1V Reference @ 1ppm

[beanflying]

When not in outside use yes. I have a hacked wine fridge at 25C. and 34970A loggers with DMM cards fitted. Still a WIP and it also has two newish LTZ1000 Refs and the bits for two more coming.

If it wasn't out of interest there is nothing I do that requires more than 4.5 digits but having that known 6.5 digit and references gives me very good confidence at that 4.5 has very good confidence. The consideration to jump to 8.5 is a total indulgence and I am still weighing up that or a modern 1/2 rack 7.5 digit option. Either way it would be kept in Current Cal.

[spec]

CROSSED POSTS

LM399 and LTX1000 [LTZ1000] based devices are POINTLESS unless you have a 'known' accurate standard or meter to set them with their initial accuracy varies widely. They will only be as 'accurate' as the device YOU measure them with but generally they are more stable than other References due to Temp but can take weeks to years to settle. They are a can of worms in the Rabbit Hole with you in free fall but always just out of reach

Isn't,  'They will only be as 'accurate' as the device YOU measure them with' true of any Vref that you make?

Both the LTZ1000 and the LM399 are oven controlled. The LTX1000 is top of the chip heap, so why not use it. OK you may not realise it's full potential, but so what. Is it because of the $50 price tag? At a lower spec (0.5ppm) and price ($12), the same goes for the LM399, or is it because the LM399 is a shunt type, which complicates the design, compared to a series type?

It would appear that the basic vref of the LM399 cant be that good if it only achieves 0.5ppm/degC performance with oven control. My thinking is that there may be big gains in performance by oven controlling one of the au natural chips we discussed, which as you say, also have an initial absolute accuracy around two orders better than the LM399.

https://www.analog.com/media/en/technical-documentation/data-sheets/199399fc.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/LTZ1000.pdf

[beanflying]

In the case of Ref's like the LM and LTZ their initial accuracy varies heaps so without a good meter they are no good for a walkup 'accurate' home built reference. The Max6350 runs rings around them for initial 'accuracy'. So if you have no 'known' source don't build one IMO there is simply nothing to be gained unless you already have a very good calibrated meter which in Raphael's case he gives no indication of.

LM399 6.75 - 7.3 V initial 'accuracy'
LTZ1000ACH 7.0 - 7.5 V initial 'accuracy'
MAX 6350 4.999 - 5.001 V initial 'accuracy' (0.02%)

[spec]

In the case of Ref's like the LM and LTZ their initial accuracy varies heaps so without a good meter they are no good for a walkup 'accurate' home built reference. The Max6350 runs rings around them for initial 'accuracy'. So if you have no 'known' source don't build one IMO there is simply nothing to be gained unless you already have a very good calibrated meter which in Raphael's case he gives no indication of.

LM399 6.75 - 7.3 V initial 'accuracy'
LTZ1000ACH 7.0 - 7.5 V initial 'accuracy'
MAX 6350 4.999 - 5.001 V initial 'accuracy' (0.02%)

Agree. But at one stage you were suggesting that the Vref be used as as a transfer standard, which implies another precision reference- or have I got that wrong.

To put a stake in the ground can I suggest that we consider two approaches

• Transfer standard (with high output R and adjustment)
• Stand-alone absolute Vref, with low output R (suitable for a load of 10M or higher) and initial calibration (if required) by resistor AOT.

I have an interest in two above.

[beanflying]

With no calibrated gear it becomes a chicken and egg type of thing you still need an 'Accurate' Reference to make that Transfer Standard. So your choices are buy or make the best one you can.

Vref best options without going broke.

1 - Buy an AD-584 with measurements attached. With no measurements it is no better than your current best meter for Accuracy.
2 - Build a MAX6350 based reference and make it YOUR volt. It wil be better than just about any 4.5 digit meter I am aware of for initial accuracy?
3 - There is 399 Refs in the for sale section at present for not to many $$ but I am not sure of what there long term aging or stability is as I haven't looked at the project other than a glance in the for sale section? Personally I would stay away unless you already own a meter capable of tracking and looking at its drift (6.5 digits)
4 - DMM check or similar other devices are also available online.

How you get to the various voltage points to transfer YOUR Volt from there is a topic in itself and really to large for here and outside basic dividers and not Beginner material in particular. Depending on the meter you want to Cal as I put a few posts back you need to hit a huge number of different points even for a couple of meters. This is why I own a couple of Calibrators both AC and DC to 1000V+ but they are not sensible for most apart from their boat anchor dimensions as per the photo below.

One other possible option is you go down the path Ian Johnson has with his LM399 based Reference http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detail using a good quality DAC/micro on the reference but this is way out of Beginner level and will cost real money.

Back shelf is how to go 'part' of the way into the rabbit hole 7.5 digit Advantest on the top shelf, HP735A, Fluke Null meter, Fluke Portable Calibrator, Bottom row HP 740B working but mid renovation 1000V DC Calibrator/Null meter and Valhalla 1200V AC Calibrator. Won't mention my 'slight' time nut addiction either. Plus other assorted 'stuff'. No I don't feel I have a problem because TEA assures me I am 'normal'  Yes I am showing off the toys a touch 

[tggzzz]

Can I ask a question of the experts in this field: When you are checking the precision of an instrument, what is the shortest time that the instrument could be connected to the reference source? I have a reason for asking.

I would suggest "until the reading is stable and repeatable", where you have to supply the two obvious definitions.

Are you thinking of a mechanical multimeter and a Weston standard cell?

My thought was to use a capacitor to lower the effective output impedance of  a precision voltage divider but, in view of your and K's, remarks it is clear that a fair amount of time would  be required to do the calibration so that idea goes down the tubes as only a film capacitor could be used. A supercap, which could allow sufficient time, would be too leaky, I think

Capacitors are crap components compared with resistors. Leakage, tempcos, non-linear voltage dependence, dielectric absorbtion, and more.

