FFT Spectrum at 0 dBFS
FFT (Fast Fourier Transform) spectrum of 1 KHz, 0 dBFS (2 Vrms unbalanced) signal. The 2nd and 3rd harmonics are below -116 dB (0.0001585 %). Note that the frequency spectrum is free from USB-transmitted noise, including 60Hz and its harmonics.
FFT Spectrum at -140 dBFS
FFT spectrum of 1 KHz, -140 dBFS signal, A-weighted. This measurement illustrates the DAC's ability to reproduce the smallest details in a 32-bit system. The -140 dBFS signal is clearly resolved above the noise floor.
Intermodulation Distortion (IMD)
IMD 19K+20KHz @ 0 dBFS (2 Vrms unbalanced). The two tones at the upper limit of the audible range. The 2nd order difference product falls at 1 KHz and is around -132dB (0.0000251%). The 3rd order IMD products produce symmetrical sidebands around 19K/20KHz and are about -120dB.
Jitter
FFT spectrum of 11.025 KHz signal sampled at 44.1KHz, 1024K points averaged 4 times. This signal exposes periodic jitter, which would be observed as sidebands symmetrically located around the signal. The measured spectrum is clean, with all the symmetrical artefacts around -150 dB. There are no visible distinct traces of periodic jitter, visible above the noise floor.
All measurements are produced with the Audio Precision APx525 system. The exaU2I USB to I2S interface was connected to a reference design ES9018 DAC. The line outputs of the DAC were terminated with 100K resistor. We used long 1024K points FFTs averaged 4 times, with the AP-equiripple window.