Design Notes

Objective: construct a reference with sufficient stability (time and temperature) and robustness that it can be mailed to other volt-nuts list members who will record their measurements and mail the reference back to me. Goal is to conservatively obtain 10ppm stability.

Two references look pretty good - the LM199 series and the MAX6350 parts. The best LM199 has a slightly lower temperature coefficient (TC) than the best MAX6350 (0.5 ppm/C vs. 1.0 ppm/C). Actually the MAX6350C specifies 1.0 ppm and the MAX6350E specifies 1.5 ppm/C. The C is spec'd over a 70C range and the E part over a 125C range - making the E the better part.

The MAX6350 has lower low frequency noise than the LM199 (3 uVp-p vs 10 uVp-p).

The Traveling Standard circuit I have come up with is shown in the attached schematic.

Design notes using an LM119AH:

LM119AH has an output voltage of about 7 V and a TC of 0.5 ppm/C. Output impedance of the zener is about 1 ohm. Zener current Iz will be held at about 1 mA, so R13 is chosen as 8.06 K.

1 ppm of the 7 Volt output is 7 uV. With a 1 ohm zener impedance a 7 uA change in Iz is 1 ppm.

Vo =~ 7 volts
TC: 0.5ppm/C
stability: 20 ppm / 1000 hrs - need to age the TS before it is used.

Ro < 1.0 ohm
Iz =~ 1mA
1 ppm = 7 uV => dIz 7 uA.

lm7815
R13 =~ 8K
lm7815 dv/dT =~ 1mv/C => 0.125 uA/C => 0.018 ppm/C
use 24 V wall supply to feed lm7815
CUI EPS240025 - +/- 1 % line regulation, +/- 0.02 %/C TC
lm7815a line regulation =~ 13 mV/V
1% change in 24 Volt => 3.12 mV change in the 15 V => 0.37 uA => 0.06 ppm
0.02 %/C in 24 Volt => 4.8 mV/C at 24 V => 63 uV/C at 15 V => .008 uA/C => 0.0011 ppm/C

R13: Vishay CMF55 Series, 1/2 Watt, 50 ppm/C, stability < +/- 0.15%
Rs TC = 50 ppm/C => 0.05 uA/C => 0.007 ppm/C
Rs stability < 0.15% => 1.5 uA => 0.21 ppm

Require Zin of VM to be >= 10M => 0.7 uA => 0.1 ppm