Oh yes I do know that, but there are capacitors and capacitors. Still overall you are obviously right. It was just a thought.

UPDATE: just noticed that the LTZ1000 does use a capacitor internally.

At voltnut level there are no individual components, only black boxes with lots of hidden and poorly specified internal components People spend careers trying to minimise and work around that.

Silicon capacitors are no different in that respect. BTW, you can get discrete silicon capacitors in the range 0.3pF to 3.3uF, but watch the availability, MOQ and price
https://www.digikey.co.uk/products/en/capacitors/silicon-capacitors/68?k=silicon%20capacitor

[spec]

xxx

Thank you for such a comprehensive summary.

In view of what you and tggzzz say, it might be folly, but I would like to have a go at designing a stand-alone, low-impedance output Vef. So if I get around to posting a schematic, your comments would be appreciated.

I have done a couple of outline schematics which, in theory, look promising, but I lack the specialist knowledge and experience that this field requires. My only qualifications are having read the threads on EEV and elsewhere on the net.   

[spec]

Back shelf is how to go 'part' of the way into the rabbit hole 7.5 digit Advantest on the top shelf, HP735A, Fluke Null meter, Fluke Portable Calibrator, Bottom row HP 740B working but mid renovation 1000V DC Calibrator/Null meter and Valhalla 1200V AC Calibrator. Won't mention my 'slight' time nut addiction either. Plus other assorted 'stuff'. No I don't feel I have a problem because TEA assures me I am 'normal'  Yes I am showing off the toys a touch 

Nice

[tggzzz]

xxx

Thank you for such a comprehensive summary.

In view of what you and tggzzz say, it might be folly, but I would like to have a go at designing a stand-alone, low-impedance output Vef. So if I get around to posting a schematic, your comments would be appreciated.

I have done a couple of outline schematics which, in theory, look promising, but I lack the specialist knowledge and experience that this field requires. My only qualifications are having read the threads on EEV and elsewhere on the net.

Folly? No. You'll have fun and learn things. But personally I wouldn't tweak anything based on its output - first attempts are rarely perfect.

You should understand what's behind the specs for professional high quality meters, especially those related to the drift over time.

You might like to consider what will/won't happen if you short-circuit your low impedance output.

[spec]

1 - Buy an AD-584 with measurements attached. With no measurements it is no better than your current best meter for Accuracy.

You have mentioned the AD584 a couple of times, but according to the datasheet, it is no big shakes (15ppm) compared to the other chips. So what is the story with the AD584?

https://www.analog.com/media/en/technical-documentation/data-sheets/ad584.pdf

[beanflying]

The Acrylic enclosed references uses the AD584KH. It is specced at 15PPM Maximum there is no typical or minimum PPM/C which is a bit strange. Based on what I measured the other warmish day I was getting 1PPM from mine so 

Interestingly the Initial Accuracy is only Maximum rated too 

Do a Search for KKMoon or AD584KH reference you should get plenty of hits here on others that have tested it.

[spec]

Folly? No. You'll have fun and learn things. But personally I wouldn't tweak anything based on its output - first attempts are rarely perfect.

Yes, well put. I don't actually need to calibrate anything, at the moment. A half-decent voltage reference would just be an interest.
 

You should understand what's behind the specs for professional high quality meters, especially those related to the drift over time.

Oh yes- got that right from the off. I have done quite a bit of precision work, but never designed/built an ultra precision Vref. I spent a year doing the development cal of a precision analog computer at one time, and had access to the highest precision voltage and resistance references, via the company- cost was not an issue.

You might like to consider what will/won't happen if you short-circuit your low impedance output.

Not much I don't think. But it may introduce a thermal shock to the V Ref Unit, which could take a day or so to recover from.

What does concern me is the secondary effects, like galvanic voltage of metals and solder joints- that kind of thing.

[spec]

Perhaps I could outline the V Ref Unit (VRU) design I have been looking at, in the background, recently:

• Ref Voltage: any from 0V to 10V
• Target Absolute Accuracy (max): TBD
• Target Tempco (max): 1ppm/degC
• Calibration: none, or resistor AOT depending on degree of precision required.
• Material Cost (max): $100US
• Development Effort (max): Infinite man hours
• Dimensions (max): Width: 300mm, Depth: 250mm, Height: 150mm
• Weight (max): 1.5 kg
• Case: DIY. Material: TBD, but probably a mixture of plastics and aluminum. The case would have at least four thermally isolated compartments. The first compartment would house the electronics that are primarily responsible for voltage precision. The second compartment would house the components that have a secondary effect on voltage precision, the third compartment would house the precision linear power supplies. The third compartment would have an efficient heatsink. The fourth compartment would house the batteries.
• Power Source: mains and battery. Mains components housed in  wall-wart or similar.
• Power Consumption (max): 100W
• Power Rails: precision low-noise using specialist linear PSU chips
• Primary Voltage Ref: one of the chips that we have discussed
• Buffer to Provide 'zero' Ohm Output Z: super precision opamp (I seem to have found one)
• Thermal Control: Possibly oven controlled primary voltage ref. All relevant circuitry in DIY ovens (simple to do, but the practical side would take some sorting)

The VRU would probably require a six-month burn-in period to reach full performance, but would be usable after a couple of days for less precise work.

Obviously, the VRU would be powered permanently, either from the mains or battery.

[Alex Nikitin]

Perhaps I could outline the V Ref Unit (VRU) design I have been looking at, in the background, recently:

Have a look at this voltage source, manufactured by a forum member here:

http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detail

Cheers

Alex