A 2pA/√Hz Current-Conveyor-Assisted Ultrasound Receiver with 25pF CMUT Parasitic Capacitance

Ultrasound (US) imaging has emerged as a promising solution for medical diagnosis with its low cost and biocompatibility. Recent demands for multi-dimensional imaging in medical diagnosis brought a challenge of operating multiple US transducers, especially within miniaturized probes. There have been some studies of US systems for the application of intracardiac echocardiography (ICE) [1] and transesophageal echocardiography (TEE) [2] that require highly integrated systems within miniaturized probes. The miniaturization led the capacitive micromachined ultrasonic transducer (CMUT) to be a desirable choice for its compatibility with CMOS and wide bandwidth (BW) [3]. One main characteristic of the CMUT is that it generates current signals, making a transimpedance amplifier (TIA) preferable to the low noise amplifier (LNA). However, the parasitic capacitance
$(\mathrm{C}_{P})$
at the input of the TIA highly affects the performance due to the nature of converting the input-referred voltage noise at the virtual ground to the current noise by its impedance. Therefore, power consumption should be increased with a large Cp to maintain low noise. Moreover, the input multiplexer (MUX) is often used for the US imaging system to reduce overall power consumption [4]. The input switches are implemented using bulky high-voltage MOS transistors to protect the low-voltage devices of the receiver (RX) circuit from the high-voltage pulse of the transmitter (TX) circuit. It also attributes additional Cp at the input. Note that using individual transducers for TX and RX requires a larger area. Hence, the TIA should be carefully optimized for CMUT-based US imaging systems. In this work, we present an analog front-end (AFE) assisted by a current conveyor (CC) to improve noise performance while maintaining the power consumption. The CC buffers the input current and isolates the Cp of the input from the high transimpedance gain stage, relieving the overall AFE burden.