This video has been supported by PCBway. Hey guys, last year we were able to give this
magnificent fluke multifunction calibrator a well deserved in depth refurbishing. Now there is nothing preventing it from doing
its job and spreading that sweet, sweet accuracy for another 30 years,
except maybe that there is no more accuracy left to spread. You see even just opening the enclosure once
and looking at the internals, lets some of those volatile ppms escape. and the open heart surgery that we performed
was much, much, much more impactful than that. So now the calibrator itself needs calibration
and adjustment. Theoretically, it's not that difficult. The 5700 can combine its functions in various
ways, allowing it to perform a full self calibration of all 41 voltage current and resistance ranges
using only three external standards. fluke calls it artifact calibration and it
needs a 10 kohm and a 1 ohm resistor as well as a 10 volt source. Practically we still depend on a calibration
service provider. And only the biggest of those can afford high
performance primary standards like a Josephson voltage standard and a quantum Hall resistance
standard. Those who can usually don't allow pilgrimage
and camping on site. just from a logistics standpoint, it doesn't
make sense to take the costly, heavy fragile calibrator to such a primary standard in person
just to let it sit there for a day or to warming up and then to lose half of the ppms on the
way back home due to uncontrollable temperatures and vibrations. What we need are rugged traveling transfer
standards with at least a good short term stability spec to import accuracy before handing
over to fluke and all other devices which happen to use the same artifact calibration
principle. Today we are going to look at the 10 volt
one or should I say two different 10 volt approaches. There is of course, the easy way of just buying
one. fluke themselves have a long history of making
extremely good battery powered voltage standards. This one the a model 732 A is one of their
earliest. It's so old in fact that it has a dedicated
1.018 volt output for compatibility with Weston cell systems. It is however Zener diode based and therefore
much more robust and less drifty. The drift of this unit has been recorded for
the last 30 years and it came out to less than point one ppm per year. an absolutely impressive track record exceeding
even the specifications of its successor or by far. until one fateful day in early 2020, where
it was noticed that it had jumped 50 ppm at once. oh no. Anyway, that's how it ended up on my bench
for some diagnosis and preventive maintenance. Oh boy,
am I excited about this, it's guaranteed to be interesting. Back here there is some seemingly mundane
power supply stuff, mains transformer, a battery charger circuit and a battery pack made from
individual six volt Lead Acid cells. These have been swapped in 2018. So most likely they are still good. I hope the old ones have been disposed of
in a safe and eel friendly manner. Their battery trickle charger according to
the service manuals schematics is actually quite primitive. In the hardware revision we are dealing with
here most of the circuit has been replaced by a ua723 adjustable voltage regulator with
adjustable current limiting. a 1980s preview of what modern electronic
engineering is going to be all about. ohhh! Hope you don't mind if I do the obvious replacements
off screen, we've got too much to do today to rerun the simple stuff. After a certain incident that you will hear
all about in one of the next videos I also added these main filters to my list of always
replace on first sight. They are not as bad as carbon composition
resistors and electrolytic caps because usually they don't take other components with them
when they die. but they used to be filled with a kind of
tar and when that gets regurgitated out, the stench never leaves your instrument again. Apart from that there is only one unusual
feature on the battery charger assembly that I would like to point out to you. I'm not talking about these playful twisted
pair traces coming from the mains voltage selector switches enclosed in what looks like
a leakage containment polygon. I don't even know if they do anything. but their destination, the guarded power transformer
is crucial. Let's say you have a sensitive mains powered
voltmeter and a mains power source in a different enclosure running on a different transformer. We've been taught that to transformer secondary
windings are completely isolated from everything else but in reality there are always leakage
resistances and inter-winding capacitances between primary and secondary. They have played us for absolute fools. If you connect to mains powered devices only
with the test leads you need for a voltage measurement or whatever those ruthless leakage
currents will do their best to flow through the test leads not caring that they influence
your measurements on the way. That's why metrology grade equipment tends
to have a separate a guard terminal connecting internal shields around to the most sensitive
circuitry and right through the main transformer. This provides a preferential lower impedance
path for those common mode effects to do their thing safely far away from my measurements. For DIY endeavors or repairs, a guarded transformer
with a shield between primary and secondary is of course not that easy to find, even though
it's pretty simple to manufacture. Here is IanScottJohnston with an anatomy lesson. I'll link the full version below and we'll
