Electronics > Beginners
1V Reference @ 1ppm
Kleinstein:
To calibrate a meter one would likely need both a stable and accurate reference. So just building a reference could result in something like 1 V with < 1 ppm/K TC , but the absolute accuracy would be poor (e.g. like 0.1% or maybe even worse).
For a good absolute accuracy chances are a used Hg Cell (e.g. Weston) is probably the best chance if you don't want to pay for a used Fluke 732 or similar. Depending on the meter it may not need exactly 1 V the slightly higher voltage mercury cell may be acceptable too. It would still need a precision buffer
beanflying:
--- Quote from: spec on January 27, 2019, 02:59:43 pm ---which is -240ppm
This seems to defeat the whole purpose of a precision voltage source, or have I missed something?
The switch from virtually no load, to a relatively heavy load of 10M has changed the whole aspect of the design.
--- End quote ---
It depends on what you want the divider to show and to what particular DUT. What Raphael needs effectively is a transfer standard using a cost effective method. Not sure of the nixie meters input impedance but as it is a 6.5 digit meter likely to be a lot higher than 10M?
My 735A Transfer Standard is sensitive to loading but it has a compensation pot available on the front panel to tweak this nominal 1V if needed. Normal procedure with this device is to Null it against a Weston Standard Cell (known voltage) or similar like I showed previously to set it's internal reference then via the internal divider network set a 1V output. Null meters in common use at the time in theory have an infinite resistance at the Null point so don't load the cell or dividers. Manual is here if anyone is interested and it goes into the theory of operation and calculations for lower impedance meters using this transfer standard http://literature.cdn.keysight.com/litweb/pdf/00735-90002.pdf?id=1870825
To do this without a known volt source other than the 'claimed' accurate voltages on the 584K and or any other reference he may makes the 'initial accuracy' is the best guide to how close that may be and then test one against the other. So the MAX 6350 will have an initial accuracy of 5V +- 0.5mV in practice it will be better than this spec. Part of the issue is we are trying to make wine from pig swill ;)
You will not get the 'perfect' resistors to divide up the 2.5 or 5V source to 1V so compensation will be needed be that adjusting the input voltage or providing compensation on one or both legs of the divider. The reason I suggested tweaking the Reference is the trimpot (generally terrible tempco) will be fairly unaffected by temperature variation due to it's already narrowed range. The alternate is to do something similar to the 735A circuit and minimize the value of the trimmers in the circuit.
If he is to build a new reference I would use the 6350 as a start point as the +-0.5mV initial accuracy is the same for the 2.5V version which would make it twice the error.
spec:
--- Quote from: beanflying on January 28, 2019, 12:56:59 am ---
--- Quote from: spec on January 27, 2019, 02:59:43 pm ---which is -240ppm
This seems to defeat the whole purpose of a precision voltage source, or have I missed something?
The switch from virtually no load, to a relatively heavy load of 10M has changed the whole aspect of the design.
--- End quote ---
It depends on what you want the divider to show and to what particular DUT. What Raphael needs effectively is a transfer standard using a cost effective method. Not sure of the nixie meters input impedance but as it is a 6.5 digit meter likely to be a lot higher than 10M?
My 735A Transfer Standard is sensitive to loading but it has a compensation pot available on the front panel to tweak this nominal 1V if needed. Normal procedure with this device is to Null it against a Weston Standard Cell (known voltage) or similar like I showed previously to set it's internal reference then via the internal divider network set a 1V output. Null meters in common use at the time in theory have an infinite resistance at the Null point so don't load the cell or dividers. Manual is here if anyone is interested and it goes into the theory of operation and calculations for lower impedance meters using this transfer standard http://literature.cdn.keysight.com/litweb/pdf/00735-90002.pdf?id=1870825
To do this without a known volt source other than the 'claimed' accurate voltages on the 584K and or any other reference he may makes the 'initial accuracy' is the best guide to how close that may be and then test one against the other. So the MAX 6350 will have an initial accuracy of 5V +- 0.5mV in practice it will be better than this spec. Part of the issue is we are trying to make wine from pig swill ;)
You will not get the 'perfect' resistors to divide up the 2.5 or 5V source to 1V so compensation will be needed be that adjusting the input voltage or providing compensation on one or both legs of the divider. The reason I suggested tweaking the Reference is the trimpot (generally terrible tempco) will be fairly unaffected by temperature variation due to it's already narrowed range. The alternate is to do something similar to the 735A circuit and minimize the value of the trimmers in the circuit.
