On Salt, 5V is taken from the on-board (that is: on the Salt PCB) regulator. This does not change when plugged over USB (so in this sense the comment is misleading and it should be "5V supply" (without the USB).
The analog DAC on Bela has a FS output of min(5V, Vcc)
, so if the 5V rail is below 5V, this will clip the effective output range of the DAC. In order to avoid hitting the 5V rail, both on Salt and the Audio Expander Capelet, we artificially limit the maximum amplitude of the output signal.
On Salt, the DAC is followed by an inverting summer with gain of < -2.2
or something like that, plus a fixed shift from the -10V reference(IIRC). Gain was set so that writing a 0 gives about +5V, and writing a 1 gives about -5V. I need to dig up the schematics and check that.
In all fairness, it just occurred to me that maybe the unit I have here has some non-stock values, as it may have been one of the pre-prod protos. We may have allowed for a wider range on purpose on the production units. I have to check the commit history for that.
Bottom line: sorry to hear that you have a -110mV offset. That cannot be fixed in software. We are happy to take the unit back and re-calibrate it(i.e.: replace resistor from the -10V voltage reference to the inverting summer).