A log-domain μbeamformer for medical ultrasound imaging systems

A fully differential, Class-AB, log domain microbeamformer has been designed in a 60 GHz Si-Ge BiCMOS process. The demonstrated microbeamformer has four input channels and four delays, though the concept can easily be extended to any desirable configuration. The log domain, Class-AB architecture is perfect for medical ultrasound applications due to the fact that the received ultrasound signal has very low amplitude during the major part of the reception period. This leads to very low power consumption because of the Class-AB configuration. The delay-line in the microbeamformer is constructed using a cascade of low input impedance allpass filter cells. A simple implementation of the zero in the allpass filter helps to keep the overall power consumption low. The delay of each allpass filter cell is programmable through the adjustment of a tuning current. Due to the Class-AB architecture used, every source signal must be shaped by a signal preconditioning circuit before connected to the filter cells. A well-known preconditioning circuit has been modified to increase the dynamic range. The modification introduces noise cancellation as well as a method to increases the maximum signal swing. The dynamic range of one preconditioning cell is shown to increase 12.6 dB compared to the classic translinear circuit at a penalty of 15% increase in the power consumption. Signal-to-noise ratio of one allpass filter cell is typically 56.5 dB, and the global dynamic range of the same cell is typically 63.8 dB at an average power consumption of 3.5 mW when 16 input signals are connected to the filter. The power consumption at maximum signal amplitude for the microbeamformer having four input channels and four delays is 3.2 mW with a supply voltage of 2.5 V. In the intended application, the quiescent power consumption is a much better description of the average power consumption. This power consumption is 1.3 mW.