Electronics > Metrology
Solartron 7081 - Reference Circuit
unseenninja:
I'm slowly losing my mind with the Solartron 7081.
I bought a 'for repair' unit, thinking that the problem wouldn't be significant... I won't bore you with tales of a multimeter which had obviously been dropped at some point in its life. Or the story of inexplicable problems and their solutions. In the end, I managed to get it working again so that I could start testing it.
I checked the 10V range against one of my most stable and quiet references - a Dr. Frank LTZ1000 build. Sadly, what I saw was 4ppm popcorn noise:
After eliminating every other component in the reference circuitry I was forced to accept that my finely aged and selected by greybeards reference diode had surrendered its ppms and stability.
I know. The answer is "put an LTZ1000 reference in there". That wouldn't be keeping the meter original though, would it? So I found a supplier of new, old stock 1N829 diodes on eBay and threw myself head first into the process of trying to find one golden 1N829 from the six that I'd bought.
I put together my first ever KiCAD project to create a PCB with six 7.5mA current sources, had it made in China, learned about not just double, but triple checking every aspect before committing a design for production, worked around my mistakes and started the selection process.
Two diodes had terrible popcorn noise and were immediately eliminated. The other four had a pair of stars and two which were OK. So I let them age for another 2000 hours... Two of them, according to my Euler Precision LFLNA-80 turned out to have around 0.5µV peak to peak noise.
I took the best one and started ramping it from 39ish°C to 51ish°C at increasing current.
I found a magic current for my best diode!
But here's where my problems start.
If I read the Solartron 7081 service manual, in the description of the reference circuit, it talks about the second order tempco correction circuit being centred around the top of the curve being at 27°C. That makes no sense at all. The average internal temperature of my 7081 is between 40 - 45°C depending on the ambient temperature. This meter is a closed case meter with no fans. There is no way that after being powered up for a day that the internal temperature would ever be even close to 27°C!
I looked at the wonderful document that Mickle T had written about his experiments with the 7081. In there, he talks about how the reference circuit works. Following his documentation, I could easily work out how to set the required "Zener Token" value and adjust the resistors involved so that I could set the correct current for my new diode. That turned out to be 7.8mA:
What I can't work out how to do is recalculate the values of resistors R351 and R354. As I now have a current for my diode which places the top of the tempco curve at between 40°C to 45°C - exactly where it needs to be for my meter, I need to know how I can work out what the values of these two resistors should be.
Can anyone help me?
Kleinstein:
The square circuit can only work with positive values. R351 sets the offset / zero for the temperature (those odd looking 27° C). One might argue this could be a bit on the low side, but it sets the minimum temperature the ref circuit works.
The exact value is not important, but it also effects the overall linear TC. If really needed it may be used to fine tune the linear TC later, though the main path for this seems to be the DAC part.
R354 (or alternatively also R353) sets the scale for the correction current and thus the T² term in the correction.
Chances are one would first have to adjust the square term via R354 and than do a fine correction of the linear term.
Overall the ref. circuit part looks rather complicated, especially the optocouplers and DAC to be set by jumpers.
guenthert:
--- Quote from: unseenninja on July 20, 2024, 12:37:04 am ---[..]
After eliminating every other component in the reference circuitry I was forced to accept that my finely aged and selected by greybeards reference diode had surrendered its ppms and stability.
--- End quote ---
I think this deserves some elaboration. How did you determine that the noise comes from the reference of the DMM and not a different component or the source (the LTZ1000)? Frankly I'm a bit skeptical that a Zener should develop popcorn noise late in life.
--- Quote from: unseenninja on July 20, 2024, 12:37:04 am ---I know. The answer is "put an LTZ1000 reference in there". That wouldn't be keeping the meter original though, would it?
--- End quote ---
Dunno. If you intend to keep the unit, it's of course entirely up to you. If you mean to sell the unit later on, it would be fair to document what you did. I doubt many would prefer an artisan-made 1N829 based reference over an artisan-made LTZ1000 reference.
