A four-channel analog front-end (AFE) transceiver chip for medical ultrasound imaging is demonstrated. The high voltage transmitter uses a 3-level pulse-shaping technique to deliver over 50% more acoustic power for the same power dissipation, compared to traditional methods. The design requires minimum off-chip components and is scalable for more channels. The receiver is implemented with a transimpedance amplifier (TIA) topology and is optimized for noise, bandwidth and power dissipation. Based on both acoustic and electrical measurements, we demonstrate the Transmitter (Tx) efficiency improvement, Tx beamformation and the pulse-echo response, revealing the system's full functionality.Semiconductor Research Corporation. Focus Center for Circuit and System Solutions (C2S2

Chandrakasan, Anantha P.

Chen, Kailiang

Sodini, Charles G.

Kailiang Chen

Anantha P. Chandrakasan

Charles G. Sodini

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Ultrasonic imaging front-end design for CMUT: A 3-level 30Vpp pulse-shaping pulser with improved efficiency and a noise-optimized receiver

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Ultrasonic Imaging Front-End Design for CMUT: A3-Level 30Vpp Pulse-Shaping Pulser with ImprovedEfficiency and a Noise-Optimized ReceiverKailiang Chen, Anantha P. Chandrakasan, and Charles G. SodiniDepartment of Electrical Engineering and Computer ScienceMassachusetts Institute of TechnologyCambridge, MA, USAAbstract—A four-channel analog front-end (AFE) transceiverchip for medical ultrasound imaging is demonstrated. The highvoltage transmitter uses a 3-level pulse-shaping technique todeliver over 50% more acoustic power for the same powerdissipation, compared to traditional methods. The design requiresminimum off-chip components and is scalable for more channels.The receiver is implemented with a transimpedance amplifier(TIA) topology and is optimized for noise, bandwidth and powerdissipation. Based on both acoustic and electrical measurements,we demonstrate the Transmitter (Tx) efficiency improvement,Tx beamformation and the pulse-echo response, revealing thesystem’s full functionality.I. INTRODUCTIONThe AFE block diagram for medical ultrasonic imagingapplications is shown in Fig. 1(a). In each AFE channel, a highvoltage pulser in Tx drives the ultrasound transducer to emitacoustic energy; a low noise amplifier (LNA) in receiver (Rx)amplifies echoes reflected back from the medium. Medicalultrasound systems use beamformation to improve image qual-ity. Tx beamformation is realized by applying different delaysacross multiple Tx channels. Similarly, the received signalsare often digitized and processed by an Rx beamformer.Piezoelectric transducers (PZTs) are commonly used asultrasound transducers. However, the Capacitive Micro-machined Ultrasonic Transducer (CMUT) emerges as an al-ternative technology for compact system integration, offeringdense arrays and flip-chip bonding to electronics [1]. Interfac-ing to CMUTs (Fig. 1(a)) is different from PZTs because theeffective load is much more capacitive and higher impedance.For the transmitter, most power loss is associated with thecapacitive load, rather than the gating and conduction lossfrom the power amplifier circuitry designed for a PZT load.Multi-level pulse-shaping techniques have been used for betterSNR, resolution and harmonic suppression in PZT ultrasoundsystems [2]. Besides these benefits, this paper presents the useof multi-level technique [3] to improve the combined powerefficiency of the CMUT transducer and transmitter.For the receiver, large input capacitance limits the band-width and degrades noise performance. At ultrasound op-erating frequencies (1-20MHz), bulky off-chip inductors areneeded to impedance match the source to a traditional ultra-sound pre-amplifier