Electronics > Metrology
Affordable Useful AC & DC references
Majorassburn:
It was suggested by other forum participants that I start a new thread to discuss specs and features of AC & DC voltage references that I make and sell on eBay (as SQWARREL. Sample Link https://www.ebay.com/itm/285789548856) where I kept things brief and non-specific. So, here is my attempt to do so. I appreciate your input and suggestions as this thread (and my device improvements) develop.
My intent is to make and sell very low-priced but USEFUL references for those of us who are not yet ready to resort to costly lab calibration. We may want to be able to perform basic, "ballpark" function checks of our DMM's to see if they're even working correctly or reasonably holding calibration. And, we can't justify spending $150 or more for higher quality devices like the DMMCheck+ and others.
AC Voltage Reference: Here's a device that generates a relatively pure, NON-FLUCTUATING, adjustable sine wave from 0 to 6VACrms @ 100Hz (fixed frequency) for those who do not have a quality signal generator or alternative. (100Hz was chosen as a DMM-friendly "sweet spot" for avoiding EMI-influence in the working environment.)
Remember, you can't do credible AC comparisons or checks by using isolation/step-down transformers, variacs or by sticking your probes into a mains socket because the line voltage is always jumping around and the sine waves don't look anything like sine waves because of all the noise and distortion.
The AC references can be pre-calibrated (buyer's choice) to between 100mV and 6VACrms @ 100Hz +/- 0.5% to cover near-full-scale readings of most popular DMM's, requiring only a single 12VDC regulated power supply or battery because they have an on-PCB ICL7662 inverter for the -12V rail.
There can be several variations (and prices) for such a device including lower sine wave amplitudes which allow using single supply op-amps and 9V batteries, etc., etc., but I would rather focus on minimum performance SPECIFICATIONS for device USEFULNESS at this time.
DC Voltage Reference: Rather than the untrustworthy cheap references, many using crappy components and USED AD586 parts and phony calibration stickers, I have developed a straightforward 10VDC reference based upon the LT1236 chip.
This DC reference currently uses a 14VAC 0.5A wall-wart transformer or other suitable 14-16VAC power supply to the input jack leading to an on-PCB 15VDC regulated power supply that then feeds the LT1236 circuitry. The power input can also be 18-24VDC through the same input jack and that also offers autopolarity protection due to the full wave bridge rectifier at the input on-PCB.
The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.
The DC reference trimming is done with the SDM3055 by powering on both for 24-48 hours before final trimming to 10.0000 which is as good as it gets until we buy a DMM6500 or similar in the near future.
However, after trimming one of the 10V references 2 months ago and keeping it powered on, and after re-checking the SDM3055 vs DMM6500 readings weekly, the 10VDC reference STILL reads exactly 10.0000, spot on, if that's worth anything.
Now, I realize that this whole discussion doesn't meet any volt-nut standards but, remember why we're here in the first place, as I set forth in my introduction.
I simply would like to improve the SPECIFICATIONS and USEFULNESS of these devices while keeping them low-cost to give the average DMM owner an alternative to spending hundreds of dollars and more on devices that may be of higher quality and accuracy but are of no more usefulness.
Thanks for any insight and help that comes to mind. A few pix attached for general reference...
:-DMM :-DMM
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schmitt trigger:
Does it have distortion specs?
alm:
--- Quote from: Majorassburn on April 08, 2024, 07:16:16 pm ---The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.
The DC reference trimming is done with the SDM3055 by powering on both for 24-48 hours before final trimming to 10.0000 which is as good as it gets until we buy a DMM6500 or similar in the near future.
However, after trimming one of the 10V references 2 months ago and keeping it powered on, and after re-checking the SDM3055 vs DMM6500 readings weekly, the 10VDC reference STILL reads exactly 10.0000, spot on, if that's worth anything.
--- End quote ---
I would try to translate this to an adjustment uncertainty specification and long term drift specification (though measuring on a 2 month time scale may require very accurate and stable equipment). Compare this to measuring the drift of continents on the time scale of a year vs a century. You need way more accurate equipment to measure low velocities by comparing the position at two points in time. You are not trying to measure drift, you are trying to set an upper bound like the drift over 2 months is less than 100 uV (which might translate in an upper bound of 600 uV/year). Compare this to a datasheet values.
For adjustment uncertainty. To me the "brand new SDM3055" says "suspect and potentially high drift". Generally voltage references start to settle in after some months of operation. And from what I read, Siglent may not be doing a very good job burning in their references before putting them in instruments. But with the weekly comparisons to the DMM6500 you should be able to get some measurement of stability over time. The uncertainty might look something like this:
* DMM6500 absolute uncertainty from datasheet (1 year or 90 days since calibration?)
