LM399 Voltage Standard

Devlog #05

The design step is finished!

I fully designed the PCB. The precision section is separated from the rest, PSU is far away and everything looks nice.

Digital section

I added an STM32, it tracks uptime, manages calibration data, measures temperature, measures heater current and all 15V rails. It can be connection to a PCB via USB.

Silkscreen

I left a few text fields on the silkscreen in order to write down initial LM399 voltage and other stuff: temperature during calibration, date and Serial Number.

Devlog #04

— PCB —

I added shielding for the precision section. All that’s left to do here is to route power, but I will do that after I route the PSU.

— PSU —

I added a 3.3V regulator for an STM32. It will use 1 16V transformer and make: 15V for reference, 15V for heater, 5V and 3.3V.

— MCU —

I am planning to add an STM32G431. To it I’ll connect and EEPROM that would store some calibration data, like date and drift.

Devlog #03

I have mostly finished reference. I’ll tell you a few interesting things about it.

— Schematic —

Not much changed. I simply added footprints. But the thing I have to point out is the added filter. I decided to go with 50ohm resistor, 10uF aluminum cap and 100nF MLCC cap.

— Stackup —

I went with a 4-layer board. That allowed me to have a dedicated ground plane on second layer. It will improve the performance.

— Layout —

I tried to keep everything together, so all the parts will have similar temperature. Also the reference part will be on opposite side of the board compared to PSU.

That would be all. It looks like I will soon finish this project, but that will take me just a little more time.

Devlog #02

— Power Supply —

I made schematic for power supply.
To make 15V for the zener, I decided to go with LT3045 and 7018 for a pre-regulator. 7818 will help with noise and ripple. Also LT3045 cannot handle input voltage so 7818 steps it down first, then it is regulated to 15V.

— Transformers —

I decided to go with 2 DP-241-4-20 20V transformers. The datasheet states that it has low capacitive coupling, which is crucial for such precision standard. Higher capacitance between windings means higher common mode currents, that could potentially make measurement inaccurate. This specific type of transformer has been tested by Marco Reps in his Extreme Isolation video, with quite good result. This transformer has lower leakage that Fluke 732A. which is much better than any standard based on LM399.
It is a 20V transformer, so after rectification there will no more than 28V - voltage drop on rectifiers without a load. That’s why pre-regulator is necessary, otherwise it would fry the main LDO.
Also I used low leakage diodes as the rectifier.
The heater supply isn’t too interesting - just a 7815.
I am wondering whether I should switch to 16V transformers or not. I am not concerned about ref supply, since it draws very little current. What may be a problem is the heater supply, but i will have to do some more calculations.

Devlog #01

I am making a precision 10V Standard based on LM399.

— Reference schematic —

I managed to make a schematic for the reference, gain stage and calibration resistors. The output can be calibrated by soldering required jumpers that connect trim resistors.
I want to achieve sub 5ppm stability, so the resistors for the gain stage must be precise and stable.

— Power supply —

Now I should focus on power supply. The heater will be powered from a separate source. Another important thing about building the PSU is isolation. It has to be isolated from mains quite well so leakage currents will not mess up the measurements. Capacitance between transformer windings is a problem.