[time-nuts] Lead acid battery noise levels

[Gerhard Hoffmann]

Am 11.07.2013 17:50, schrieb Mike Feher:
> A long time ago, when I was concerned about a phase noise issue, I found an
> old NBS article. It was on measuring phase noise and included a schematic of
> an ultra-low noise amplifier. In that amplifier they used Mercury batteries.
> I also glanced at the referenced article, stating NiCad is the lowest noise,
> and, NiCads were available for a long time, yet they used Mercury.
A decent low noise amplifier should feature a nice power supply
rejection ratio. For example, the ADA4898 has abt. -100 dB, so the
amplifier's own power supply should not be a factor, unless it is
_really_ bad.

I have built a preamp that averages 10 pairs of ADA4898-2 and I get
abt. 200 pV/sqrt Hz, 20/40/60/80 dB gain (plus optional 6 dB boost to
make up for 50R-terminations), 0.1 / 10 Hz to 1KHz/120KHz/1 MHz.
That works for low source impedance only.


Since the design is stable now, I have made a compact layout
and will publish it. FR4 is in production currently.

It looks like this:
< 
https://picasaweb.google.com/103357048842463945642/Tronix?authuser=0&feat=directlink 
 >


The first thing to measure were my lab supplies & batteries, of course.

There must be huge differences among NiMH. Ansmann 4 * AA 2500mAh
were really bad with regard to noise while Sanyo Eneloops approached the
limit of the preamp. They seem to be good otherwise, too :-)

Unexpectedly, cheap zinc-carbon batteries were at 0.5nV/sqrt Hz,
but if I would buy them tomorrow I might get completely different 
no-name stuff.
All measurements up to now are without load.

regards, Gerhard