Should anyone want to make a white noise generator, here is
a way to do it.
We used to use this method to determine S/N in receivers and look at
bandwidth ( with things like AVC turned on ).
It seems that zeners have two modes of operation, one is a knee, which
is a soft change from conducting to not conducting.
Typically zeners below about 6.8V are knee type action.
Above this above, it tends to be avalanche breakdown. This means the voltage
will go a little above the zener voltage and will then conduct.
The voltage will snap back to the zener voltage.
It turns out that if you run a 6.8V zener with a light current, you
can have it dancing back and forth, making nice noise. It may need
a little amplification for the A/D use.
One can play with the bias current to get the maximum amplitude.
Using such noise has many purposes.
Making truly random numbers is one of them.
Another source of such a zener is the reverse bias of the emitter base
junction of a silicon transistor.
So, should one ever need noise, you now know how to find it.
Dwight