Using Negative-R Assisted Integrators in Wide-band Delta-Sigma Modulators

Active RC-Integrators are one of the main building blocks of continuous-time Delta-Sigma-Modulators (CTDSMs). The DC gain and gain-bandwidth product (GBW) of their Operational Ampifiers (OAs) play an important role in the modulator stability and performance in terms of signal-to-quantization-noise ratio (SQNR) as well as spurious-free dynamic range (SFDR). Negative-R assisted integrators have been introduced to reduce the CTDSMs sensitivity to OA non-idealities, where the negative-R has shown linearity improvements in low-power CTDSMs for relatively narrow bandwidth applications. However, the negative-R concept has not been widely applied for wideband applications. Wide-band CTDSMs in particular are more sensitive to the non-idealities of the OA GBW and especially in the associated phase-shift compared to their narrow-band counterparts. This paper provides insight into the effect of negative-R assistance on the GBW of the OA, where it is shown, that the phase-shift of the integrator due to finite GBW is not reduced by the negative-R assistance, and still remains an issue to be considered in the design of a wide-band CTDSMs.