More than one reference all together? why?

[Conrad Hoffman]

Drifting OT, but last time I built an RF noise bridge, I used the emitter-base breakdown voltage of a small transistor. That seemed to have a better noise spectrum than a zener. Might have been a 2N2222, but try several different types.

[MK]

Consequently I am working on a non metrology related project which requires an ample amount of Zener noise. I just actually bought like 20 different zeners to characterize the noise with, and pick the one with "best noise"  .

OT, but you might like this set of links:
http://www.ciphersbyritter.com/
he is going the opposite of Volt nuttery, he wants noisy zeners by running at as low a current as is practical.

[chuckb]

Other people have discussed and explained flicker noise so here is a visual to support the understand of flicker noise floor limitations in Voltage Standard measurements.

I used a K2182 Nanovoltmeter to monitor the 10V output voltage of two Fluke 732A zener references. This is similar to what you would do to calibrate a known Voltage reference to an unknown reference. For a real calibration the interconnect leads would be reversed to average the thermal EMFs. During this test the temperature was stable and I did not see any correlation between error voltage and temperature.

The raw data was sampled every 30 seconds and it has a 2.4 uVpp noise level. If that data is post processed with a 10 minute moving average 
the noise level only drops to 1.8 uVpp. With an extreme 2 hour moving average the noise is reduce to 1 uVpp. At these time levels the drift of the zeners starts to add to the noise. You can see a slight downward slope to the traces after 100 hours.

With 2 or 4 independent references you can get some noise reduction and you reduce the influence of one bad device.