that assumes a low-impedance source [4].LNAPulse Shaping LogicTx Beamformation Logic0.1uF1MΩ80VPulserRx SwCMUTCMUT BiasingHigh Voltage(a) (b)Fig. 1. (a) AFE Block diagram for medical ultrasound applications usingCMUTs. (b) The four-channel AFE chip die photo.To achieve inductor-less design, the TIA topology is used toimprove the trade-off between gain, bandwidth and noise [5].II. TRANSMITTER DESIGNA. Multi-Level Pulse Waveform for Higher EfficiencyThe small signal model for a CMUT element is representedby a capacitor and resistor in parallel [1]. The C is the parallel-plate capacitance between the membrane and the commonnode. The R is the medium’s mechanical load at the CMUTsurface, transformed to the electrical port. The power dissi-pated by R, due to the electrical pulse’s fundamental frequencycomponent, models the useful acoustic power delivered to themedium. The CMUT transducer used in this work is modeledas 40pF ||80KΩ per element (size 300µm x 3000µm).The Tx efficiency is defined as the ratio between theuseful acoustic power and the total power dissipated. Resonantdrivers can be used to achieve high efficiency, but off-chipinductors for each channel are once again needed due tothe low operating frequencies. Alternatively, the multi-levelpulse-shaping technique reduces power dissipated to chargethe capacitor and increases efficiency [3]. It also requires theleast off-chip components.To show how multi-level pulse-shaping increases efficiency,assume conventional 2-level square-wave pulses (30Vpp,3.3MHz) are used to drive a 40pF ||80KΩ load. The usefulpower on R at the fundamental frequency is calculated to be2.3mW, while the dynamic power wasted on C is 119mW.An N-level waveform, using (N-1) voltage sources to chargeand discharge the capacitor, reduces the wasted power toCV 2f/(N−1) while useful power decreases only slightly [3].A 3-level pulse with 3rd order harmonic cancellation delivers30V15Vφ1φ1φ2φ2M4M3M2M10.1uFM5M6M7M80VShared DC-DC Converter(off-chip capacitors)0.1uF1MΩ80V3-level Waveform Generation & Tx Beamforming Control0.1uFHVDD=30VCMUTCMUT Bias Circuitry(off-chip)Fig. 2. Circuit schematic of the four-channel 3-level pulsers with the middle-voltage generation (all transistors here are high voltage devices).1.7mW power on R, wasting 59.5mW dynamic power. Thus,efficiency is improved considerably.B. 3-Level Pulser Circuit DesignThe 3-level pulser is implemented on the four-channel testchip, as shown in Fig. 2. The three pulse voltage levels are 30V(HVDD), 15V and 0V (GND). The middle voltage is generatedfrom a 2:1 parallel-series switched-capacitor DC-DC converter(M1-M4), which is shared between channels. The only off-chip components are two 0.1µF capacitors. Because the loadis much smaller than 0.1µF (about 0.2nF for four CMUTelements combined), the converter is switched at a very lowfrequency (10-100Hz) to save power, consuming less than 1%of the total four-channel pulsing power.3-level pulse-shaping is implemented with four high volt-age switches (M5-M8) in each channel. The relative timingdifferences between each channel’s control signals is digitallyadjustable and effectively implements the Tx beamforming.This design of multi-channel pulsers with a shared voltageconverter can be extended to more voltage levels or channels.III. RECEIVER DESIGNA. LNA Optimization MethodologyFig. 3(a) shows the small signal model of the CMUT andLNA; Fig. 3(b) plots various circuit transfer functions to helpthe analysis. The closed-loop TIA gain is:ZCL =Rf ·(11+s·Rf ·Cf)1 + 1F ·AOL, (1)where F = Zi/(Zi + Zf ) is the feedback factor, and AOL isthe op-amp open-loop gain. From (1), the LNA bandwidth isdetermined by the smaller of the following two poles:fp =12piRfCf, (2)fi =√fc · fz =√fc · 12pi (Rf ||Ri) (Ci + Cf ) , (3)where fi is the intersection between 1/F and AOL, whichis approximately the geometric mean of 1/F ’s zero (fz) andthe op-amp’s unity-gain frequency (fc), assuming 20dB/decroll-off. When fi < fp, an increase in Rf always im-proves LNA’s gain-bandwidth product (GBP). Because DCFig. 