Buyers beware, misleading 34470A reference modules (clones) on eBay.

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"Temperature in chamber was swept gradually from +18 °C to +50 °C with speed around 0.03 °C per minute with static step at 50% of sweep (+34 °C) for 1 hours and 1 hours for peak temperature. Ambient room temperature and humidity stability was maintained at +23 °C ±1°C and 15% RH ±2% respectively. "

Hello Illya,

You show temperature at alpha=zero at 237.565C. That's hot! Is this a result of the quadratic regression?

Why did you use quadratic on data that look amenable to linear regression with a pickup of one degree of freedom in the regression?

By definition, one degree in Celsius is equivalent to one Kelvin. In other fields of science, temperature-related comparisons are based in Kelvins; why not here? Does it matter?

If anyone would like a raw data file for any of of my temperature/voltage measurements related to the Keysight reference, I would be happy to post here. Reciprocity would be nice.

[TiN]

Using Celsius scale is just convinience in regular room-temperature metrology, so we don't have to say that temperature was changed from 292.15 K to 323.15 K, while room temperature kept at 296.15 K?

As for math, I used same approach to calculate alpha and beta coefficients just as it is common and well understood in resistance metrology.

This particular LTZ1000A reference does look fairly linear but it is not given and many other (LTZ-based) references are not as linear. Quadratic law fits bigger picture better. Sure, +237.6 C "zero" point does not have significant value here, more of an artifact number from my automated math script. 

If you want to share RAW data, it is always good to include more details about actual experiment and methodology. I was interested in tempco of this 34470 clone and impact of different resistors on tempco results. Might try some other resistors in future as well, but for now I got my answers here. Full disclosure : don't have any 344* LCD-based instruments in my lab to contribute anything else or make tail or horns out of ACAL tests posted above and alike, sorry.

 To me adding extra delays and pauses or running ACAL so often (which is not ACAL like on 3458A) sounds bit strange. Typical experiment run is using DMM for logging data for some time, and then if temperature or condition changes a lot, perform corrective ACAL. Usually always try to collect at least day or more of data when its about stability or noise parameters. But without more clear idea what you testing or expect to see in results it is hard to suggest anything specific.

One important thing forgot to mention - measurements of the 34470 module output were made with wires (both power and LTZ output) directly soldered to the module. Original connector with female sockets was desoldered. Voltage output wire was twisted pair PTFE-insulated copper with shielded connected to GUARD port of 3458A. Cable on the DVM end are terminated to bare copper spade lugs. LTZ1000A chip was covered with some packing foam on both sides with help of kapton tape  . Typical stuff when dealing with LTZ1000A-designs.

[Kleinstein]

For the test of the reference in the 34465 it makes sense to have both a run without any autocal calls and a rund with a relatively frequent (though ideally not as often as shown - need to have at least enough time to fully settle) autocal calls. One wants a low TC / low drift in both cases. The case without ACAL is limited by the ADC gain but still needed as normally one would take a curve without frequent autocall calls. The case with autocal would be relevant for the long run. The difference would tell on how much drift is from the ADC gain plus maybe taking the reference low side from the "wrong" pin.

The slightly higher than normal TC may be due to not so good resistors, but also from the way the negative side is sensed.  With the separate pin Dr Frank may be able to a test where the extra ref- signal is not connected - so one would be sure the DMM would use the normal ground like with the LM399. If there is no difference, chances are the meter does not use the extra ref.- pin.

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The case with autocal would be relevant for the long run. The difference would tell on how much drift is from the ADC gain plus maybe taking the reference low side from the "wrong" pin.

The slightly higher than normal TC may be due to not so good resistors, but also from the way the negative side is sensed.  With the separate pin Dr Frank may be able to a test where the extra ref- signal is not connected - so one would be sure the DMM would use the normal ground like with the LM399. If there is no difference, chances are the meter does not use the extra ref.- pin.

The schematic posted in this forum shows the -REF LTZ connected to ground on the ref board and J1 P5 connected to ground, presumably on the mainboard. I agree, the reference is probably operating as designed, albeit in a different venue.

I have a longer (24 Hour) test underway and will post here when completed. Preliminary results show PDVS2 voltage stable in a less than 10 micro-volt range over a 12 hour period when measured internal temperature decreased from 29.5C to 26.5C.

