Incorrect DAC AFE Output Range

[linuswck]

From #5, the DAC AFE Output Range is expected to be +-1V (Differential output) or +- 0.5V (Single-Ended output)

On actual hardware, after removing the feedback capacitor, the DAC AFE output range is 0.03+-0.3V(Single-Ended Output)
This issue may be related to #64.

[mkiepiela]

The ~30 mV offset in output voltage range is due to the type of amplifier used in the AFE.

ADA4930 is differential output amplifier optimized for driving ADC with 0.9V common mode voltage.
The output common mode voltage is set by Vocm input (in FastServo tied to GND).

The offset voltage between Vocm pin and actual output common mode voltage is specified in datasheet for Vocm = 0.9V (assuming single +5 V supply).

1. In FastServo, only one (positive) of the differential outputs is used as a single ended output reffered to ground.
   This means that the Vocm offset adds to the output signal and is a source of asymmetry in the output voltage range.

2. Since ADA4930 is powered by +/-2.5 V and the Vocm pin is tied to GND, it is 2.5 V above the negative supply voltage. This is even outside the range specified in the datasheet (0.5 - 2.3 V above the negative supply). For this reason, the observed common mode offset (effectively a shift in the output signal range) is much higher than that specified for Vocm = 0.9 V (-25 - +15 mV, typically +0.35 mV).

As an exercise, I changed the positive and negative supply voltage values to move Vocm closer to the negative supply voltage.
I then measured the output signal (Vocm offset) for the DACs set at mid-range (0x2000, expected output signal value of 0 V). Vocm offset decreased as expected.

Vs- [V]	Vs+ [V]	Vocm (for both DAC channels)
-2.5	+2.5	28-30 mV
-2	+3	~20 mV
-1.22	+3	15-18 mV
-1.22	+3.73	7-10 mV