3. (a) Small signal model and noise sources of the CMUT element andthe TIA. (b) Transfer functions used for LNA optimization.TIA Gain=Rf , while bandwidth (=fi) is approximately pro-portional to 1/√Rf . This improvement stops until fi = fp,at which point the LNA achieves maximum bandwidth whileexploiting full op-amp GBP. The phase margin is 45o. Furtherincreases in Rf no longer improve GBP because fp (∝ 1/Rf )starts to limit the bandwidth. But phase margin continues toimprove at the expense of bandwidth reduction [5].Fig. 3(a) also shows noise sources. The noise figure is:NF = 1 +RiRf+V 2opI2in · |Zi||Zf |2+I2opI2in+2 · |Vop · Iop|I2in · |Zi||Zf |. (4)From (4), a large Rf is desired to reduce its thermal noisecontribution. Moreover, the noise gain (Gn), from op-amp’sinput-referred voltage noise (Vop) to output, has a peakingeffect due to a drop in |Zi| at higher frequencies. Both (5) andFig. 3(b) suggest that the peaking is minimized when fi = fp,the same condition for maximizing the GBP and bandwidth.Gn =∣∣∣∣ AOL1 + F ·AOL∣∣∣∣ = ∣∣∣∣ 1F ||AOL∣∣∣∣ ≈ min(∣∣∣∣ 1F∣∣∣∣ , |AOL|)(5)B. LNA ImplementationThe LNA optimization starts with a 5MHz bandwidth targetand the optimal condition: fi ≈ fp ≈ BW . Rf is maximizedwhile the corresponding Cf , estimated from (2), is kept largerthan parasitic capacitances to maintain control over circuitstability. The op-amp unity-gain frequency is estimated from(3). Further design adjustments keep phase margin above 60o.Fig. 4 shows the LNA schematic. The input stage devices(M1, M2) are biased at the boundary of strong and weakinversion for high transconductance and low noise. The Millercompensation leg (M9, Cc) keeps the op-amp second pole wellbeyond the closed-loop bandwidth for good phase margin.The source follower (M7, M8) lowers the op-amp outputimpedance for accurate feedback. To protect low voltagedevices during high voltage transmissions, the high voltageVIN VIPM1 M2M0M4M3M6M5CcM8M7OUTRf=76KΩCf=0.45pFVB1VB2M9M10TXONRXONRXONTOCMUT1.8VVIPRXONS1S2 S3 S4 S5S6R1R2C1HV Rx SwitchFig. 4. The LNA schematic, implemented in the TIA topology.Rx switch (M10) is opened and the low voltage switches (S1-S6) are closed. This also enforces the sleep mode; only thereference current remains conducting for fast wake-up.IV. MEASUREMENT RESULTSA. Acoustic Power MeasurementTo measure Tx efficiency, the total pulsing power can beobtained electrically. However, the ultrasonic power transmit-ted into the medium requires acoustic measurements. From(6), acoustic power is the product of the acoustic intensity attransducer surface (I) and the transducer surface area (A). I iscalculated from the fundamental frequency component of theRMS acoustic pressure at the transducer surface (prms) andthe acoustic impedance of the medium (Zm).Pacoustic = I ·A = p2rmsZm·A (6)In practice, the acoustic pressure at the transducer surfacecannot be directly measured. Instead, it can be reliably back-calculated from a pressure measurement at another location.According to [6], when the transducer aperture is close to asquare or circle, the pressure magnitude profile along the axialdirection reaches its maximum at the boundary between thenear and far field. This magnitude is roughly twice the pressuremagnitude at the tranducer surface.Fig. 5(a) shows the test setup. Eight CMUT elements areconnected in parallel to form an aperture of 2.4mm x 3mm,approximately a square. The solid curve in Fig. 5(b) is the cor-responding acoustic simulation of the pressure field using theField II software [7], verifying that surface pressure (z=0mm)is about half maximum pressure (z=5.9mm). Furthermore,a hydrophone (ONDA HNC0400) is used to measure theacoustic pressure magnitude along the axial axis. The resultin Fig. 5(b) shows good agreement between measurement andsimulation. In near field, there are deviations likely caused bythe hydrophone tip distorting the pressure field. This, however,does not affect calculating the transducer surface pressure.B. Tx Efficiency MeasurementFixing the hydrophone at the near and far field boundary(5.9mm away), Tx efficiency is measured with the aforemen-tioned method. Different pulse shapes are generated to evaluatethe efficiency improvement. The pulse shape is defined by the∆/T ratio as shown in Fig. 6(a). When ∆/T = 0, 2-levelpulses are generated. As ∆/T increases, the pulses turn intoFig. 5. (a) Acoustic power measurement setup. (b) Normalized RMS pressurealong the transducer axial axis, measurement vs. simulation.Fig. 6. (a) Tx efficiency measurement setup and pulse shape definition. (b)Measurement results using different pulse shapes. (c) Measured time-domainwaveform of the optimal 3-level 3.3MHz pulses, ∆=20ns, ∆/T=0.0673-level, reducing the dynamic power from CV 2f to CV 2f/2and increasing the efficiency. But as ∆/T increases further,the acoustic power starts to decrease because less energy iscontained within the pulse shape. Since the dynamic power iskept at CV 2f/2, efficiency decreases. Therefore, there is anoptimal pulse shape to maximize the Tx efficiency.Fig. 6(b) shows the measurement results. Under the sametotal power consumption, the optimal 3-level pulser delivers56% more acoustic power than the 2-level pulser at 2.5MHz.The relative efficiency improvements of a 3-level pulser overa traditional 2-level pulser at 3.3 and 5.0MHz pulses are50% and 43%, respectively. Efficiency improvement is lessfor pulses with a shorter period, because the same RC settlingtransition distorts shorter pulse shape more severely, reducinguseful acoustic power. Moreover, as higher frequency pulsesraise dynamic power proportionally, the overall efficiencycurve shifts down. HVDD value determines pulse amplitudeand total power consumption, but does not have a big effect onTx efficiency. Fig. 6(c) is a time-domain waveform of optimal3-level 3.3MHz pulses. Table I (top part) lists optimal 30Vpp3-level pulsing power dissipation and efficiency.C. LNA CharacterizationTable I also summarizes the LNA performance. The noiseefficiency factor (NEF) [10] commonly used for instrumen-tation amplifiers is revised to compare the CMUT LNAs.The term Vrms,in/√BW is approximated by Vn,in (the mid-TABLE IMEASURED PERFORMANCE SUMMARY AND COMPARISONPulser (Optimal 3-lvl) 2.5MHz 3.3MHz 5.0MHzPower (mW) 39.4 52.4 77.6Tx Efficiency 1.3% 1.0% 0.5%LNA Specs This Work [8] [9]Target CMUT Area 300x3000 63x1037 250x250(µmxµm)Power [Ptot] (mW) 14.3 3.81 4.0Bandwidth (MHz) 5.2 20 10TIA Gain (dBΩ) 96.6 94.0 112.7Input-referred Noise 0.56@3MHz 2.18@10MHz 1.8@5MHz[Vn,in] (mPa/√Hz)Noise Figure (dB) 10.3@3MHz 10.5@10MHz N/A(CMUT Model)Output P1dB (mVpp) 618@3MHz 84.2@8MHz N/ANEF ′[Vn,in ·√Ptot] 2.1 4.2 3.6(mPa√mW/Hz)Other LNA Specs This WorkSleep Power 1.5mW4-Ch Gain Mismatch <0.11dBΩCrosstalk <-47dBc@3MHz;<-35dBc@10MHzWake-up/Sleep Time < 1µsFig. 