I will use this post to give some updates for the status of openDCM project for the last 4 months.
During this period I tried to collect information about calibration process and had contact with two National Measurement Institutes about possibility to calibrate direct DC voltage reference standard. The prices were reasonable for calibration against Josephson voltage standard (around 500 Euro for ±0.02 ppm uncertainty) and against Fluke 7010N/T (around 100 Euro for ±0.8 ppm uncertainty).
Voltage reference standards on the market costs several thousand USD (even for the second hand) and they are not affordable for me. Voltmeter calibrators costs even more. So I started digging into this area and found the excellent thread about LTZ1000 based voltage reference. Even with the most expensive Vishay HZ series resistors, stable 7 to 10V amplifying, gold plated tellurium copper banana jacks, and low EMF cables it will cost less then 500 EUR. The cost here is very important, because for the good calibration practice I have to make 3 or 4 voltage references. For the voltmeter calibration and linearity testing, I'm thinking to use a PWM voltage divider and the LTZ1000 based voltage reference.
One more thing, which I'm thinking to build is a null voltmeter detector. It is used to compare two voltage sources if they are equal. Using this tool I can transfer the hot calibrated voltage reference to other voltage references and to check drift between them.
My plan for 2017 year is to finish the low accuracy openDCM prototype and to start working on the calibration process (voltage standard, null meter, PWM voltage divider) and the high accuracy prototype based on the LTZ1000 reference and multi-slope architecture.
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