talk about DIY friendly methods in the second part of this video. For now, back to fluke 732 a where guarding
is already taken care of. there is one more small board in the rear
hiding a huge population of illegal carbon composition resistors. I was so relieved when I saw that our board
was not yet charged to a crisp. This is called to the regulator PCB assembly,
because it's most important to job is to regulate input voltages from mains or battery down
to 18.6 volt, which is what the reference assembly likes. Personally, I don't really know what it sees
in 18.6. I'm more of an 18.0 kind of guy. Oh well, whatever floats your boat. The electrolytic caps on here all measured
fine. But after over 30 years of service, I think
it's time to retire them anyway. Just like all their carbon composition henchmen,
which will now become 1% metal resistors. Again, there is a noteworthy sub circuit on
this board. It's for the calibration indicator on the
front panel. Using q7, a programmable unijunction transistor,
it's a kind of a reverse thyristor that becomes conductive when the anode voltage exceeds
the gate voltage, it powers the in cal led permanently until the supply voltage falls
below a certain threshold. In that case, it is assumed that something
bad happened like battery empty or on fire. Either way, the internal temperature couldn't
be guaranteed anymore and a new cal is in order. Alright, time for us to leave the boring management
area behind and to unbury the business end the reference assembly. Fundamentally, this product is actually based
on the same SZA263 Zener diode that's largely responsible for my calibrators good performance. So how is it that this DC voltage standard
used to be the official recommended calibration source for that calibrator? what makes it better? Well, it's more than likely that they are
doing binning at the factory. With the creme de la creme going into DC voltage
standards, the second best going into calibrators, the third into multimeters and the rejects
somehow service on eBay eventually. But the underlying Zener diode is only part
of what makes the 732A So good. Look at the size of this package, all that
to put out three DC voltages. Of course it's all about temperatures again,
this aluminium coffin surrounded by 40 millimetres of insulating foam material on all sides is
kept at 48 degrees C precisely. far away from expected ambient temperatures. even if those are not terribly stable At least
there is no powerful fan to inhale the changes greedily. There are only these holes for the trim pots
breaching all the wonderful insulation. I don't like those at all. Even if they are meant to be covered by Cal
stickers eventually. Strange construction, the cables all feel
a bit brittle. I hope I can snoop around in here without
breaking any. There are three major thermal effects at play
that have to be managed for this assembly to achieve best performance. First our old friend the thermocouple effect
that can generate microvolts and therefore ppms when junctions of dissimilar metals are
at different temperatures. tinned copper circuit board gold plated kovar
component leads nichrome resistance wires and platinum resistance thermometers. None of these can be engineered away easily. So care is taken to heat everything evenly
with these orange silicone heat pads on all walls. Who would have thought that there were applications
for these other than 3d printers. Then there is of course the actual Zener voltage
temperature coefficient of around plus two millivolt per Kelvin in this case. that Is elegantly compensated by a matched
transistor base emitter junction in the same package, guess what, minus two millivolts
per Kelvin. And that's the SZA263 for you, and that transistor
actually has another purpose in addition to that. And third there is of course, the resistance
tempco of all the surrounding configuration, scaling and trimming resistors. other than using the finest fluke branded
hermetically sealed the wirewounds, there's not a whole lot one can do about that I'm
afraid. vishays bulk metal foil technology was not
yet around at the time. But with precise enough oven control even
these few ppm barrels will do their job superbly. oh no! what the fluke happened here? I thought we were looking at a world class
DC voltage standard and not a look mum no computer circuit bending victim. Either way, I don't think I'm going to attempt
to freeing the reference assembly from its tethers. Nah, I just swapped all the carbon composition
enemies in situ. Basically all blue resistors are new, and
should be good to go for more than three decades this time. The Originals were mostly out of tolerance,
but none drastically. the only obvious problem I was able to find
in here is this heat pad that has decided to separate itself from its wall partially. I don't think that that can account for the
50 ppm jump that brought us in here in the first place, but I'm going to reattach it
anyway. I've been told that after the 50 ppm jump,
everything was perfectly stable Once again. that matches what I've found inside, nothing
worth repairing. I've got no explanation either, maybe incorrect
handling or a mistake during the last calibration. Either way, I don't want to inconvenience
the sleeping ppms in here any further. With all the perishables out of the way I'm
going to leave it alone and observe a bit more. Just a few more quick thoughts on design details
