How to correctly calculate dBv using voltage sample points?
You're measuring the output DC offset, which is 0.8V regardless of any signal. With an oscilloscope, you would need to measure the RMS voltage amplitude of the analog output signal to get the correct voltage.
With your oscilloscope, use AC coupling, and then measure the RMS amplitude. If you can't get a suitable RMS reading, it may be easier to measure peak to peak. For a sine wave, the RMS voltage is 0.707 times the peak-peak voltage.