--- Quote from: unseenninja on July 20, 2024, 12:37:04 am --- So I found a supplier of new, old stock 1N829 diodes on eBay
--- End quote ---
1N829 or 1N829A? The former seems to be still easily available (as NOS, e.g. at Anchor Electronics), the latter harder to find and considerably more expensive. Afaiu the latter are selected for lower tempco. Not sure if there are other differences or whether there are some 1N829 just as good as 1N829A.
I tinkered a bit with 1N829 and while they perform as specified (specifications I found were 'thin'), I found them too noisy and poor in terms of long term stability to be all that interesting. I know that HP (in the 735A), Solartron and Datron used 1N825(A?)/1N829(A?) much more successfully, but I suspect they put a lot more effort into the selection process than I could muster the patience for.
tggzzz:
--- Quote from: unseenninja on July 20, 2024, 12:37:04 am ---I checked the 10V range against one of my most stable and quiet references - a Dr. Frank LTZ1000 build. Sadly, what I saw was 4ppm popcorn noise:
--- End quote ---
If you are looking at noise, then measure the voltage of some form of chemical battery. While you will probably see drift as the temperature changes, any noise you see will be within the meter - e.g. the voltage reference diode.
I've used a 7081 with a (1949, still in-spec!) saturated Weston standard cell. While the cell made a reasonable thermometer, it was effectively noiseless.
https://www.eevblog.com/forum/metrology/weston-cells/msg4093978/#msg4093978
unseenninja:
--- Quote from: guenthert on July 20, 2024, 08:06:36 am ---I think this deserves some elaboration. How did you determine that the noise comes from the reference of the DMM and not a different component or the source (the LTZ1000)? Frankly I'm a bit skeptical that a Zener should develop popcorn noise late in life.
--- End quote ---
I'm sure that the popcorn noise was coming from the reference Zener. I also have a perfectly quiet and reliable Datron 1271, and my Dr. Frank reference, measured on the 1271 produces a completely acceptable result, changing by around +/- 0.4ppm over temperature excursions of 2°C. It has no popcorn noise.
I was also sceptical that a diode would develop popcorn noise later in life, but after changing every other active component in the reference circuit and checking all the passive components, the only thing left to account for the popcorn noise that I saw was the diode.
--- Quote from: guenthert on July 20, 2024, 08:06:36 am ---1N829 or 1N829A? The former seems to be still easily available (as NOS, e.g. at Anchor Electronics), the latter harder to find and considerably more expensive. Afaiu the latter are selected for lower tempco. Not sure if there are other differences or whether there are some 1N829 just as good as 1N829A.
--- End quote ---
The difference between the non-A and A version of the 1N829 is the maximum Zener impedance. 15 Ohms for the non-A version and 10 Ohms for the A version.
Both have a data sheet specified tempco of 5ppm/°C at 7.5mA current. My replacement is a 1N829.
--- Quote from: Kleinstein on July 20, 2024, 07:02:16 am ---The square circuit can only work with positive values. R351 sets the offset / zero for the temperature (those odd looking 27° C). One might argue this could be a bit on the low side, but it sets the minimum temperature the ref circuit works.
The exact value is not important, but it also effects the overall linear TC. If really needed it may be used to fine tune the linear TC later, though the main path for this seems to be the DAC part.
R354 (or alternatively also R353) sets the scale for the correction current and thus the T² term in the correction.
Chances are one would first have to adjust the square term via R354 and than do a fine correction of the linear term.
Overall the ref. circuit part looks rather complicated, especially the optocouplers and DAC to be set by jumpers.
--- End quote ---
It is indeed complicated. The DAC is set by the firmware based on the value set for what Solartron call the "Zener Token". This determines the voltage generated by the DAC and therefore the current added to that generated by the fixed resistor R304.
From what you are saying, I guess the only thing I can do is implement the calculations that Mickle shows in his document:
I already have a spreadsheet which allows me to calculate the setting of the Zener Token and the value of R304 which is needed. (The Iz(token) part from Mickle's calculation). Then I'll need to implement the rest of his calculations so that I can plot the compensation curve and adjust R351 and R354 until they match the tempco curve that I derived for the diode in testing.
Or, just give in and just put a KX reference in the machine!
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