Thanks to Harvs and Andreas for this optic stuff. I didn't even know it was used outside science and big scale communication systems. But it has made me think. If I follow the pursuit of noise reduction to the bitter end, I will have no time for other things. (There is imo no end - only the frustrating work that yields smaller and smaller improvements the more you have succeeded until now. I think economists call it
diminishing returns).
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I read that it is impossible to get rid of the USB ground connection without a USB isolator. If this is correct and I understand it correctly I want to try these two things:
- use optocouplers with the two-wire Rx/Tx of RS232. If the ADC+uC is battery powered and have good ground design (star ground as in the LT schematic in a post over), and the 4-wire USB cable stops at a USB-to-RS232 breakout board at the Arduino, then one can galvanically isolate the ADC+uC from the PC by using 2 optocouplers (Rx/Tx).
- buy or make a USB Isolator based on ADuM3160 or similar. This will be a more general thing that I also can use for other measurements for example measuring 1 V differential at common mode 50 V DC with an op amp, collect the 1 V measurement on analog I/O on the Arduino, and then transfer the 1 V measurement from the Arduino that now has floating ground 50 V DC to the PC via USB cable.
Alternatively - since I very seldom need any precision measurements, I could take away the USB cable completely. When I want to collect measurements that will be sent to PC, I can store them in a software declared array, in I2C EEPROM or in SD card, and transfer them after the measurements are finished. I also have bought EEPROM chips, but never used them.
I don't know - really. But I do know that there has to be a limit somewhere. And I am happy to know that my limit is before beginning with one more technology (optic fiber) when I haven't even tested more traditional solutions.
There is so many other things I should have made in analog and mixed signals. Thanks for making me realize that I have to make priorities ...
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And I also must rethink the whole less-than-1-ppm 'philosophy' (from a pragmatical point of view 10 ppm is very good and 25 or 50 ppm seems to be realistic most of the time). I knew this from before, but he states it very clearly:
"Although the chip has 24-bits of reso-
lution, it isn’t really a 24-bit converter.
The speci?cation sheet says the typical
output noise is 1.5 uV RMS. Gaussian
random noise has a crest factor (peak
divided by RMS) of about ?ve so 1.5 x 5
x 2 = 15 av pk-pk. At 5 V full scale, the
least-signi?cant bit isjust 0.3 uV so the
converter noise effectively “uses up” the
lower ?ve hits; at best we have a 19-bit
converter, which is really quite good for
an SO-S 1C!"
Source:
www.tdl-tech.com/hires-vm.pdf?