This paper proposes a third-order low-distortion deltasigma modulator (DSM). The third-order noise shaping is achieved by a single opamp (excluding the quantizer). In the proposed DSM structure, the timing limitation on the quantizer and dynamic element matching (DEM) logic in a conventional low-distortion structure can be relaxed from a nonoverlapping interval to half of the clock period. A cyclic analog-to-digital converter with a loading-free technique is utilized as a quantizer, which shares an opamp with the active adder. The signal transfer function (STF) is preserved as unity, which means that the integrators process only the quantization noise component. As a result, the opamp used for the integrators has lower requirements, as low-distortion DSMs, on slew rate, output swing, and power consumption. The proposed third-order DSM with a 4bit cyclic-type quantizer is implemented in a 90-nm CMOS process. Under a sampling rate of 80 MHz and oversampling ratio of 16, simulation results show that an 81.97-dB signal-to-noise and distortion ratio and an 80-dB dynamic range are achieved with 4.17-mW total power consumption. The resulting figure of merit (FOM) is 81.5 fJ/conversion-step.
A third-order low-distortion delta-sigma modulator (DSM), whose third-order noise-shaping ability is achieved by just a single opamp, is proposed. Since only one amplifier is required in the whole circuit, the designed DSM is very power efficient. To realize the adder in front of quantizer without employing the huge-power opamp, a capacitive passive adder, which is the digital-to-analog converter (DAC) array of a successive-approximation-type quantizer, is used. In addition, the feedback path timing is extended from a nonoverlapping interval for the conventional low-distortion structure to half of the clock period, so that the strict operation timing issue with regard to quantization and the dynamic element matching (DEM) logic operation can be solved. In the proposed DSM structure, the features of the unity-gain signal transfer function (STF) and finite-impulse-response (FIR) noise transfer function (NTF) are still preserved, and thus advantages such as a relaxed opamp slew rate and reduced output swing are also maintained, as with the conventional low-distortion DSM. Moreover, the memory effect in the proposed DSM is analyzed when employing the opamp sharing for integrators. The proposed third-order DSM with a 4-bit SAR ADC as the quantizer is implemented in a 90-nm CMOS process. The post-layout simulations show a 79.8-dB signal-to-noise and distortion ratio (SNDR) in the 1.875-MHz signal bandwidth (OSR=16). The active area of the circuit is 0.35 mm 2 and total power consumption is 2.85 mW, resulting in a figure of merit (FOM) of 95 fJ/conversion-step.
This paper presents a comparator-based OTA first-order discrete-time low-distortion sigma-delta modulator. A split data weighted averaging (DWA) algorithm logic is proposed to release the heavy burden of digital circuit while a 6 bit DAC is implemented in this work. In addition, a comparator-based OTA is used to reduce the power consumption. On the top of that, to achieve lower power consumption, a power efficient SAR quantizer with embedded analog passive adder is proposed to eliminate additional operational amplifier for summation. The modulator core occupies an active area of 0.0275 mm 2 in TSMC 90-nm 1P9M CMOS process. Experimental results show that the proposed modulator achieves 59.90 dB SNDR with 0.58 mW power consumption under 1.0 V supply voltage, an OSR of 16 at 65 MHz sampling frequency and 500kHz input frequency.
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