations around the meters- and I am sure a laundry list of other things.I usually do this the lazy way, and tweak parameters until everything lines up as best I can get it.So for each column of readings, add at the top of the spreadsheet:Offset Gain - two adjustments for the whole columnCorrected - an extra column that has the corrected reading, calculated by something like:Then, Corrected = (reading -offset) * gain *temp deltaYour goal is to tweak the offset and gain for minimum deviation.

author: BradC
author_rank: Super Contributor
timestamp: July 06, 2018, 07:11:54 am
content: Right, after being away all day yesterday and coming home to a graph with a hockey stick worthy of an IPCC report I decided to throw caution to the wind and break the cal seals to pop the top off and figure out what is so sensitive to thermal changes.As one would assume after looking at the circuit for the meter, it's the U101 input hybrid. Hardly a surprise as that has most of the input conditioning and divides the 30V range by 100 before it is multiplied by 33.3 back into the ADC (it is a perverse input path). I wandered around the rest of the analogue board, and while temperature variations resulted in short term excursions, things settled very quickly. Whereas temperature changes to the input hybrid were almost linear and most permanentSo it could be either the resistive 1:100 input divider, the 33.3:1 divider or a combination of both.As mentioned in previous posts, this drift is well within specification, but I can't help wondering if I built a little thermally regulated heater on the top of the hybrid (given it has such a huge flat top surface) if I could at least reduce the level of drift over ambient. It'd be interesting to see the results of that when it goes for a cal later in the year.Or of course I could just save up and buy a newer meter that doesn't drift as much, or stop worrying and learn to love the bomb. It has been an interesting process.Slightly infuriating, in that I don't actually need the accuracy, but I'd like to have some more confidence in my readings. Awkward when my office is subject to such ambient variation.edit : I tried a quick squirt of contact cleaner onto the same hybrids in the 3478a and got about the same reading swing on a 10V reference. I'm increasing the resolution of those meters by averaging 10 readings to give me an extra decimal, and the swing is about the same as I saw on the 3457a, so it appears characteristic of those hybrids (3 meters, 3 different manufacturing dates, 3 different hybrids and roughly similar results). So a new hybrid isn't likely to change things. Next to sort a 3V reference and see if it's the divider or the multiplier.

author: e61_phil
author_rank: Frequent Contributor
timestamp: July 06, 2018, 06:53:52 pm
content: I think here in the forum somebody showed his own divider to bring 10V to 3V and use the 3V range of a 3457A to measure 10V.