Abstract
This work reports a CMOS flash analog-to-digital converter (ADC) for capacitive micromachined ultrasonic sensors. The ultrasonic sensing element is fabricated by post-processing in a conventional 0.35-μm CMOS process. The sensing circuit of ultrasonic sensor is composed of the capacitive sensing element and a feedback capacitor in the feedback loop of a wideband Opamp. Once the sensing element receives the ultrasound, the ratio of capacitances is changed and a sensed signal is read out at the output. The measured signal bandwidth is approximately 15 MHz. The flash ADC is introduced to in the backend. To increase the linearity of the system, a bootstrapped sample-and-hold circuit is used. Discrete-time sampled signals are quantized by a comparator array, which consists of two-stage pre-amplifiers and regenerative latches. The input-referred random offset voltages of comparators are suppressed by resistive array averaging. A 6-bit 135 MS/s flash ADC is designed with INL/ DNL peaks at ±0.2 LSB and ±0.15 LSB, respectively. The digital backend is a logic-gate based 1-out-of-64 code generator with bubble correction and fat tree encoder. The power consumption is 110.3 mW at 135 MS/s with 3.3-V power supply. This ADC is fabricated in the TSMC 2P4M 0.35-μm CMOS process.