A low noise single-ended to differential linear charge sampling SC-VGA for second harmonic cardiac ultrasound imaging

A low noise single-ended to differential linear switched capacitor variable gain amplifier (SC-VGA) is designed in a 0.18μm CMOS technology for 4MHz center frequency (fs) 26-MHz ultrasound imaging. To fit the higher source impedance from gradually scaled piezo-electric transducers (PZT) in ultrasound imaging systems, a charge sampling amplifier with a fixed integration time as the first stage exhibits the lower noise, and higher sensitivity compared to the conventional voltage sampling amplifiers. The second voltage sampling stage converts the single-ended input to differential outputs with an exponential gain control which exploits an 8b binary capacitor (CAP) array, and the gain varies dB-in-linear from -14dB to 14dB. To reduce the capacitance spread for a binary-weighted 8b CAP array, the array is segmented between the upper 4b and lower 4b by a divider capacitor. Simulation results show the analog part of proposed amplifier consumes 1.25mA at 1.8V, has HD2 -62dB, HD3 -79dB at 150mV output Vpp, and the input referred noise (IRN) is 6.56pA/√Hz at 4MHz and 25.3nArms at a sampling frequency (fs) of 30MHz. The layout size is 310μm×370μm.