Realization of the 2nd-order NTF enhancement in a time-encoded continuous-time sigma-delta modulator using passive elements

Embedding time encoding into the quantizers, has been proven to be as a promising technique to overcome the data converter´s resolution problems in low-voltage CMOS circuits. In this paper, an NTF-enhanced time-based continuous-time sigma-delta modulator (TCSDM) with second-order noise-coupling is presented. The structure takes advantages from a combination of an asynchronous pulse width modulator (APWM) as a voltage-to-time converter (VTC) and a time-to-digital converter (TDC) as a sampler to realize the time quantization. By using a novel implementation of the noise-coupling technique, the modulator´s noise-shaping order is improved by two. The concept is elaborated for an NTF-enhanced second-order TCSDM and behavioral simulation results are presented to verify the performance. To further confirm the effectiveness of the structure, the circuit-level implementation of the modulator is provided in TSMC 90nm CMOS technology. The simulation results show that the proposed modulator achieves a dynamic range of 84 dB over a 30 MHz bandwidth while consuming less than 25 mW from a 1 V supply voltage. With the proposed time-based noise-coupling structure, both the order and the bandwidth requirements of the loop filter can be relaxed, which in turn the analog complexity of the modulator is significantly reduced.