Cancellation of spurious spectral components in one-bit stimuli generator

This work presents a cancellation technique of non-linear distortion components of one-bit digital stimulus sequence which is generated in software by a SA modulator. The stimulus is stored in a cyclic memory and applied to a circuit under test through a driving buffer and a simple low-pass reconstruction filter. The distortion components originate from buffer imperfections which result in a possible asymmetry between rising and falling edges of a NRTZ waveform representing the encoded stimulus. We show that the distortion components can be cancelled by using a simple predistortion technique. In addition an on-chip DC-calibrated ADC can be used to identify the second-order nonlinear products of the driving buffer. This procedure allows for cancellation of all the second-order distortions before the actual test and it can be extended to the third order terms as well.