Hi @ja-tech, the dBFS and dBV outputs assumes single-ended output. So, if you set the output for -5.88 dBFS (0 dBV) then you’d measure 1Vrms (single ended) or 2V rms (diff)

The primary purpose of the dBFS button is two fold: If you are intimately aware of the ADC/DAC characteristics of the QA401, then it helps to know how far away from full scale you are operating. The second is so that when you are using mirror mode to drive a DAC, you can know the level being generated on the PC relative to full scale.

For the dBFS RMS versus peak question, the answer is RMS just like dBV. The internal maximum is +/- 1 (normalized) and then RMS is computed and scaled. You can have signals greater than 0 dBFS if their crest factor is is greater than the crest factor of a sine. If you pump in a square wave, your RMS will be +3 dBFS. At first blush this seems impossible because 0 dBFS is the max. But the common RMS relationship to peak (sqrt(2)) only applies for sines.

Finally, on the display, you PEAK L or PEAK R you see (3rd column) on the display can be confusing initially. This is showing the RMS of the signal peak. That is, the RMS value of the “strongest” frequency. The RMS field in the second column will give you the RMS of the entire signal (over the specified bandwidth). Consider the plot below, with GEN1 at 1 kHz 0 dBV, and GEN2 at 2 kHz -3 dBV. The PEAK is shown as 0 dBV (the 1 kHz) and the RMS is shown as 1.7 dB (the sum of 0 dBV and -3)

In short, everything is RMS.

Hopefully this didn’t muddy the waters!