An AC Coupling Ultrasound Analog Front-End Architecture With a Three-Stage DCOC

This paper presents an AC coupling ultrasound analog front-end (AFE) architecture with a three-stage DC offset correction (DCOC) circuit. In ultrasound systems, the low noise amplifier (LNA), time gain control (TGC), and low pass filter (LPF) constitute the AFE, which achieves low noise, time-varying gain compensation, and filtering for the received ultrasound signal. The inherent asymmetry in LNA, layout asymmetry and the process variation introduce DC offset and the TGC changes it into low-frequency offset drift. The proposed DCOC circuit for LNA is composed of a transconductance amplifier and an off-chip capacitor, while a fully differential operational amplifier and a pseudo resistor are used for other amplification stages. The AC coupling scheme is also used to reduce the offset and drift. The simulation result shows when the DCOC and the AC coupling are adopted, the offset and drift are almost perfectly suppressed. The proposed AFE has been fabricated by a 28-nm CMOS process, and it achieves an 85 dB gain range with low input-referred noise of 2.43 ${\rm{nV/}}\sqrt {\text{Hz}} $ at 5 MHz, and it also has a tunable bandwidth of 15/30 MHz and switchable input impedance of 50/100 ohms.