before we move on to act two of this video. The resistive heaters are all made for 110
volt mains. Here they are driven by 24 to 30 volts DC,
which slows them down significantly. Additionally, there is a shaping circuit made
from capacitors and mega ohms, low pass filtering the proportional temperature controller even
more. The reference assembly has three NTC thermistors
arranged in a modified to bridge configuration. My brain can't process this crazy construct
and neither can a simulation. So I'm afraid we've got to take flukes word
for it And trust that after one day warm up time it reaches some kind of stable state. The reference circuit is also pretty complicated,
but only because it's meant to be adjustable to 10 volts precisely. Most of the expensive precision resistors
are only in here on an if needed basis. They are used to add or subtract fractions
of ppms from the output voltage without adding temperature coefficient or long term drift
that cannot be done only by trim potentiometers because those just don't exist with sufficient
stability specs. The fundamental circuit over here in the corner
is a lot more simple, and I bet it would be a lot more stable without all the adjustment
clutter next to it. The SZA263 is often called ref amp instead
of just Zener diode because with its series transistor and its most important divider
on the side, it can compare a voltage to its own in house zener diode while putting out
a differential error signal. Now who's the expert at eliminating differential
error signals? Of course, an operational amplifier gets to
generate that initial compare voltage at exactly the right level that satisfies our ref amp. This arrangement has an inherent immunity
against all sorts of errors, and it needs so few external components to generate 10
volts directly. That might be its biggest advantage over the
competition. The LTZ1000 whose list of mandatory external
components is long and much fought over. Okay, that segway came a little early. I'm going to put everything back together
and we'll start data logging with my most stable voltmeter the
similarly organized HPM7177. Let's hope that we won't catch any more jumps. And while we wait, I'll introduce you to my
own fluke 732A alternative. The SZA may be nice and compact and easy to
scale. But our old friend the LTZ1000 has a few compelling
advantages up its sleeve too. for example, you can officially buy it ... uhm from time
to time. There is a lot of publicly available knowledge
about it out there regarding the most questionable thermal cutout designs, top secret burn in
recipes, and of course the all time favorite: creative applications that differ from what
the manufacturer intended. Officially, I don't claim to be any wiser. But I've seen a few well working applications
now, and I think it's time for me to try my own. I'm going to build an LTZ1000 based voltage
reference with its normal drive circuitry, a fixed gain buffer amplifier to turn its
7.1 volt into 10. And I'm going to make it a battery powered
and ovenized for mobility. As far as possible, I tried to use readily
available ingredients. But because this plan started to take shape
last year, the current shortages have not been taken into account. There are these mu metal cans for shielding
sensitive components against a broad band of electromagnetic interferences. Those can be combined with these hermetic
feedthroughs that my friend Gigabecquerel has donated to the good cause. These are the real deal kovar glass kovar
and gold plated for maximum fanciness. With these we can fortify our EMI shielding
cans and make them resistant against moisture and other chemicals. With a hermetically sealed module we might
get around using vishays sealed luxury resistors. we do still have to rely on vishay in the
end because they also happen to make the lowest tempco SMD resistors, which I wanted to use
instead. But those are at least stocked by the larger
distributors and don't have to be custom ordered. Having these superb vsmp foil resistors available
in many different values on short notice is awesome and a major threat to my bank account. They might not have the unparalleled temperature
coefficient of their VHP series or the sempiternal long term drift of their VHA series but they
just fit wonderfully inside my mu metal cans and inside my budget. What worries me a bit is that they don't have
flexible leads. So thermal expansion of PCB materials and
solder joints is handed right over to them. Not that their ceramic substrate couldn't
handle that. But there is a chance of losing fractions
of a ppm to stress. I can relate but luckily This videos sponsor
PCBway has a very elegant solution to that: flexible circuit boards. These are the first ones I've designed and
ordered ever. At first I thought I was opening Pandora's
box with new design rules an unfamiliar purchasing process and a lot of caveats, but in reality,
it was exceptionally easy. The result is as beautiful as I had hoped. It's flexible, yet durable, the substrate
as well as the coverlays are polyamide with much better dielectric strength, heat and
chemical resistance than traditional fr4 material. PCBways intuitive ordering form makes it no
different than preparing a traditional board With all the familiar settings. they can manufacture up to 8 layer flexibles
with ultra fine 0.06 millimeter features on every one. What's more, they can attach these directly
to normal rigid boards, giving you the designer immensely powerful tools for high density,