If he is to build a new reference I would use the 6350 as a start point as the +-0.5mV initial accuracy is the same for the 2.5V version which would make it twice the error.
--- End quote ---
Thanks for reply :)
(I do appreciate that most approaches would require an initial adjustment to get the absolute voltage spot on.)
I see, a transfer standard- that puts a whole new complexion on the design. What I see is an issue though is that relying on the input resistance/characteristics of the meter to be calibrated may wreck even the performance of a transfer standard as the characteristics of the meter input are not known (to the 1ppm level that is). Or are you proposing adjusting the transfer standard every time it is used? Perhaps that is the normal procedure.
Interesting stuff about your 735A transfer standard.
I wonder if a ref voltage of 1V25 would be acceptable because, if so, an LTC6655 based design could be used and no potential divider would be necessary. Admittedly the LTC6655 is 2ppm but, maybe, keep the LTC6655 at a constant temperature to fix that in terms of the overall performance of the voltage reference unit.
On the other hand, if a 1V reference was essential a voltage divider for the LTC6655 could be made to have a lower output resistance and that would reduce the effect of loading delta/unknowns.
It is a shame this is not a volume production job, because then we could just phone up the manufacturer, Maxim/LT/AD etc, and get them program a 1V Vref version for us. :)
https://www.analog.com/media/en/technical-documentation/data-sheets/6655ff.pdf
beanflying:
Just to complete the sort of usage method here is what I did today.
Agilent 34401A set to 10Meg Impedance.
Adjust the HP 735A Transfer using the meter to 1V (actually ran out of cal range on the front panel so 0.999993)
Take that know 'known' (as known as I can make it) voltage and apply it to the 10Meg inputs of the DUT in this case the 121GW.
Result the 121GW is horrid and should never be trusted to read anything for reasons at least according to some ..... :-DD
For interest the 735A is well under 1ppm/C (done by longer term logging) and the tests were done at about 28C as per the121GW.
The procedure using a homebrew solution would be fairly similar prepare the source as best you can for the test you plan for the DUT in question.
beanflying:
--- Quote from: spec on January 28, 2019, 03:32:17 am ---I wonder if a ref voltage of 1V25 would be acceptable because, if so, an LTC6655 based design could be used and no potential divider would be necessary. Admittedly the LTC6655 is 2ppm but, maybe, keep the LTC6655 at a constant temperature to fix that in terms of the overall performance of the voltage reference unit.
On the other hand, if a 1V reference was essential a voltage divider for the LTC6655 could be made to have a lower output resistance and that would reduce the effect of loading delta/unknowns.
It is a shame this is not a volume production job, because then we could just phone up the manufacturer, Maxim/LT/AD etc, and get them program a 1V Vref version for us. :)
https://www.analog.com/media/en/technical-documentation/data-sheets/6655ff.pdf
--- End quote ---
Generally for Cal points they are fairly specific. So Apply this then tweak that scenario is normal. So you would need the 1V.
Most meters would take the form of 1V, 10V and maybe 100V and 1kV (34401a needs all of these for a full cal) depending on how it is designed to be calibrated just for DCV. The 121GW for example needs 5,50,500 & 600 for the DCV and a couple of others for the mV range for a full Cal of just the DC volt side of the meter. This is why in part you see DC Calibrators to 1000V.
The thing to bear in mind is you were to use say the higher accuracy version of the LTC6655 at 1.25V then it's initial accuracy (0.025%) is 1.25 +-0.3mV so divided down the best 'known' accuracy will be higher than the MAX6350 not a deal breaker but if you don't have known volt it is an issue. A little care needs to be taken regards the long term stability figures of both devices and both will tend to settle with use to value X and then need an additional tweak but to what is the question ;)
Now we have scared or driven off everyone to or away from Metrology for ever come on in the Rabbit Hole is large and very very deep >:D
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