A High-frequency Transimpedance Amplifier for CMOS Integrated 2D CMUT Array towards 3D Ultrasound Imaging Xiwei Huang 1, Jia Hao Cheong 2, Hyouk-Kyu Cha 3, Hongbin Yu 2, Minkyu Je 4, and Hao Yu 1* 1. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 2. Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research) 3. Dept. of Electrical Engineering and Info. Tech., Seoul National University of Science and Technology, Seoul, Korea 4. Dept. of Info. and Communication Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Korea 02-Oct-2014
Outline 1. Introduction 2. CMUT-array based Ultrasound Receiver 3. TIA Circuit Design and Implementation 4. Measurement Results 5. Conclusions
3D-UBM Introduction Glaucoma imaging by 3D ultrasound bio-microscope: High-frequency (>30MHz) high-resolution CMOS readout with integrated CMUT array CMUT+ Analog front-end IC and supporting electronics Targets 3-D Imaging High-frequency 2-D Transducer Array High Bandwidth AFE Transmitting Receiving
Ultrasound Imaging System Analog Front-End Transmitted acoustic wave Received reflected acoustic wave CMUT Transducer array HV Tx/Rx Switch HV pulser Preamp Digital Signal Processing and Control ADC Image Processing/Display TGC (VGA) LPF Key components: CMUT array + analog-front-end (AFE)
Capacitive Micromachined Ultrasonic Transducer CMUT Device A transducer that converts ultrasound acoustic waves into electrical signals and vice versa The energy transduction is due to capacitance change between membrane and substrate Easier CMOS integration with wider bandwidth R=4.785kΩ L=31μH i C=44pF C=1.6aF Equivalent simulation model for CMUT Table I. Design Parameters for in-house fabricated CMUT Parameter Values CMUT array (elements) 16 16 CMUT cells per element 20 20 Width 28μm CMUT cell Depth 28μm geometrical profile Thickness 3μm Gap size 0.1μm CMUT element dimension 600μm 600μm CMUT excitation voltage (V P-P ) 20V Bandwidth 17.5-52.5MHz Capacitance variation 2.12aF/Pa Capacitance per element (deflated) 44pF (d) (a) Diagram of CMUT array, (b) one CMUT element, (c) one CMUT cell, (d) cross-section view of CMUT cell, (e) top view of CMUT cells. (e) Trench Connection
CMUT-array based AFE Receiver CMUT1_EN Pulser1 (Transmitter) CMUT1 R f OUT_EN R B V Bias C B TIA CMUT2_EN Pulser2 (Transmitter) CMUT2 C parasitic R B V Bias C B 1. One preamplifier shared by two AFE channels considering bonding area constraint for 600μm 600μm CMUT element 2. Additional parasitic capacitance of 1pF included in simulation considering bonding for CMUT element and preamplifier 3. HV protection switch using HV double-diffused lateral MOS (DMOS) transistor to isolate preamplifier and avoid possible breakdown in transmission mode
AFE Preamplifier Circuit Specifications Preamplifier: trans-impedance amplifier (TIA) with specs by CMUT device and system dynamic range Table. Design specs for preamplifier Parameters Supply Voltage Gain Specs. 6V 61.18dbΩ Receiving Bandwidth: 100% fractional bandwidth of the CMUT center frequency 35MHz 3dB Bandwidth 52.5MHz Gain: output of the preamplifier able to produce Input Referred Noise 1.15uArms a maximum of 1V P-P voltage to the TGC in next Max Output Voltage 1V P-P stage considering the maximum CMUT Output Load 3.2pF//310KΩ capacitance variation Input referred noise: determined by the case when the minimum acoustic-wave pressure echo signal is received Output load: determined by the input impedance of the next stage TGC on PCB AFE Receiver DR Attenuation rate: -0.5 db/mhz/cm Target focal depth: 1.2 cm Input signal DR: centre frequency + focal depth (back and forth) = 35MHz*2*0.5*1.2= 42dB 256 gray-scale display DR: 20*log(256)=48dB => ADC: 6.02*10+1.76=61.96dB, TGC=90-61.96=28.2dB =90dB
AFE Preamplifier Circuit Design Resistive feedback TIA VDD Low-noise detection Ease of biasing high bandwidth capability Transimpedance Gain R f = 1.15KΩ => Gain=20*log(1.15K)= 61.2dBΩ M SW1 M P1 M P2 M P3 Ibias RX_IN_EN1 CMUT1 M N1 R f M N3 OUT_EN M SW3 3dB Bandwidth 1 ωtia, 3dB = R + IN ( C C ) CMUT Input Referred Noise parasitic RX_IN_EN2 M SW2 2 2 2 2 1 i N _ in _ total = in _ amp + ir + vn _ amp + ωc f Rin _ amp M N2 M N4 CMUT2 GND Resistive feedback TIA schematic in + 1 R f 2
AFE Operation Principle Basic timing diagram for ultrasound analog front-end (AFE)
AFE Implementation and Measurement 400μm 250μm CMUT Array TIA testing chip photo TIA testing PCB photo 1. Tapeout Process: Global Foundry 0.18-μm Bipolar/CMOS/DMOS (BCD) 2. A unity gain analog buffer is included on chip for driving external load of the probe with over 280MHz bandwidth 3. CMUT array wire boned on PCB within a barrel glued on the PCB (QFN24 package) 4. External power supply of 6V and 80μA input bias current
AFE Preamplifier AC + Noise Measurement Results Simulated closed-loop frequency response AC simulation vs. measurement Results Parameters Simulation Measurement Transimpedance Gain 61.18dBΩ 61dBΩ -3dB Bandwidth 75MHz 100MHz Input Referred Noise 16.8pA/ Hz 27.5pA/ Hz Input referred noise simulation result Input referred noise measurement result
AFE Acoustic Measurement Setup 1. Immerse CMUT array in the vegetable oil contained in the barrel to mimic the underwater testing environment 2. Choose one CMUT element from the CMUT array for transmitting and provided it with 20V DC bias voltage 3. Choose one other CMUT element for receiving the acoustic wave resulting from the reflection at the oil-air layer interface 4. A hydrophone was immersed into the oil to measure the acoustic pressure as a reference to the TIA output voltage signal
AFE Preamplifier Acoustic Measurement Results (a) CMUT transmitted acoustic pulse signal captured by hydrophone (b) TIA received echo signals from CMUT. 1. The delay of the received echo can show the pulse-echo distance, which is the depth of the oil inside the barrel 2. Our in-house fabricated CMUT device successfully generated a 6mV acoustic pulse with the triggering from external pulser 3. The peak-to-peak voltage of our first echo signal was about 7mV, which also successfully demonstrated the functionality of the developed TIA of the analog-front-end receiver
Conclusions A CMOS analog front-end (AFE) receiver integrated with CMUT array is demonstrated (0.18-µm BCD process) for high frequency 3D ultrasound imaging The primary component, a transimpedance amplifier (TIA), achieves 61dBΩ gain with 17.5MHz to 100MHz bandwidth, and low input referred noise of 27.5pA/ Hz The TIA was successfully integrated with CMUT and the receiving functionality has been demonstrated with a pulseecho acoustic testing Our future work is to demonstrate the whole 3D ultrasound imaging system with digital image processing
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