OK, after I had learned, that power supply had a certain influence, I revised my setup, and TATAH!
Power supply to Daisy Seed Rev7 via USB coming from my laptop. (A 9V battery was not better).
Use differential amp to cut ground loop.
Essential: Take output ground directly from Analog_GND Pin!!!
2kHz 100% output => set Zoom R8 input sensivity
2kHz, 50% output => take measurement,
As Audacity does not display <-90dB, I set output to 1%=-40dB.
Comparing relative rms values 620 for 100% amplitude and 0.024 for 0% gives S/N=88dB and ENOB= 15.7bits.
Nevertheless, when I had a look at the Datasheets of LP2985, TI recommends an input voltage more than 1V over the output voltage (4.5V), and there is a diode before this chip. So if you have a weak and noisy usb supply, there might be problems?
USB connection via a USB-HUB:
If the 9V Battery at Pin 39 is used in parallel to the bad USB:
The influence of output level to distortion is:
Level= 1%: THD=0,05%
Level= 50%: THD=0,02%
Level= 80%: THD=0,03%
Level= 90%: THD=0,1%
Level=100%: THD=2,6%
Lessons learned:
Beware of ground loops! Use analog GND very closely to the PIN! If you have a bad USB supply, then an additional 9V Battery supply helps! Reduce output level to 90% or so for low distortion.