We are doing 5.000V, 4.096V, 3.000V, 2.500V and 2.048V, and have stock of each, although most of production was the 5.000V version. We'll list the other variants soon on the website.
The situation with EU sales - we are happy to ship to anywhere in the world! Also happy to talk with anyone who wants to distribute our products in the EU - BREXIT is not making things easier would be an understatement.
With a low-cost product like the REF50X the extra import duty management fees are a disproportionate extra cost. Not alot we can do about that, and we wouldn't be able to factor in a distributor margin into the REF50X price to support a dealer (so they would have our blessing to sell it at EUR27+their margin) - likely end up costing the same as if we ship it direct... It's on eBay and our website, and we make a little bit more if you buy direct but we are happy to sell via both.
The REF50X calibration is fairly horrific as it is semi-automated at the moment and represents a good chunk of the cost. We've found calibrating at 8C through to 32C, with upto 10 points seems to keep it within a +/- 4ppm band. Our acceptance criteria for release is +/-10ppm. Pictures from the EPIC REF50X sw during calibration.
Here's some of how it works - the processor is a PIC12F1572, and the code was written in assembler. There are 128 locations at the top of program memory that have HEF (high-endurance flash), these are treated as 64x 16 bit memories. The PIC ADC is 10 bit and is fed from a differential amplifier that amplifies/offsets the reference IC temperature sensor - some maths takes these values, uses the table (64 entries) and calulates the required PWM.
PWM is 1200Hz, which was a chosen as it's a multiple of 50 and 60Hz. The PWM drives an analog switch that toggles between 0V and a low-pass filtered output from the reference IC, the output of the analog switch is then low pass filtered, buffered and low pass filtered again before being fed to the TRIM on the reference. The range of adjustment on the TRIM can be quite extreme, from 4.9 to 5.3V, so the amount of movement in the PWM is pretty small over the compensated range.
Coating the reference IC in silicone conformal coating reduces humidity effects somewhat and we age the reference for at least 4 days at an elevated temperature. We'd need to sell a few thousand of these to pay back development costs, but the project has been a useful path-finder for potential future products.
We will publish the communications protocol at somepoint, and the sw is included in the next update to EPIC (Electron Plus Instrument Control) - it's under Instrument>Change Instrument on 21.007 onwards. The 6 pin-header (TX/RX/LED/+V/0V/0V) will probably grow to 10 pins on the next revision for various reasons.
*Just noticed pix rotated - pix from my phone the other day..