The ADC3683-SP includes an internal auto-zero front end amplifier circuit which improves the 1/f flicker noise(noise near DC).
Figure 7-6 FFT at 25MSPS with Input Frequency of 3MHz (Auto-Zero Feature Enable vs Disable, 4M Point FFT)
Figure 7-8 FFT at 65MSPS with Input Frequency of 5MHz (Auto-Zero Feature Enable vs Disable, 4M Point FFT)
Figure 7-7 FFT at 2 MSPS with Input Frequency of 3MHz (Auto-Zero Feature Enable vs Disable, 4M Point FFT) zoomed in near DC
Figure 7-9 FFT at 65MSPS with Input Frequency of 5MHz (Auto-Zero Feature Enable vs Disable, 4M Point FFT) zoomed in near DC
Note: Auto-Zero enabled (plots on the left) does not affect AC performance as graph above shows. Auto-Zero enabled is comparable and slightly better than when Auto-Zero is disabled. Auto-Zero enabled (plots on the right) improves the 1/f flicker noise (noise near DC) compared to when Auto-Zero is disabled.