The graph on the left shows a fast fourier transform (FFT) of an input signal at 70MHz folding back into the first Nyquist zone of the LTC2261 14-bit 125Msps ADC. The scalloping of the noise floor and spurs in the output spectrum are caused by digital feedback, which is accentuated by the low level signal at the input where the digital outputs are switching between all ones and all zeroes. The FFT on the right shows the SFDR performance improvement using the alternate bit polarity (ABP) mode and digital output randomizer. The noise floor is flat and the highest spur has been reduced by 12dB.