-
Capacitors within 100ppm going to be real challenge here
.
I'd say loose USB input (it's very noisy), loose dc/dc converter and power the linear preregulator with some decent quality linear brick mains power supply.
Glass fuse will not do anything there too, if you reach currents that will break even smallest fuse (plugging mains into output?
), then whatever is behind fuse would be already charcoal dead.
Thanks for your input
Forget about the capacitors, bad idea and not that useful.
Yes USB wall-wart supplies are noisy and so are dc/dc converters but I do like the idea of having all of the references powered thru one single power input and having something standard like USB means I don't have to ship a dedicated PSU with it.
Also having isolated dc/dc converters means I can connect outputs in series.
This power supply section can be housed in a separate metal compartment inside the metal reference box.
Personally I'm not to worried about induced HF noise (not really HF but I'm a audio guy so anything above 20KHz is HF to me..
).
Alternative could be to use separate mains transformer for each reference but then leakage current may become troublesome when references are wired in series and 50hz induced mains frequency noise is harder to get rid off (its much closer to DC obviously).
Or, 9 volt battery's, huhm, have to work out the math for that.
Yes the fuse in combination with the diodes won't help when the output is connect to mains but it does help when it get accidentally connected to
a bench supply or some other silly event, just few cents in costs and from my calculations no measurable negative effects, so why not.
-
Capacitance to 100 ppm is hard to do. The most accurate rlc meters are only 0.02% accurate, with a bridge going from 0.01(1620), 0.001(GR1621 / AH2700) to 5ppm(AH2500/2550) Which I would guess as being really pricy to calibrate.
I notice one sold on e-pay lately. Tin did you buy it?
-
Supplying a reference with a DC/DC and lots of filtering can be done, if the ADC the Ref is for is in the same box. But since you plan to check external meters with it, the common mode noise of the DC/DC and the USB supply will kill any precision and stability. Go for battery power and conventional mains transformer PSU.
Characterising a low common mode noise DC/DC is very demanding for that application.
There is a reason that they went for this classic approach in the 3458A -
Capacitance to 100 ppm is hard to do. The most accurate rlc meters are only 0.02% accurate, with a bridge going from 0.01(1620), 0.001(GR1621 / AH2700) to 5ppm(AH2500/2550) Which I would guess as being really pricy to calibrate.
I notice one sold on e-pay lately. Tin did you buy it?
No, not Tin, was me. May do a teardown at some time later, currently still sealed with warranty tags (for what its worth). Testing it at the moment, looks good so far. Plan to use it in conjunction with the GR 1620 (and 1689) bridges for increased accurracy and faster calibration of capacitance standards. May take a little, need to get down a bit with internal lab standards uncertainty. -
No, not Tin, was me. May do a teardown at some time later, currently still sealed with warranty tags (for what its worth). Testing it at the moment, looks good so far. Plan to use it in conjunction with the GR 1620 (and 1689) bridges for increased accurracy and faster calibration of capacitance standards. May take a little, need to get down a bit with internal lab standards uncertainty.
Whoa, cool. It was bit sad to see that unit go, not to see it again. Glad it's still in community. I can't have it all anyway
.
I'd love for the separate thread about it, once you ready.
-
Very cool, I look forward to a teardown. The theory of operations is rather interesting. The msbs are achieved by a normal bridge balance, with the remainder calculated with an adc (calculating the difference). With the internal references beig ovenized. You got it for a good price, one sold a few years ago for about 2-3x that. I'm not sure what they go for newCapacitance to 100 ppm is hard to do. The most accurate rlc meters are only 0.02% accurate, with a bridge going from 0.01(1620), 0.001(GR1621 / AH2700) to 5ppm(AH2500/2550) Which I would guess as being really pricy to calibrate.
I notice one sold on e-pay lately. Tin did you buy it?
No, not Tin, was me. May do a teardown at some time later, currently still sealed with warranty tags (for what its worth). Testing it at the moment, looks good so far. Plan to use it in conjunction with the GR 1620 (and 1689) bridges for increased accurracy and faster calibration of capacitance standards. May take a little, need to get down a bit with internal lab standards uncertainty. -
Reference chips are on their way, ordered 10 of them just to be sure, buried zener in a hermetically sealed package so 2,5 volts is out the window...
I'm not a voltnut, not a voltnut.
-
Hello,
believe me it is absolute normal to have at least 4 references!
Its like having clocks.
How many clocks do you have in your household?
with best regards
Andreas
-
How many clocks do you have in your household?
Two that I use, one on my phone and one on my computer they're pretty much in sync, we also have a bunch of analog clocks around but I don't look at those..
-
Reading this thread I'm wondering whether you can expect a 4.5 digit meter not to wander more than 100ppm over time. The references in those won't be as stable as in (for example) a 6.5 digit meter.
-
Reading this thread I'm wondering whether you can expect a 4.5 digit meter not to wander more than 100ppm over time. The references in those won't be as stable as in (for example) a 6.5 digit meter.
Correct, however these two meters I have are pretty old(metrawatt ma5d and keithley 178),
and therefore I assume that they don't drift much more over time, the fact that they are consistently no more than a couple of counts
apart from each other supports that idea.
And if not the internal references will be replaced.
-
The references have arrived this week, 10pcs LT1021-DMH10.
https://www.arrow.com/en/products/lt1021dmh-10/linear-technology
These are the worst spec D grade with poorer tempco (max 20ppm/C) and initial accuracy (10V +/-50mV).
Long term stability specification is the same for all varieties, so at 3 euro each these are pretty cost effective in my opinion.
I did just trow something together with a random picked one and the MA5D is steadily reading 10,001V for a couple of hours.
