A 64-fJ/Conv.-Step Continuous-Time $\Sigma \Delta$ Modulator in 40-nm CMOS Using Asynchronous SAR Quantizer and Digital $\Delta \Sigma$ Truncator

A third-order single-loop continuous-time sigma-delta modulator (CTSDM) with 6-bit asynchronous successive approximation register (ASAR) quantizer and digital ΔΣ truncator for WCDMA/GSM/EDGE cellular systems is presented. The proposed ASAR-based quantizer reduces the area and power of the modulator dramatically by utilizing the digital truncation technique. By using the 6-bit ASAR quantizer, the sampling frequency is lowered, which reduces the design efforts not only in system level but also in the modulator. In addition, the ac-coupled push-pull stage is employed to improve the high-frequency driving capability of the first integrator. Sampling at 65 MHz, the modulator achieves 83.4 dB dynamic range (DR) and 80/79.6 dB peak SNR/SNDR with 1.92 MHz bandwidth in WCDMA mode. In GSM/EDGE mode, the DR is 96.2 dB. Fabricated in 40-nm CMOS, the modulator occupies 0.051 mm
²
and consumes 1.91 mW from a 1.2-V supply. A 64fJ/conv.-step figure of merit is achieved.