This thesis discusses the architecture, circuit implementation and simulation results of an amplitude-tunable bipolar high voltage (HV) pulser for ultrasound imaging. This design is capable of providing a relatively high-resolution amplitude modulation and allows bipolar pulsing under a unipolar supply with a small die size.
The pulser design starts from the driving circuit of the HV output stage. A driving circuit based on a current mirror structure is implemented to drive the ultrasound transducer element with different slewing currents. These currents levels are generated by6-bit current DACs controlled by a calibration loop. This calibration loop senses the pulse output voltage through a capacitive divider. The resulting attenuated voltage is compared with a reference voltage by a low-voltage (LV) dynamic comparator. Based on the comparison result, the input code of the current DACsare adjusted by a successive approximation register (SAR).
Several techniques are proposed to flexibly change the pulse amplitude and reduce the die size. First, the pulser is driven in a current mode which allows the transistors at the output stage to operate in the saturation region, so that the currents through the transducer elements always follow the input codes. Second, a separate calibration phase is applied at the beginning of the transmitting and receiving cycles, which will generate flexible and accurate input currents for the corresponding transmission phase.
The pulser has been implemented in TSMC180nm HV BCD technology. The simulation results show that the prototype is able to transmit HV pulses with a4-bit amplitude modulation with 0.5V inaccuracy. The transmission power consumes less than 0.2mW per channel when driving a transducer with a capacitance of18.29pFat a pulse repetition frequency (PRF) of4kHz. The estimated die size of the pulse is around 0.04mm2.