Analog output will output the measured velocities on a linear scale usually from 0-5V. The outputted voltage must manually be converted to velocities. More information about Analog Inputs can be found in the Comprehensive Manual.
To enable this option in the instrument software go to Configuration > Analog output and click the checkbox. "Full range" must be selected according to the flows you are expecting.
Screendump from the instrument specific software
The velocity is given by:
(vel_range/V_range)*V_out - vel_min = velocity
Where vel_range = full range of velocity, V_range = usually 0V to 5V, V_out = voltage outputted, vel_min = minimum velocity.
In the screenshot above and a measured V_out of 1V the velocity equation would be: (2/5V)*1V-1m/s= -0.6m/s
Alternatively set up two equations (2 unknowns, slope and intercept) using mx + b =y, where x is voltage and y is velocity. Knowing voltage and velocity in two scenarios will give m and b (use max and min velocity and voltage). This gives you the full linear fit.
At zero flow the outputted voltage should display 2.5V. If not, there may be issues with grounding.