7. (a) Measured ultrasound lateral beam profile, steered to the center(broadside). (b) Measured beam profile, 30ns delay between channels.band pressure noise density, input-referred at the transducersurface), leading to the revised NEF ′ = Vn,in ·√Ptot. Thiswork achieves better NEF ′ and linearity1.D. Tx Beam-Steering and Pulse-Echo MeasurementsUltrasound Tx beam-steering is demonstrated by the four-channel pulsers driving four CMUT elements with 3.3MHzpulses. Fig. 7(a) and (b) shows the beam profiles in far fieldwith 0 and 30ns delays between channels, respectively. Bothmeasurements match Field II simulations well. In (b), the delaytd (30ns) and the beam lateral displacement x (1.0mm) isrelated by: td·cpitch ≈ xz , where sound speed in oil c=1460m/s,CMUT element pitch=300µm, and depth z=7.387mm. Besidesthe above examples, various delay patterns for beam-focusingand steering are also measured and match simulations well.The pulse-echo measurement tests the complete ultrasoundsignal chain. The pulser drives the CMUT with a widebandpulse (20ns duration, 2-level shape, shown in Fig. 8(a)). Theultrasound wave then propagates through an oil medium andreflects back at the oil-air boundary 26mm away. The echois received by the same CMUT element and amplified bythe LNA (Fig. 8(b)). The echo’s FFT (Fig. 8(c)) shows thetotal channel characteristic, including CMUT, the oil medium1Limited published results can be found on CMUT LNA designs, in whichthe CMUTs are different in size, intrinsic bandwidth and application, leadingto different LNA design targets. The NEF’ is used here for fair comparison.Fig. 8. The pulse-echo measurement of the complete ultrasound channel.and LNA. It mainly reflects the band-pass characteristic ofthe CMUT device, with a center frequency of 4MHz and afractional bandwidth close to 100%.Finally, Fig. 1(b) shows a die photo of the test chip fabri-cated in TSMC 0.18µm high voltage process. Each channeloccupies an area of 300µm x 1100µm.V. CONCLUSIONThe multi-level pulse-shaping technique is successfully ap-plied to the CMUT transmitter for increased power efficiency.The design is highly modular and extendable to increasedchannels or more voltage levels. Meanwhile, the TIA topologyoptimizes receiver performance in the presence of a highlycapacitive source. The combined electrical and acoustic mea-surements demonstrate complete system functionality.ACKNOWLEDGMENTThe authors acknowledge the support of the C2S2 FocusCenter, one of six research centers funded under the FocusCenter Research Program (FCRP), a Semiconductor ResearchCorporation entity. The authors thank Prof. Khuri-Yakub’sgroup in Stanford University for providing CMUT devices andthe TSMC University Shuttle Program for chip fabrication.REFERENCES[1] O. Oralkan, “Acoustic imaging using capacitive micromachined ultra-sonic transducer arrays: devices, circuits, and systems,” Ph.D. disserta-tion, Stanford University, 2004.[2] K. Kristoffersen and H. Torp, “Method and apparatus for generating amulti-level ultrasound pulse,” Apr. 4 2006, uS Patent 7,022,074.[3] L. Svensson and J. Koller, “Driving a capacitive load without dissipatingfcv2,” in 1994 IEEE Low Power Electronics Symposium.[4] E. Brunner, “Ultrasound system considerations and their impact on front-end components,” Analog Devices, 2002.[5] Graeme, Photodiode Amplifiers: Opamp Solutions. McGraw Hill, 1995.[6] J. Bushberg, The essential physics of medical imaging. LWW, 2002.[7] J. Jensen, “Field: A program for simulating ultrasound systems,” in 1996NordicBaltic Conference on Biomedical Imaging, 1996.[8] A. Nikoozadeh, “Intracardiac ultrasound imaging using capacitive mi-cromachined ultrasonic transducer (cmut) arrays,” Ph.D. dissertation,Stanford University, 2010.[9] I. Wygant et al., “Integration of 2d cmut arrays with front-end electronicsfor volumetric ultrasound imaging,” Ultrasonics, Ferroelectrics andFrequency Control, IEEE Transactions on, 2008.[10] M. Steyaert and W. Sansen, “A micropower low-noise monolithicinstrumentation amplifier for medical purposes,” Solid-State Circuits,IEEE Journal of, vol. 22, no. 6, pp. 1163–1168, 1987.

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Ultrasonic imaging front-end design for CMUT: A 3-level 30Vpp pulse-shaping pulser with improved efficiency and a noise-optimized receiver

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