A low noise single-end to differential switched-capacitor VGA for PZT-Xducer ultrasound imaging

A low noise single-ended to differential linear two-stage switched capacitor variable gain amplifier (SC-VGA) is designed in a 0.18μm CMOS technology for 4MHz center frequency (f_c) ultrasound imaging. To simplify the clock generator and improve linearity, the voltage sampling technique is adopted to replace the charge sampling for sake of the source impedance of piezo-electric transducers (PZT) not being as high as capacitive micro-machined ultrasonic transducers (CMUT) in ultrasound imaging systems. The two-stage VGA based on a single-stage OTA in each stage to save the power, and controlled by the 10-bit digital signals, has the maximum dB-in-linear gain varied from -14dB to 32dB. The first stage converts the single-ended input to differential outputs with a 2-bit 6dB/step coarse gain control varying from 0dB to 18dB, and the second stage has a fine gain control which exploits an 8b binary capacitor (CAP) array varying from -14dB to 14dB. For reducing the capacitance spread for a binary-weighted 8b (1:256) CAP array, the array is divided between the upper 4b and lower 4b by a divider capacitor. Simulation results show the core analog part of two-stage VGA consumes 900μA at 1.8V, has HD2 -61dB, HD3 -77dB at 190mV output Vpp, and the input referred noise (IRN) is √ at 4MHz at the maximum gain and a sampling frequency (f_s) of 30MHz. The layout size is 387μm×502μm.