A wide dynamic range multi-mode band-pass continuous-time delta-sigma modulator employing high-Q single-opamp resonator

Abstract

Continuous-wave (CW) Doppler signal, which utilizes pencil transducer with the center frequency of a few MHz to tens of MHz range, is suitable for measuring the high blood velocity in the ultrasound imaging system. The CW Doppler received signal consists of a strong signal from stationary tissue and the much weaker Doppler signal from moving blood. Since the Doppler signal is 40–60 dB below the strong signal, CW Doppler receiver is required to achieve a wide dynamic range (DR). The RF-to-baseband mixing process, which limits the performance of the CW Doppler receiver, can be moved from analog domain to digital domain by using a band-pass continuous-time delta-sigma modulator (BPCTDSM). Since the signal processing for correcting I/Q mismatch can be easily implemented in the digital domain, the CW Doppler receiver that uses BPCTDSM is more efficient than the conventional one. The performance of the BPCTDSM depends on the quality factor (Q) of a resonator. When Q falls below the ratio of the center frequency to the signal bandwidth, FC/FB, the performance degradation becomes significant. In CW Doppler receiver, the center frequencies are in the range of up to tens of MHz and the signal bandwidth is typically 200 kHz, which results in large FC/FB and therefore requires high Q resonator.<br/> Recently, a single-opamp resonator is widely used to decrease the power consumption. However, compared to a conventional active resonator, which uses two opamps, the single-opamp resonator is more vulnerable to the effect of finite gain-bandwidth (GBW) of an opamp that results in the degradation of Q. Using a multi-stage opamp can be used to achieve high GBW, but requires enhanced specifications for an opamp as FC/FB increases and therefore increases power consumption. Compensating the coefficients of the loop filter that restores the modified noise transfer function of the modulator due to the GBW can be utilized to alleviate to the effect of finite GBW. Additional feedback path can also compensate the loop delay caused by the finite GBW. Although these approaches can be used regardless of FC/FB, the compensated coefficients are not guaranteed to be implemented using unit resistance and unit capacitance, which is important to reduce the mismatch between the passive components. Also, an additional feedback DAC is required to implement the compensation feedback path.<br/> To alleviate the effect of finite GBW of an opamp to Q of the resonator, we propose a high Q single-opamp resonator, which compensates the degradation of Q by using a positive feedback resistor while not burdening to the design of an opamp. Also, the passive components that used in the single-opamp resonator are designed by using a unit resistance and a unit capacitance to minimize the effect of mismatch. In this thesis, a multi-mode wide DR BPCTDSM for pencil probe application is presented. Our BPCTDSM can be applied to measure the blood flow in the heart, vessel, and gingiva, which uses the center frequencies of 2 MHz, 5 MHz, and 20 MHz, respectively. By utilizing the proposed high Q single-opamp resonator, it is available to achieve wide DR despite the finite GBW of an opamp. The prototype is implemented in 180 nm CMOS technology with an active area of 0.845 mm2. The experimental results show DRs of 88.15/88.42/90.39 dB and peak signal-to-noise distortion ratios of 70.2/70.03/68.58 dB in cardiac/vascular/ gingival-modes. The power consumptions are 14.73/18.77/25.75 mW in cardiac/vascular/ gingival-modes from the supply voltage of 1.8 V.