high reliability, interconnects and so much more creative freedom. It's a fun material to work with and much
less prone to de-lamination. only modification after the fact is a bit
tricky because it's easy to break through to layers below. Just don't make mistakes Kappa. In addition to my reference and buffering
circuits, I also ordered these heaters which rely purely on trace resistances. based on some napkin math, I expect their
room temperature resistance to be 35 ohms. And sure enough, all of them came out within
10% of that theoretical value. Weird flex but okay for PCBways precise materials
and processing. This is what I made these laser cutter thermal
interface adhesive pads for in the last video. I hope nobody saw that. I'm going to attach a heater to the mu metal
can with one to repslicate flukes proven thermal design. But not really. This package is different in almost every
way, and I expect it to need some significant fine tuning later. For example, we can sweep the oven temperatures
in an attempt to find the flattest, tempco neighborhood where slight deviations have
the least amount of influence on the output voltage. Oh, well,
let's get it to work with a more heavy handed temperature control method first. What do you think I should call this project
by the way? I was thinking either ADR1002 as a nod towards
a certain semiconductor rumor that inspired this whole thing, or LTZmu, but then I would
feel the need to keep you waiting for a few years before releasing. I'd love to hear your suggestions in the comments. The mostly SMT construction makes population
quick, easy and scalable. You know if we were to turn this into a real
product. the two leaded components the LTZ1000 and
its most important temperature configuration voltage divider I couldn't really optimize
away but they need some fingertip-Feeling as Germans like to say, to solder them correctly
anyway. that's not great for production at scale,
but not as bad as the finishing touches. Here's that mu metal bottom cover. for lack of a better method, I'm going to
have to hand solder each and every one of those hermetic feedthroughs. And I'm afraid I have to overdo it because
I don't have a way of testing hermetic seals for leaks. The LTZ1000 for this module has been carefully
selected, aged and tested in a different circuit before. This way, if my measurements reveal a problem,
in the end, I can be reasonably confident that the buffer is to blame and not the voltage
reference itself. Not a terribly unlikely scenario, if I'm being
honest. the fundamental circuitry that generates ultra
stable 7.2 volt is pretty much the same that everybody else uses. ultra stable amplification However, with fine
trimmability and short circuit protection was a bit of a challenge. basically a voltage divider with a 2:5 ratio
gets you reasonably close, but more zeros after the decimal point are costly. I've reserved a lot of extra resistor footprints
on my flex PCB, as well as two jumper links to configure either trim up or trim down. This trim resistor, if needed can be connected
from the outside to two of our hermetic pins. Fair warning though, these guys are all very
influential. So if they ever decided to drift, it will
be detrimental. It's likely that we could achieve better stability
without all this trimming stuff at a lower price too. those blue SMD resistors are the 0.2 ppm per
Kelvin vishay ones. They are between 10 and 20 US dollars in single
quantities and the plastic molded Z202 ones are even more expensive, but I had those lying
around before. Of course no revision 1 circuit board is complete
without a few botches. For example, the LTZ driving op amp must be
a single supply one, otherwise the circuit won't start. it took me a while to fine tune this thing
to 10 volts with three zeros behind the decimal point using the limited selection of precision
resistors that are available immediately. If needed, further adjustments are possible
from the outside. Outside is also where I happen to store the
goodest of the good stuffs, which cleans circuit boards better than anything I've tried to
before. it can clean plastic ic packages right off
the board, so I wouldn't recommend it unless absolutely necessary. I will apply a few fixes to the project then
I'll upload and link it below. But don't build one just yet. Instead, let me collect a few weeks worth
of data first to see if a doubly ovenized LTZ1000, stress free mounted, hermetically
sealed SMD foil resistors do actually work as nicely as they sound. I'm quite confident in the short term stability
for which I made this thing. That way I could ship it around to friends
in Europe or take it to meetings myself, but I'm not delusional enough to think that I
can beat the fluke 732A recorded performance as easily as this. I'm not exactly loading my scaling resistors,
but I am storing them at elevated temperatures. So I think we have to expect some load life
drift after 1000s of hours. Let's press on regardless. with thicc boi thermal insulation being such
a prominent feature of fluke 732A I was sure I needed that too. So I bravely started CNC machining some styrodur. just letting a hot mu metal can sink into
the material would have worked too, although that leaves some dense molten material around
the can which diminishes the insulation capabilities a little bit. it wouldn't have mattered because turns out
of my first insulation attempt was way too good. embedded in this foam shell a single LTz1000