Pretty promising so far.
Will hook up a second reference and measure the voltage between the two with 10uV (1ppm) resolution and see how stable that is.
-
The first one reading 10,001V must have been pure luck, the second one reads 9,982V.
Now measuring the voltage differential between the two, after 15 minutes its close to steady 18,58mV at 15Volt input
and 21 degrees Celsius.
Will let it run for 24 hour and see how things change.
-
The first one reading 10,001V must have been pure luck, the second one reads 9,982V.
Now measuring the voltage differential between the two, after 15 minutes its close to steady 18,58mV at 15Volt input
and 21 degrees Celsius.
Will let it run for 24 hour and see how things change.
It's been a bit more than 24 hours, three weeks actually, running the references 24/7 and now back at 21 degrees Celsius (we have had some nice weather) all readings now are exactly the same as before.
I've learned from individually warming these references up (between my fingers) that these two references have approximately the same temp-co but opposite from each other.
At highest ambient temperature 27 dgr. C. the change between these two was 20ppm.
So reference nr1 (10,001V) has a temp-co of approx. 1,67 ppm/dgr. C.
And nr2 (9,982V) has a temp-co of approx. -1,67 ppm/dgr. C.
Assuming they both contribute to the 20 ppm over 6 dgr. C. by the same amount.
Either way this data points out that this pair would be sufficient to use as a travelling reference for my purpose.
Next up building a enclosure..
-
Not close!If someone is near me (west Drenthe) ..
And now he is also luring the German Voltnuts out, because that is close to their border
BTW, I am not too close, mid-south.
You live in a tiny country, where nothing is much more than 200 km from anything else, probably much closer.
I once drove from my hometown to the Altantic ocean coast, never leaving my own country. It took five days to travel those >6000 km... and that was one-way. Another five days to drive back home! -
Not close!If someone is near me (west Drenthe) ..
And now he is also luring the German Voltnuts out, because that is close to their border
BTW, I am not too close, mid-south.
You live in a tiny country, where nothing is much more than 200 km from anything else, probably much closer.
I once drove from my hometown to the Altantic ocean coast, never leaving my own country. It took five days to travel those >6000 km... and that was one-way. Another five days to drive back home!
Well when you drive a car perhaps, not when you're riding a bicycle with wooden shoes, 200km here also takes 5 days.
-
If you can get a good measurement on it, some NP0 caps are very stable. Multi-layers are even good and will get you a higher value, say up to 0.01 uF or so. The most accurate mechanical traditional bridge, save for the ratio transformer types, was the later production GR1608 at 0.05%. You need to find somebody with a GR1615 (or the big 1621) and a calibration standard to confirm it.
-
Recently I was able to buy a General Radio 722 precision condenser.
But I decided not to
, as I have no real purpose for it as of yet, nor any capacitance standard.
It would looked nice together with my GR 602n decade box tho.
-
.............And two years later..........
My references are long finished and the voltage references (lt1021-dmh10) have many hours on them.
Out of the ten I've purchased four are still in the original packaging, one decided it was happier being a 12k Ohm resistor,
five undergone further tempco testing (passive dummy load in a cardboard box method) out of which three where chosen:
Two are in a portable self powered (dual 9v battery) enclosure. and may have around 1000 hours on it.
One is in a bench-top enclosure with a passive switchable voltage divider (10x ESI 1K Ohm)
with trimmer networks to match each resistor to 1ppm.
I've haven't rechecked the resistors lately but any check from the outside makes me assume they are still spot on.
It has been powered on 24-7 for close to two years, (15000 hours?).
Oddly the dual "travel reference" never reads more than 2 ppm apart from each other/it self, amazing after all that time being on and mostly off.
The 24/7 "mother reference" remains within a range of 7 ppm of the travel reference most of that as far as I can tell is due to
tempco.
The Metrawatt ma5d only reads one lsd low when cold, after a 30 min warm-up it is spot on.
So it did not drift visually in the last two years.
Some poor quality pics are attached.
-
In the resistance department I have build a box with five terminals and four ESI resistors:
black-100R-green-998R-bleu-9K98-yellow-99K8-red (absolute values not sure as of yet..)
The terminals are gold plated hirschmann pki10Au and can be used with kelvin connections, two spades, two bananas.
In the picture from the previous post a 10mA current source is connected to the 100R and the voltage drop across it is measured
with the metrahit 2+.
And another (this time plastic) box with hirschmann safety sockets and three values:
black-1M-green-9M-bleu-90M-yellow
The 1M is a single 0,1% tolerance generic (cheap..) metal film resistor chosen from a bunch of ten for having a value closest to the average of the ten.
The 9M are the remaining 9 in series.
The 90M are made of 9 high voltage (500V) 1% resistors and one or two additional resistors to bring them as closely to 9x the sum of the 1M and 9M as feasible, so also within 0,1% of the supposedly absolute.
Should also come in handy measuring (very low energy!!) high voltage.
-
Another high accuracy capacitance bridge is the ESI 707A/B, ±.02% or better depending on range. Not that bad for calibration either.
-
Another high accuracy capacitance bridge is the ESI 707A/B, ±.02% or better depending on range. Not that bad for calibration either.
Thanks Edwin, but at the rate I'm going it will take another two years before I've build a couple capacitance standards and probably
also diy a bridge, these old bridges are fantastic but out-dated and to big for my "lab".
-
I have 3 ESI videobridges that claim .02% midband. The references are resistors so easier to make and make stable (for ESI). I have no clear way to verify its accuracy. Ease of use is definately good. Old CRT display not great today and the fans are noisy. But a good value on eBay.