In the meantime, I could use some advice concerning my newly acquired Keithley 182 Sensitive Voltmeter. How sensitive? Well, really too sensitive and embarrassed that I have posted the attached photo of its input jack stripped down to show all. Noted as made of unobtainium, is it possible that the pins could be available somewhere? If no one offers a source, I can make them on my lathe. Which leads to my second question: what is the material? I presume some sort of copper alloy? I don't want to waste time making something unsuitable for the next phase: low voltage testing on the 34465 '70 Keysight ref.

Illya, if you are done LoL at my pure copper short, how can I make a proper one?

[TiN]

Would recommend radical method - before you spend hundreds $-$-$ ditch the obsolete unobtanium connector on 182 and replace it with something modern like LEMO connector used in 2182/34420A (still few $-$-$, but at least you are more futureproof). But I'm lost on what you planning to do with nanovoltmeter and LTZ reference? 
Connector is primary reason why I nearly never use my 182.

My PDVS2mini (prototype 5, if I remember right) has very good tempco and p-p noise under 5uV.

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Would recommend radical method - before you spend hundreds $-$-$ ditch the obsolete unobtanium connector on 182 and replace it with something modern like LEMO connector used in 2182/34420A (still few $-$-$, but at least you are more futureproof). But I'm lost on what you planning to do with nanovoltmeter and LTZ reference? 
Connector is primary reason why I nearly never use my 182.

Good advice! I watched your video on the battery replacement, which led to having a look inside my 182-M--missing its battery--now it has one. Thanks. I also have a 181--that how I came by the connector--so I'll look for the LEMO connector for the 182.

Nanovoltmeter and LTZ ref? I might be "lost" as well. I was thinking about doing some millivolt verification on the PDVS2 (presently connected to the 34465) using the 182, however looking back, it was a dumb idea. Using an unknown 182 anywhere in the process of testing an unknown implementation of the Keysight ref in the 34465 just add to confusion.

Thanks for the assist.

[MiDi]

Noted as made of unobtainium, is it possible that the pins could be available somewhere? If no one offers a source, I can make them on my lathe. Which leads to my second question: what is the material? I presume some sort of copper alloy?

K-181/182 Connector:
Amphenol 97-3106A-16-11P & 97-3057-1008-1 only awailable with silver plated pins.
For nickel finish add (689) to order number.

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Preliminary results--overnight 21:00 to 09:00 on Sunday. PDVS2 set at 10VDC into KS34465 w/ '70 Keysight reference.

[Kleinstein]

The curve looks good for a LM399 base reference. It does not tel very much about the meter or LTZ based refernce - the LM399 alone could explain all the noise. The temperature coefficient looks good, but the temperature range is to small to definitely see something.

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The curve looks good for a LM399 base reference. It does not tel very much about the meter or LTZ based refernce - the LM399 alone could explain all the noise. The temperature coefficient looks good, but the temperature range is to small to definitely see something.

Good point about the source of noise.

My reasons for buying a reference from Keysight rather than the China knock off stemmed primarily from having a reliable baseline comparison installed in a 34465 as I knew there would be interest on this forum. And also curiosity, sloth (didn't want to DYI), lust (Keysight delivered in less than a week), maybe getting better performance from the meter. So my focus was on limiting the effect of temperature on the voltage measurements while determine whether the replacement of the reference was beneficial.

TIN showed an excellent test across a broad range of temperatures for both the China ref and the PDVS2 mini. Is it safe to assume that the "best case" for performance of a 34465 with a '70 reference from any vendor is the performance of a Keysight 34470? From your comment, the voltage source should be two levels above the meter--9.5 digits for a 7.5 digit meter.

I would love to have a 9.5 digit voltage source . . . I have an option 2 board for a 3458A wired up in a metal box that has been run for a few months. I haven't verified performance on it, however I'm sure others on the forum have tried this as a reference. Time for some research . . ..

[Kleinstein]

No need to have the external reference much better than the one to test. The same grade reference would be about the minium - higher grade is better, but not that easy at that level.
An extrernal source of lower quality make the life very difficult and limits the result.

With the upgraded reference I would expect the performance of the 34470 at best - though by luck the DIY thermal cover may be better than the original. There may still be some other higher grade or better tested in the '470 - not just the reference.
I have a slight suspicion that there may be some mathematical correction active in the '470, that is not used with the '465 and this may be the part that sometimes / after some drift in other parts cause the ACAL problem.

