Thermal mechanical noise based characterization of CMUTs using monolithically integrated low noise receiver electronics

Monolithic integration of CMUTs and CMOS electronics minimizes interconnect parasitics which is essential for low noise receiver design. Ideally the system noise should be dominated by the CMUT thermal-mechanical noise, which is shaped by CMUT electrical impedance. In addition to improved signal to noise ratio (SNR), a thermal mechanical noise limited detection system can be helpful to reveal information on CMUT characteristics. A transimpedance amplifier is custom designed in 0.35μm CMOS to be monolithically integrated with a forward looking volumetric imaging CMUT array element. TIA design is optimized to minimize noise taking advantage of the minimized parasitics provided by CMUT-on-CMOS approach. Output noise spectrum of the TIA connected to the CMUT element in air with and without DC bias clearly shows that the noise spectrum has a peak at the CMUT resonance frequency when a DC bias is applied. This demonstrates that noise measurements of CMUT elements in air can be used to characterize uniformity and functionality of array elements without the need for any external electrical or acoustic inputs. Transducer noise dominated detection in immersion is demonstrated through output noise spectrum measurement of the integrated system in water tank. A noise figure of 1.76dB is obtained in the CMUT bandwidth. In addition, the relation between the CMUT impedance and noise spectrum can provide means for implementing other types of sensors using CMUTs.