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Part selection, schematics, PCB assembly and start of long term testing

A project log for voltage reference assembly

Precision ovenized voltage reference assembly

LucasLucas 10/22/2021 at 03:210 Comments

I chose the MAX6226 for this voltage reference specifically for its low noise performance. My options for references in the performance range I wanted were the LM399, LTZ1000, or MAX6226. I didn't want the expense and difficulty of using an LTZ1000, and the MAX6226 is lower noise than the LM399. However, the LM399 has a much better temperature coefficient owning to its internal oven. I decided to see if I could match the temperature stability by ovenizing the MAX6226 myself.

Here is the schematic. It includes the reference, a buffer, and a closed loop heater control for the reference.

To thermally couple the reference, temperature sensor, and heater I stacked them on top of each other. 

The reference is the purple ceramic package on the bottom, the heater is the SMD resistor in the middle, and the TO-92 package is the temperature sensor. This was a pain in the ass to solder, and I will definitely just use an LM399 in the future.

I 3d printed a cover which both thermally insulates the reference and presses the whole assembly together for better thermal coupling. I didn't have thermal compound on hand and did not feel like waiting to buy some. I stuffed some foam into the cover for insulation and to make contact with the sandwich.

After lots of wrangling and troubleshooting, I finally got the board to power up and stabilize in temperature.

Here is the long term test setup. I have a raspberry pi that logs the voltage from the 6.5 digit DMM, the oven temperature, and ambient temperature. All data is uploaded to an webpage for remote viewing. Don't @ me about poor fixturing, I know it is very bad. Solutions are welcome though.

Some problems I already know about:

That's all for now, see you guys in 1000 hours when it is done burning in.

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