The resolution / noise of the 34470 also limits on how easy a test of the refernce can be made. There are parts in the ADC circuit that act like reference noise (e.g the first OP where the ref signal goes to).

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Attached are plots showing "Serial Correlation of Voltage Readings from an LTZ1000 Reference (HP3458 A9)" as measured by an HP3458 and for comparison, a KS34465 with a genuine Keysight 34470 reference. The testing comprised about 14 hours of data collection. Discussion and setup photos here:

https://www.eevblog.com/forum/metrology/making-do-with-what-you-got-an-hp-3458-opt002-reference-with-no-heater/msg3430502/#msg3430502

From what I can gather from the time spent installing testing, adding the official '70 was worth the effort. (mostly for the fun of it; if someone were paying me, buying a 34470 would be the better choice.)

Oh, the plots on top of the three PPM plots are of the regression residuals. ("Nothing to see here folks, move along . . .")

[branadic]

I had a play with our 34465A at work and measured some LTZ1000 on it to see, if this unit suffers from popcorn noise and indeed it does, about 2.5 ... 3 ppm. So I followed Frank's advice and covered the area around the reference with foam to prevent any air current. The unit is currently warming up and then we will see, if that improved anything.

-branadic-

[branadic]

By the looks of it the foam slightly improved noise by preventing air current, but other than that this LM399 is pretty volatile and suffers from strong popcorn, worth to throw it away.

-branadic-

[Kleinstein]

The amplitude of the jumps looks pretty high. The expected size would be some 6 or 8 µV and not 20-25 µV. It could also be something different from the normal popcorn noise (e.g. contact problem, or maybe a cell phone).

So this may indeed be a reason to look for an exchange - maybe even one of the new ADR1399 as a slight upgrade if the current provided to the reference is high enough.

[dietert1]

In other discussion it was proposed that LM399 popcorn noise may be caused by minor movements of the plastic hat and/or by RF instability caused by directly wiring a resonator made of traces and a capacitor parallel to its output. The two states would then be oscillator/non-oscillator.
Another difference between LM399 and LTZ1000 with its external temperature control is that one is a P controller, while the other one is a PI controller. I don't think the integrated temperature control of the LM399 has an integrator as a proper capacitor doesn't fit the scheme.

Regards, Dieter

[branadic]

It's neither a contact problem (checked that when I installed the foam, by removing the reference from its socket) nor a cell phone. But I agree, this jumps are rather high, even for the average LM399.
A moving cap can't be the case here, the legs are pretty short and the reference originally sits in this "precision sockets" so that the cap touches the socket and can't move. Also with the foam the cap is prevented from moving.
RF instabilities should have been reported by other customers before I guess, if that was the problem.

ADR1399 would be an option, as Frank reported lower noise with LTZ board installed, but that would require a full adjustment and at least the change of one additional resistor on the board.

In summary the unit is nice, but the GUI needs more work, there are still some issues such as auto scale that doesn't fully work as expected.

-branadic-

[Kleinstein]

The ADR1399 does not need that much more zener current than the LM399. It depends on how much current is actually used and in some cases the current can be enough also for the AD1399.  It would thus only be the extra RC (or tantalum cap) connected in parallel to the reference - either free floating or a tiny PCB.

The meter would not be able to tell the difference from LM399 or ADR1399 - so I don't think it would need extra adjustments. One would still need to account for the different absolute voltage, but that would also apply to a normal LM399 replacement.

[randymatt444]

That also allowed me to use a copper plated plier during soldering of the LTZ1000A, to avoid strong heating of the chip, ...

I know this is an old thread, but what would be the harm in using a good socket for the LTZ1000?   

[Kleinstein]

The extra socket would introduce possible extra thermal EMF from thermal gradients. To a large part this would be constant, bit still avoidable. For the current setting path the contact resistance would add.
Worst case an open contact could cause the ref circuit to apply high than normal current or temperature.
Usually the LTZ1000 is used in conbination with the resistors as a set and not real need to remove it. Except for a separate burn in or test circuit for selecting chips I see no use for a socket.
As a normal reference it is more a disadvantage.

The chip is also not that sensitive to heat, as it is normal to run it at some 60-90 C. So unless one uses excessive long soldering on all pins I would not expect an effect of the soldering heat. This is especially with the full length pins and not mounted flush to the board.