Modeling the pulse-echo response of a 2D CMUT array element

Motivation/Background: Front-end integrated circuit design for 2D ultrasonic array elements is challenging due to reduced element sensitivity. The pulse driver and return echo amplifier circuits can be optimized by the use of a precise circuit model of the transducer array at the design stage. An equivalent circuit that models transmit and receive operations of the array enables the matching of the IC to the transducer. This helps the designer to optimize the front-end IC before fabrication. Statement of the Contribution/Methods: In this paper, we present an equivalent circuit model for a 2D CMUT array element to asses the overall pulse-echo behavior. The circuit is a modified version of the conventional model in which the radiation term in the Mason equivalent circuit has been replaced by an RLC network to model the finite size of the transducer. The new circuit includes (i) an RC network to model the frequency dependent diffraction losses and attenuation in the transmitted and reflected acoustic waves, (ii) transmission lines for propagation delays, (iii) a VCVS to model the reflection from the imaging surface (which is the oil-air interface in our experiments). Component values of this model circuit were calculated using analytic expressions for the electro mechanic transformer ratio of CMUTs, based on the work in [1] and [2]. Pulse-echo measurements were carried out using a front-end IC designed in AustriaMicroSystems H35 high-voltage CMOS technology. An array element of 14×14 CMUT elements of 20 µm radius and 1 µm membrane thickness was wire-bonded to the IC. The resulting pulse and pulse echo response responses were then compared to the simulation results based on the developed model. Results: The equivalent circuit model was verified by running simulations on Cadence Spectre. The post-layout extracted netlist of the IC is combined with the proposed equivalent circuit that models the wire bonded array elements. The elements were fired with a unipolar pulse of 20 Volts and the signal at the amplifier output was than compared to the experimental data. Figure 1 shows a comparison of the experimental data and simulation result. Discussion and Conclusions: We demonstrated that the information obtained from the pulse-echo model is consistent with experimental results and can be used for further IC designs to enhance overall system performance. [1] Yaralioglu G. G., et al, “Finite-Element Analysis of Capacitive Micromachined Ultrasonic Transducers”, Tran UFFC. Vol. 52, No: 12, pp.2185-2198, December 2005. [2] Wygant, I. O., Kupnik, M., “Analytical Calculation Membrane Displacement and the Equivalent Circuit Model of a Circular CMUT Cell”, Proc. IEEE Intl. Ultrasonics Symp. 2008.