* Uncertainty of transferring from DMM6500 to SDM3055 (noise, temperature fluctuations / coefficients for both meters, repeatability)
* 1 week stability of SDM3055 (you can estimate this from repeated comparisons with the DMM6500 taking into account the uncertainties of the DMM6500 and the comparison)
* Uncertainty of measuring the voltage reference with the SDM3055 (noise, temperature fluctuations / coefficients for DMM and reference, repeatability)For the ones that are not normal distributions (like specs from a datasheet), you multiply them by the factor from The Guide to Uncertainties in Measurement (see page 13 for how to rectangular distributions, e.g min/max specs). Then you add them all up (linear is the most conservative), and multiply by a coverage factor k=2. This should give you a best estimate for the uncertainty of the reference just after adjustment. Probably not all components will contribute equally. Feel free to simplify and use conservative estimates for those.
Then using the same procedure at different points in time, you can calculate an upper bound to the drift based on the uncertainty of each measurements. I would show a table with uncertainty calculations so anyone who doubts the figures can verify.
Ideally you would build a small thermal chamber that would allow to to characterize the temperature coefficient of the circuits. Temperature and time are probably the major variables affecting the output of the standard. Humidity would be next, but is difficult to control and also difficult to measure to to the large time constants involved.
bdunham7:
--- Quote from: Majorassburn on April 08, 2024, 07:16:16 pm ---The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.
--- End quote ---
I typically think of the errors rather than the accuracy, so I avoid terms like 'spot-on'. To me 'spot-on' just means you don't have enough resolution to see the errors. It's like someone asking me to machine something with a dimension of "one inch". So I ask "to what tolerance?". They reply "I want it exactly one inch". I spend a week setting up my most precise measuring tools and another week lapping the surfaces so that they are 1 inch apart to a precision of 0.00001", the best I can do. Customer comes in to pick it up, pulls out a measuring tape and says "yep, looks like exactly one inch". :palm:
That's not a true story, of course, but it is sort of based on one. The question I have first is about your 'comparison calibrating". I'll presume you are not adjusting anything but rather just using a stable voltage source to take measurements with the two meters in parallel and comparing the results. Correct me if that is wrong. If that is the way you are doing it, are you saying that your SDM3055 always matches the DMM6500 exactly to the last digit? And then your Fluke 87V matches the SDM3055 to the last digit, each and every time? If so, that is truly exceptional. If not, my recommendation would be to start by logging and stating the errors in each of those steps.
Majorassburn:
--- Quote from: bdunham7 on April 08, 2024, 09:14:03 pm ---
--- Quote from: Majorassburn on April 08, 2024, 07:16:16 pm ---The LT1236 is trimmed to exactly 10.0000VDC. How does this happen? I use our brand new SDM3055 5.5-digit DMM (new production, direct from Siglent) which is "comparison calibrated" to a new DMM6500 that I have access to. Then, the SDM3055 is the basis for "comparison calibrating" my Fluke 87V's.
--- End quote ---
I typically think of the errors rather than the accuracy, so I avoid terms like 'spot-on'. To me 'spot-on' just means you don't have enough resolution to see the errors.
-snip-
If that is the way you are doing it, are you saying that your SDM3055 always matches the DMM6500 exactly to the last digit? And then your Fluke 87V matches the SDM3055 to the last digit, each and every time? If so, that is truly exceptional.
--- End quote ---
Both good questions. First part, the display of "10.0000" on the SDM3055 is the limit of its resolution. So, all further errors (and there must be some) are masked. However, four zeroes is probably as good as it's going to get with a $25 reference with today's technology.
More importantly, four zeroes is certainly adequate enough for the purposes outlined in my intro although certainly not anywhere near good enough for verifying the calibration of a 34465A or DMM6500.
The second point, is also a good question. So, far the "comparison calibration" method I referred to is simply attaching both the DMM6500 and the SDM3055 to the same 10V reference (after warmup) and reading the displays. No intervention or actual calibration. The SDM3055 agrees with the DMM6500 to four zeroes. I have NOT recorded any further errors or deviation of the 10V reference revealed by the DMM6500's extra resolution during these comparisons.
As for the Fluke 87V in the photo, it agrees with the SDM3055 only SOME of the time because it's running in Hi-Res mode where the offset fluctuates +/- 1LSD. It can read anywhere from 9.999 to 10.001 with 10.000 most of the time. That's not good enough but it is a curiosity.
I'll buy either a 34465A or DMM6500 if these reference devices prove salable. If not, I'll stick with the SDM3055 for awhile as long as I have access to the new DMM6500.
So, considering that these are affordable, low-cost references, I think accuracy to four zeroes is adequate for the purposes which they are designed to fulfil. Maybe I'm missing something but offering free periodic re-cals of the 10V & AC references once further detailed specs are developed along the lines that poster ALM suggests above, might keep buyers happy and provide the value that is intended for the prices paid.
What I'm getting at is developing a spec for example, the 10V reference, that says something like, trimmed to four zeroes, plus or minus 2 LSD's over a 6 month period? Maybe?
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