Download AN124 775 Nanovolt Noise Measurement for A Low Noise Voltage Reference (2009, Linear Technology, 12 pages, 462 KB pdf file)
In this app note Williams discusses the techniques he had to apply to measure the (extremely low) noise of a voltage reference. If you have to make precision voltage measurements too, maybe one of his clever ideas might be useful for you.
The techniques he uses include the selection of FETs so that their VGS matches within 10%, close thermal coupling between two FETs by enclosing them in epoxy, careful shielding, using J-FETs that are connected as diodes (because their leakage current is much lower than that of normal diodes), using polypropylene capacitors because of their low dielectric absorption (for more information about dielectric absorption please see Understand Capacitor Soakage to Optimize Analog Systems by Bob Pease). Another capacitor has to be selected for 5nA leakage current. The test sequence to verify this low leakage current includes a delay time of 24 hours because otherwise the effects of dielectric absorption might introduce unacceptable errors.
You may also be interested in some other Application Notes by Jim Williams.
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