Stop blinking LD1 on STM32F4 Discovery
After realizing that there was very little hope beyond actually flashing the ST-link with improved code, I pulled R4, disabling the red portion of LD1 while still having the green light to confirm programming, and the issue goes away.



The noise looks dramatic, but that's about 2 uV input-referred, merely double my theoretical ideal given my analog chip set, and I haven't even bothered to account for Johnson noise on the 500K patient-protection resistor on my driven reference electrode.  Also, I haven't done any high-pass filtering yet, which will be necessary for the EMG signals I need to collect-  so any 60Hz should go away.

Some lessons learned.
1- there is an opacity to high-resolution ADC design that I had not anticipated.  Standard bench equipment is not up to the job of looking for 100 uV sags.  Certainly, start with as bullet-proof of a power supply design you can, because its a bear to debug it later.

2- Do it right, or do it again.    In retrospect, the choice to use an on-board DAC didn't make much sense.  I should have put more thought into where I chose to isolate Vusb.  It would have made much more sense to keep the Discovery on the Vusb side of things, and to use an external DAC instead of the STM32F407 on chip DAC.  Instead of isolating the UART communication, I should have isolated the SPI comms and kept ALL the analog stuff on my own board, isolated.  Not only does this make more sense, but it would have entirely avoided the issue at hand.  I fell into some funny design choices because of a need for speed, and the knowledge that this was just to be a full-functional prototype and not a final version.  I got lucky, because now I have that full-functioning prototype, but that was far from a sure thing.