1 Introduction

One of the most demanding clinical diagnostic tools employed in ultrasound systems is continuous-wave Doppler (CWD). Unfortunately, designing CWD system is not easy since the received CWD signal is a combination of strong signals from body tissue, commonly referred to as clutter and the much weaker Doppler signals from moving blood. The typical clutter signals on the input of a receive channel can be as large as 200mVpp, whereas the reflected Doppler signal is weak and has a small signal amplitude in uV to mV range. To receive CWD signals, a mixer is needed to demodulate CWD signals first and then demodulated CWD signals is summed from each received channel to enhance SNR. TI’s AFE has integrated high performance mixer and summer in the chip. However, in actually design, there potential has 4 approximately 16 PCS AFE in a typical ultrasonic equipment. The designer has to summing the CWD signals external of AFE. So, designer has to design a summing circuit with low noise and high bandwidth. For simpler summing circuit, the full differential amplifier prefers to be used in here since the output CWD signals from AFE is a full differential signal. This is not straightforward for most designers. Quantization noise of ADC is another factor potential impact system performance since the weaker Doppler signals. With this said, this is challenged for a designer. This application note proposes four designs with different performance, size, and cost to meet customers miscellaneous requirements.