and three op amps were enough to push the internal temperature beyond 45 °C just by
self heating in normal operation. uhm ... I was thinking of setting my oven
temperature to about 40. How about some audio foolery to give our voltage
standard that full body tonality and a clear uncolored timbre that fluke can only dream
of? we already have the magically aged components, oxygen free copper cables, pricey high performance
mains filters and arcane geometries on the circuit boards. This type of R-core power transformer is available
under various stickers from China directly or at DIY hifi shops. Its construction suggests that it is ideal
to insert a shield between primary and secondary for our fluke Inspired anti common mode guard
terminal. But unfortunately on closer inspection, it
turns out that it is not. There are primary and secondary windings on
top of each other on both coils for easier reconfiguration of 110 and 220 volt and also
to improve the coupling coefficient which makes the transformer more efficient. It's a pretty cool transformer, no doubt,
but its inter winding capacitance is not going to be anything special, I
think, Oh, nevermind, there actually were shields between the primary and secondary
windings. They were connected to a green yellow lead,
which usually indicates protective Earth. they could have easily made them larger to
cover everything, while still not creating a shorted turn. But this is certainly better than any old
off the shelf transformer. For maximum performance with least manual
rewinding, I would just look at eBay where you can pick up beauties like these for 10
bucks or so. There are also companies that wind Transformers
according to your specifications, including shields, of course. the easiest solution however, and yet one
of the least obvious ones: if it's battery powered, you can just unplug it while in use. Where there is no connection to mains, There
are no common mode troubles from means hopefully. instead of worrying about my standard burning
down itself and the logistics company, multimillion dollar warehouse due to lithium battery failure,
I think I'm just going to go with the safer gel batteries. there are a lot fewer shipping restrictions
on those. But in general, the shipping powered on Electronics
is always problematicI think. The batteries as well as my isothermal reference
Cube fit perfectly inside this schroff ratio pack pro air desktop enclosure. can you tell that that product name has been
chosen by Germans? the whole package will be a lot smaller than fluke 732a, and I'll
let it take its place in the long term measurement Now. A few more thoughts and details and then we
can jump right ahead to the first results. the LTZ1000 data sheet application as seen
in 3458A multimeters And cern ADCs for example, looks a bit intimidating at first, but it's
not all that different from the SZA263 we've seen before. It uses this first transistor purely as a
temperature sensor. The 13k 1k divider sets the base voltage to
around 500mV. When the transistor temperature rises it draws
more and more current exactly until the collector voltage reaches 500 milli volts too. that satisfies the oven driver op amp and
it turns off the heater. The oven circuit is neither perfect nor almighty. So to make what little temperature variations
there are less influential The LTZ uses the same compensation trick where a minus two
millivolts per Kelvin base emitter junction is added to the plus two millivolts per Kelvin
Zener diode. It's neat, widely used and trusted, and clearly
good enough for some of the best multimeters in existence. But what about DC voltage standards? Are there any trustworthy models with LTZs
inside other than my recent creation I mean? ah glad you're asking. there is a certain wavetek 7000 series and
it is immensely interesting. Unfortunately, the company was bought by fluke
and the model 7000 soon discontinued. There is not a whole lot of info about them
out there. But the EEVBlog forum members are digging
deep. their answer to the common mode issue seems
to be a special DC to DC converter with a custom transformer whose primary windings
and cores are encased fully in a conductive plastic shell. The conductivity has to be high enough to
provide good shielding against inter winding capacitance, but it can't be too good lest
it become a shorted winding that loads down the whole transformer. In the LTZ and buffering department, they
are relying heavily on these arrays of equal resistors. By connecting multiple elements in parallel
slight deviations of the individual are of little consequence. And if all are truly equal, ratios are maintained
no matter what. when I was talking about my heavy handed temperature
control method earlier, I was thinking about this: at wavetek they were able to get great
performance out of mostly affordable components, even without a brute force ovenizing the whole
thing, much less hermetically sealing it. A big part of why that works so well for them
is a resistor connected to LTZ pin three, the Zener cathode. To test this I disabled completely the heater
circuit on one of my LTZ boards, and instead took control over it with the Arroyo temperature
controller. It was so brutally hot here at the time, it
hardly needed to supply power to reach those higher temperatures. But this made it possible to sweep the package
temperatures while recording the LTZ output voltages with different resistance values
upstream. Here's what I got with 20 ohms above and 120
Ohm below. at around 61 degree one can clearly see that
