Analysis of effect of feedback current variation in CT Delta-Sigma modulators with Gm-C integrators

In design of Continuous-time Delta-Sigma modulators, the feedback DAC (Digital-to-Analog Converter) is typically implemented with a switched current circuit. It is desired that these DAC current circuits provide constant current across clock cycles for each modulator output level. However, when Delta-Sigma modulators are implemented with Gm-C integrators, the DAC feedback current circuits are directly connected to the integrator outputs that may have high-swing voltages and this results in varying DAC feedback current. In this paper, we analyze the effect of feedback current variation caused by high-swing integrator output voltages in Delta-Sigma modulators built with Gm-C integrators. It is shown that feedback current variation may significantly degrade the performance of Delta-Sigma modulators mainly due to the non-linearity of current variation dependence on the integrator output voltage, while mismatch of current variation has minor effect.