Do I spot a certain LTZ1000A trace in Dr. Franks diagram?  ;)

My reference circuit will be shipped to another volt-nut this week (off-band for this forum) for further comparison. Also I will start building a LM399AH-based reference at work, or to be exact, prepare a design to be made by our trainee, will be used for performance tests of our 34401A at work (we have a scheme of buying fresh calibrated 34401A in place of a calibration sometimes and do a in-house comparison as performance/fitness check for the older ones, and some smaller fitness tests inbetween)

BR

Hendrik

Dr. Frank:
The stability of the output voltage over time and temperature is determined mainly by the LTZ1000 chip, and the 5 resistors, R1-R5 (see datasheet).

Let’s first consider ageing (stability over time).

For the LTZ1000 chip and all of the resistor technologies there exists an exponential law over temperature, (Arrhenius), saying that ageing rate doubles every 10°C.

Such dependencies from operating temperature are often encountered in deterioration of material, e.g. the decay of LED luminosity and the life time of light bulbs, which is always driven by the operating temperature. The underlying process is the increase of defects in the crystalline structures, or the acceleration of diffusion processes. Therefore, reducing the operating temperature will reduce the ageing rates.

LT claims typical ageing of 2µV/sqrt(t/1000h) @ 65°C, that gives 0.8ppm/yr.
By lowering the temperature to 45°C, typical values of 0.2ppm/yr. may be achieved.
Therefore, the LTZ1000 version has to be used, with R4=12k, R4=1k. The reference then can be operated at an ambient temperature up to 35°C only.

Spreadbury (1) and also Pickering (2) have confirmed this experimentally for the LTZ1000 reference chip. The typical drift rates really apply in most cases. Intermittent operation of the LTZ1000 also will reduce its annual drift.
In the HP3458A the potential ultra-stability of the LTZ1000A is spoiled, by running it at 95°C, which leads to an 8 times higher ageing rate (continuous operation).

The 5 resistors also have to be operated on lowest possible temperature. This is fulfilled by Tamb. < 35°C and limitation of the self heating effect: Pmax. < 10mW. For those conditions, a Shelf Life ageing parameter is specified for precision resistors. In the LTZ1000 circuitry, R4=12k, 4mW, and R1 = 120, 2.5mW fulfill this requirement.

Additionally, there exists an influence from oxygen (and other reactive gases) and humidity on the crystalline structures, causing a sort of corrosion on resistors. Molded components, and to a lower degree conformally coated components suffer from that. Therefore all high quality / precision resistors as Thin Film, Wire Wound and Metal Foil types have similar specified ageing rates of 20...35 ppm/yr., (typical or maximum).

The LTZ1000 chip is hermetically sealed; therefore oxygen and humidity have no influence.

The resistors influence the reference voltage by changing the temperature set point (R4, R5), or by changing the current through the zener (R1).
The instabilities of R1, R4 and R5 are attenuated by a factor of 100; R2 and R3 by 300 and 500, respectively (see datasheet).
If all five resistors have the same instability values (over time or temperature), the total impact will be 0.035 times their instability.
Resistor ageing of 20ppm/yr. will add 0.7ppm/yr maximum.

Only the hermetically sealed, oil filled types (e.g. VHP202Z) give a big advantage. Their rate is typically 2ppm/6yrs., and therefore will add about 0.02ppm/yr only.
In picture 1 you’ll find long-term stability monitoring of 3 EA of my 5 VHP202Z. After 2 years, they really remain within < 0.5ppm of their initial value, so that is obviously no fake advertisement.
(Remark: The measurement stability was improved also during that time.)
 

Second, the temperature coefficient can be calculated in the same manner.

As the LTZ1000 chip is heated and thermally isolated, there is no further mechanism that would directly change its output voltage by change of ambient temperature.

The resistors will contribute with 0.035 times their own T.C.

Thin film resistors of 10ppm/k would give 0.35ppm/k, wire wound and metal foil have 2..5ppm/K max. T.C., which will yield 0.07... 0.18 ppm/K.

The metal foil resistors have no appreciable advantage over the wire wound types. Even the hermetical sealed VHP202Z, advertised as 2nd level standard and with typical T.C. of < 0.05 ppm/k will not give better results, because this extraordinary low value will not be achieved in reality. Also, the parabolic shape of the T.C. does not exist.

Picture 2 shows a high precision T.C. measurements on one of my 5 VHP202Z.
I measured values of 0.3, 0.6, 0.7, 0.8 and 1ppm/K, which is below the max. value of 2ppm/K, but far beyond the advertised typical 0.05ppm/K.

Therefore, an improvement can be made only by measuring the individual T.C. and selecting the ones with the lowest value. If R4 and R5 are matched concerning their individual T.C.s, this would improve the overall T.C. the most.

Conclusion: Using the LTZ1000, temperature set at 45°C, and using wire wound or metal foil resistors yield around 1ppm/yr. ageing. Using hermetically sealed, oil filled resistors may improve ageing to around 0.3ppm/yr.
Both resistor technologies yield a T.C. of < 0.2ppm/K.

Rem.: Sample variation of the LTZ1000 and other effects will lead to lesser stability figures.

(1) “ The Ultra Zener ... is it a portable replacement for the Weston cell?”,
P J Spreadbury, Meas. Sci. Technol. 1 (1990)
(2) “Setting new standards for DC Voltage Maintenance Systems. A Solid State DC Reference System”, John R Pickering, Metron Designs and Paul Roberts, Wavetek

quarks:
Here is a short update, I received some more parts of my Vishay order, but still wait for the VHP101T. This feels like waiting forever.

Unfortunately (or better luckily) I am also quite busy right now, because I upgraded my lab with some gear that is planned to replace some/most of my actual calibration gear.   

@Mickle T., can you share where you found the Fluke 8508A details?
@Dr.Frank, are you planning to make a new PCB for your new ordered parts?

BTW Very interesting to see what is going on meanwhile.

@Dave + all interested, what do you think of the idea to try to design/make/build a kind of "EEVblog 10V Master Reference" based on LTZ1000 for this community?