the LTZ output voltage reaches a plateau, a peak if you will, where the neighborhood
is as flat as it gets and small thermal variations have little impact. So the most populer configuration with a 13k
1k divider that just so happens to configure the oven setpoint to 60 degrees C is popular
for good reason. Otherwise I'm seeing approximately a 10 ppm
shift in output voltage for every degree Celsius. That's not too special, but significantly
better than what I was getting with the data sheet recommendation of no ohms above and
120 Ohm below: 33.5 ppm per Kelvin on average. Now let's take a look at this completely random
configuration of 25 ohms above and 120 Ohm below. Whoa grafanas automatic scaling makes that
look unremarkable. But lo and behold, that is one ppm per Kelvin. So it looks like wavetek and everybody else
who is determined enough can influence the LTZ1000 tempco. EEVBlog forum member branadic has demonstrated
that that is possible down to fractions of a ppm. if a nearly perfect zero ppm was reached,
one could forget about the LTZ1000 heater altogether. I think this works by balancing the positive
contribution of the zener tempco versus the negative one of the base emitter junction. we are taking influence away from the Zener
diode by increasing the resistance in front of it exactly until the natural mismatch between
the two semiconductors is accounted for. putting it that way, it should be obvious
that this method is a bit of a double edged sword. I wouldn't splice it into all existing LTZ
circuits generally, because the influence that we are taking away from the Zener diode
now rests on the shoulders of resistors. The 13k 1k voltage divider that used to be
the most critical part no longer matters all that much. instead the circuit is now dominated by I
guess the 70 kohm one and our new so called above resistor. I guess it would really work best in combination
with waveteks statistical resistor network method. I just wanted to mention these fascinating
ideas briefly, and leave a mental note for myself that I'll have to try them someday. For now, though, my heavy handed flex PCB
employs none of them. Alright, then let's get to the results Finally. the fluke 732a preventive maintenance actually
took place last year, I'd have monitored it intermittently since then, both using voltmeters
and calibration reports generated by fluke 5700. Not even a hint of another jumper appeared
since then. If anything, the two flukes seem to agree
with each other too closely, as if they are conspiring to save each other's skin. This, by the way, is why I was questioning
the external trim capabilities of both of the standards we've seen today. All devices that matter give you the opportunity
to enter the precise value of the reference that used. it has been only seven days since the last
calibration this time, but this is kind of sus Don't you think? Only something like a handheld multimeter
will ask you to deliver the calibration values on point. And then that isn't something that one would
use a fixed standard for. In the 0.1 to 10 Hz noise discipline we are
getting 1.88 micro volts peak to peak or 192 nano volts RMS that's about 1/5 of their datasheet
specification and therefore slightly suspicious again. But then again, the calibrator had 2.1 micro
volts peak to peak so we are in the same ballpark. Here are some graphs over the nine most temperature
stable days in recent history. We are seeing day to day variations of a little
more than half a ppm. The blue markers are self calibrations of
the voltmeter are used. Could I perhaps interest you in a video about
that? in the end, we are arriving at exactly the same value we started out with. that is looking very promising, but ideally
I should have an entire coterie of standards to compare to. Unfortunately, my flex PCB has not yet recovered
from the shock of having been assembled a week ago. it is still happyly losing ppms on a daily
basis, but at a surprisingly steady rate. If it never settles down at least it's predictable. I endured the suspense for another week and
was rewarded with a capture of the one and only turning point. At this time the reference was powered by
a benchtop PSU so it's a bit more noisy than it has to be. Here I switched over to power from an imbalanced
pair of batteries and was promptly rewarded with a huge jump. We've got a poor a power supply rejection
ratio and probably need a better pre regulator like fluke. that also contributes to what looks like long
term drift in this plot. The positive battery discharges faster it
makes the imbalance worse. In the next version, I am Probably just going
to ditch the 4 wire force and sense outputs. that way, I'm going to need fewer expensive
hermetic pins, one less op amp and one less power supply rail. That will also make the self heating a lot
more manageable. If only I could stop changing stuff for a
few days, I'm pretty sure I would have made it to 0.4 ppm per day already. Oh, well. This is the best I have for today, but I'll
keep you updated. What do you think we should do next? resistance standards or a multimeter repair? We had quite the information density today
so if something needs more clarification, please leave a comment. Sometimes I miss the forest for the trees. Thank you for watching :) See you next time.
Gosh Marco is awesome
One of my favorites is the video about his low noise amplifier. It’s just pure analog goodness. Hope he makes an update video to that one some day