Download Fully Integrated TX/RX HV ASIC design for CMUT

Fully Integrated TX/RX HV ASIC design for CMUT ultrasound application
Kangqiao Zhao, Peng Wang, Surya Sharma, Rune Kaald, and Trond Ytterdal
Department of Electronics and Telecommunications, NTNU
Ultrasound imaging is a non-invasive technique for medical diagnosis, which becomes a common practice for
surgeries, and is used more and more for medical investigation purposes. It demands for a highly sensitive,
accurate, and portable mechanical sensor system. A capacitive micro-machined ultrasonic transducer (CMUT) is
a strong candidate for a MEMS resonant sensor system with several key advantages [1]. The existing
piezoelectric transducer fabrication technology relies on meticulous and labor-intensive steps such as hand
lapping, polishing, and high-precision dicing. However, the CMUT technology takes advantage of mature silicon
integrated circuit (IC) fabrication techniques, which makes CMUTs to gain more and more attention from both
scientific and industrial fields as they can be integrated with CMOS front end electronics [2], which greatly
reduces the cost of ultrasound imaging systems.
The main challenge for CMUT is the high impedance due to the small size. Circuitry for impedance
transformation is then required to drive the cable that connects the CMUT frontend and the backend system. To
transmit the optimal energy to the object from CMUT surface, the CMUTs have to be biased as introduced in [3].
Here, we discuss important challenges in the design of a TX/RX HV ASIC for interfacing with the CMUT array.
The design was performed in a state-of-the-art 0.18um CMOS technology with 50V support. Important
challenges include:
1. Provide very low input impedance to accept current input signal and very low output impedance to
drive the voltage output signal across the cable and into the backend system with as small as possible
noise factor.
2. Interface low voltage (LV) circuitry, i.e. Rx amplifier, with HV bias and transmission pulse signal.
3. HV circuitry using HV transistors with gate—source voltage limit of only half the required signal swing.
a. Pass/stop the transmitted signal from backend to center and/or edge transducers (XD)
b. HV logic control and HV level shift.
As shown in the figure, Rx amplifier resides in the 30V deep n-well to interface with reflected signal from
transducers biased at 30V; Back to back HV transistors are used for isolation switch (‘HVsw’ in the figure); and
switched complementary source follower is adopted for the Tx buffer design to have flexibility in Tx voltage
swing and pass/stop control without violate the operation condition for the process.
ASIC
Tx_e
Edge XD
0V
75(match) or larger (power saving)
HVsw
Vctrl_e
30(+/-)20V
30V
Tx_c
(+/-) 20V(Tx).1V(Rx)
Center XD
0V
HVsw
Vctrl_c
75
47n
HVsw
HV Pulser
100k
Rx
31(+/-)1V
30V
47n
Diode Bridge and AA
filter and so on
References
[1] K. K. Park, H. Lee, M. Kupnik, Ö. Oralkan, J.-P. Ramseyer, H. P. Langc, M. Hegner, C. Gerber, B. T. Khuri-Yakub, "Capacitive
micromachined ultrasonic transducer (CMUT) as a chemical sensor for DMMP detection," Sensors and Actuators B: Chemical, vol. 20, no. 1,
pp. 1120-1127 , Dec. 2011.
[2] Nobel R. A., et al., "Low temperature micromachined CMUTs with fully-integrated analogue front-end electronics," IEEE Ultrasound
Symposium, pp. 1045-1050, 2002.
[3] I. Ladabaum, B.T. Khuri-Yakub, “The microfabrication of capacitive ultrasonic transducers,” Journal of Microelectromechanical Systems
vol.7 (3) p. 295-302, 1998.