For measuring low voltages especially the differences between two Voltage References I build the DC Null Voltmeter from Conrad Hofman.
The Mini Metrology Lab, Part 2
This is a switchable chopper based (LTC1050) DC Amplifier with a max. Gain of 10000!
For testing i build a Voltage DIvider with an Vishay CNS471A6.
It is amazing to measure 10µV with common Multimeter in the 100mV Range.
Here Pictures from the Bench.
No Housing yet.