dc offset in cmos mixer
In practice, DC offsets cannot be reduced to sufficiently low levels by improved circuit design without any compensation. Therefore, circuits that remove the DC offsets have to be used. DC offset removal is not necessary in the analog domain if the baseband gain is low and the dynamic range in the analog-to-digital conversion is sufficient to tolerate the DC offsets, which can be orders of magnitude larger than the desired signal. Then, the DC offsets can be removed in the digital domain.
The component mismatches and the LO self-mixing produce constant DC offsets. In cellular systems having a continuous reception, the constant DC offsets can be filtered out using capacitive coupling in the signal path or a low-frequency DC feedback loop, i.e. servo. The feedback is a first-order lowpass filter or an integrator. The feedback may be in the analog domain or partly in the digital. Both DC offset removal schemes form a highpass filter in the signal path. In spectrally efficient modulation schemes used in modern cellular systems, the maximum of the baseband signal spectrum is at DC. Highpass filtering removes part of the signal and causes inter-symbol-interference (ISI) to the signal. The –3-dB frequency of the highpass filter must be small compared to the signal bandwidth to avoid significant signal degradation. Because of the low –3-dB frequencies, large silicon areas are required to implement these time constants on-chip. The highpass filters suffer from long settling times. If the baseband gain is limited to such a low value that the DC offsets cannot saturate the ADCs or ΔΣ modulators, the DC offset can be removed in the digital domain. In addition, the constant DC offset can be measured and removed from the input signal before the ADC, which improves the dynamic range of the back-end of the analog baseband circuit and ADCs. In burst-mode operated systems, like GSM, the DC offsets can be measured during idle modes. The stored DC offset can be removed from the signal during the receiver burst since the DC offsets, because of mismatches and LO self-mixing, in practice remain constant during a single burst. During a receiver burst there is no highpass filter degrading the signal quality. However, the dynamic range in the back-end of the receiver has to be sufficient to account for the changes in the DC offsets. Slowly changing DC offsets can be mitigated using highpass filters if the rate of change is low enough.