WarrenS wrote: > Bill Hawkins > > >From a theoretical standpoint, Zero Volts is the lower noise limit > > If you want to reduce the measurement noise of a system you need to do one or > more of the following: > > Lower the source impedance, by reducing the resistance of the thing you are > measuring > Lower the Bandwidth, by filtering over a longer time period > Lower the temperature, by making it colder. > > Doing even any one of these things enough will in theory let the noise > approach zero, When you do two (or all three) at once the noise will approach > zero sooner. > see "Johnson-Nyquist noise" for the details > > >From a practical standpoint, 1 nV of resolution is doable by comparing the > >difference between two voltage sources if one uses a lot of care and applies > >some form of extra filtering. > A 1nV (1e-9) is way below the noise level of any voltage standard that puts > out volts. > This means reference measurements are not limited by the noise level when > using a good but simple setup until the references gets to be in the 3e-10 > precision range. > > Modern voltage references are somewhat noisier than standard cells and batteries (at zero load current). For the adventurous, recipes for constructing stable Weston standard cells are readily available. > The answer to "What can an amateur do to get a good low noise reference for > less than, say, $500" > IS shop at the US eBay site. > > WarrenS > > Bruce
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