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@ -28,10 +28,11 @@ Let's review the design features one by one:
### Humble beginnings and early learnings
This project began some time ago when I was new to the world of precision analog design and metrology. After enthusiastically reading about various designs, I made the decision to forgo further reading and instead try my hand at designing my own. I began by creating a simple and compact design to serve as a testbed for experimentation. The philosophy behind the design was straightforward: keep it simple and cost-effective.
[picture]
### Testing it
Upon testing it was found that the voltage reference could only be trimmed to be within about 40mV of the setpoint. This being more than the usual range used for voltage standard comparisons Such as the Keithley 2182A and the Keysight 34420A.
With some help in testing due to lack of available equipment at the time, a few findings were made. Testing also revealed that the temperature coefficient on the initial design was unacceptably high at 1uV/K on the 10V output range. Further there was also an issue with hysteresis being quite high, related to a fault in the circuitry.
### Better tools, better learnings
Some time ago I upgraded from the slower Solartron 7061 to an Advantest R6581T. This enabled faster readings with more resolution. When measuring the LTZ-A variant with the Advantest it was observed that very low frequency oscillations were present. Removing components and probing around showed that the root cause was a foolishly played capacitor in feedback for the heater loop.
@ -39,7 +40,17 @@ Some time ago I upgraded from the slower Solartron 7061 to an Advantest R6581T.
### Improving
In the new version, I aimed to address the issues of the previous version by integrating everything onto one PCB. This involved implementing a new trimming method to achieve a voltage closer to 10V, allowing for the use of high-end nanovoltmeters.
The concept evolved to include two high stability voltage dividers with a divider in between to finely select a voltage between the two. Initially, I considered using a digitally controllable potentiometer for this purpose, as they are typically well matched. However, I decided against this due to the additional requirement of a microcontroller to program the PCB. I wanted to avoid the use of microcontrollers and the need for firmware development and flashing. While microcontrollers could have simplified issues such as thermal conditioning and output voltage control, I preferred to keep the project free of these dependencies.
The trimming concept evolved to include two high stability voltage dividers with a divider in between to finely select a voltage between the two. Initially, I considered using a digitally controllable potentiometer for this purpose, as they are typically well matched. However, I decided against this due to the additional requirement of a microcontroller to program the PCB. I wanted to avoid the use of microcontrollers and the need for firmware development and flashing. While microcontrollers could have simplified issues such as thermal conditioning and output voltage control, I preferred to keep the project free of these dependencies.
![OutputStage](img/OutputStage.png)
Further to compensate the issues of trimming temperature coefficient a copper meander was added to this trimming array. The idea being that it now becomes possible to slightly vary the resistance of this piece of track, thus allowing you to vary the effect of the temperature coefficient copper brings with it. Effectively allowing one to finetune the temperature coefficient of the output stage.
### Evaluation of the new voltage reference
With all this new fancy gear available to me testing and validating became quite a bit easier. The first test was seeing if the trimming worked as intended, and indeed it was functional. Evaluation has shown that the most effective trimsettings were 6,7 and 8 resistors. With these two a trimstep of less than 10mV is easily achievable.
With this functional it was time to test the temperature coefficient trimming, this proved to be a little tricky to test due to the test taking quite some tine and ambient temperature not being extremely stable. Nevertheless from this chart it is possible to make out the temperature coefficient to be roughly -0.034ppm/K with some instability in the measurement device used being visible. This could have been improved with even more trimming but I wanted to move on with testing.
![TCVMK2](img/TCVMK2.png)