Time-Shared DAC in SAR ADC for Tx Beamforming of Ultrasound Application

Abstract

A new circuit configuration of a Tx beamforming of 3D ultrasound probes is described. In the 3D ultrasound probe, Tx and Rx operate in a time division manner. Therefore, Tx and Rx do not operate simultaneously at the transducer. Although Tx has a quiet, shorter operation time compared with that of Rx operation, fully beamforming circuits, a DAC, a buffer, a delay controller circuit, and a transmission amplifier are required. In this paper, a new circuit configuration that enables beamforming without adding circuits in Tx mode is achieved by applying the DAC and the buffer in a successive approximation register (SAR) ADC used in Rx beamforming for signal generation during Tx mode. That is, the DAC is time-shared by Tx and Rx beamforming. The buffer in the SAR ADC when used as a part of ADC is necessary to operate at high speed. On the other hand, in the Tx mode, the buffer requires suppressing the second order distortion rather than the speed to get high quality image. Therefore, a switching method was adopted that switch between source follower that can operate at high speed and super source follower with reduced distortion with Tx and Rx. This circuit design was designed using a 65-nm CMOS process, and a Tx DAC was constructed using a 12-bit SAR ADC. Simulation results show that the second harmonic distortion of 47.8 dB was realized at 5 MHz with 1.1 mW / CH.