dipankar nag, jia hao cheong, and minkyu jehere gm and gds represent transconductance and output...

Abstract—This paper presents the design and

implementation of a fully integrated trans-impedance amplifier(TIA) as the pre-amplifier of analog front-end for ultrasoundneedle intervention application. The application demands verylow power solution for the receiver operating in the frequencyband 1.3 MHz to 3.9 MHz. The design adapts low powerarchitecture to create low noise electronic interface forCapacitive Micro-machined Ultrasound Transducers (CMUT)sensor. Using a 0.18-µm CMOS/DMOS process, simulationresults of this interface IC show that the proposed TIA canprovide 104 dBΩ trans-impedance gain, 1.44 nArms inputreferred noise integrated over frequency band 1.3 MHz to 3.9MHz, while consuming 200 μA current from 1.8-V supply.

Index Terms—Analog front-end, capacitive micro-machinedultrasound transducer, trans-impedance amplifier.

I. INTRODUCTION

Ultrasonic needle intervention shows tremendous potentialin enhancing safety and confidence level in many diagnosticprocedures like amniocentesis, chorionic villus sampling,epidural anaesthesia, liver biopsy etc [1]. In this approach anultrasonic trans-receiver embedded in the intervening needleprovides the real time image of the internal organs therebyproviding greater feedback and control over the needle,leading to better safety for the procedure. The systemcomprises of a 1-dimensional (1D) capacitiveMicro-machined Ultrasound Transducer (CMUT) array,supporting front-end circuit and portable image display thatprovides real-time ultrasound images as shown in Fig.1.

The associated trans-receiver architecture for thisapplication is shown in Fig.2. It includes a transmit path and areceive path. The receiver consists of a preamplifier and atime gain compensator (TGC) that compensates for signalattenuation as function of depth. The TGC is implemented bya variable gain amplifier (VGA) where a time varying controlsignal is applied to the VGA gain control input such that thesignal strength at the VGA output is constant over time. TheTGC is followed by low pass filter (LPF), analog-to-digitalconverter (ADC) and image processor to generate real-timeimage. The transmitter chain consists of a pulse generator, aprogrammable delay block, and a transmit driver to drive thetransducer.

Manuscript received June 6, 2012; revised August 27, 2012.This work is funded by the Biomedical Engineering Program (BEP),

Agency for Science, Technology, and Research (A*STAR), Singapore(SERC Grant no. 1021480005).

The authors are with Institute of Microelectronics, A*STAR (Agency forScience, Technology and Research) 11 Science Park Road, Science Park II,CO 117685 Singapore (e-mail: [email protected],[email protected], [email protected]).

Fig. 1. Schematic illustration of ultrasound needle

The pulse generator produces 10-V pulse in the frequencyrange of interest to the transducers through programmabledelay chains to enable electronic beam focusing. Atransmitter/receiver switch is placed in between thetransducer and the transceiver IC in order to control thetransmit/receive mode and also to protect the receiver fromthe high transmit voltage pulses used to excite the transducerarray. Multiple trans-receiver channels and transducers arerequired for ensuring desired image quality. Owing to thereal-time operation of the front-end circuitry inside thepatient’s body, the application demands very low powerbudget from the analog front-end. Also the requirement to fitthe entire system inside the needle sets very stringentrequirement on chip area.

Fig. 2. System diagram of ultrasound imaging system

Due to high electrical impedance of CMUT devices,wide-band current amplification is required at thepre-amplifier stage. Trans-impedance amplifier (TIA), owingto its low input impedance has been a preferred choice in thiscase. So far different TIA topologies have been used byresearchers for ultrasound preamplifier application [2-6].Among all the topologies reported cascaded common-source,source-follower structure with resistive feedback appears tobe the most common choice due to its obvious merits [2-4]. Inthis paper, we have analyzed that resistive feedback around

Low Noise Low Power Preamplifier for IntegratedUltrasound Needle Intervention Application

Dipankar Nag, Jia Hao Cheong, and Minkyu Je




I. INTRODUCTION










1D cMUT Array

Front-End Circuit

Portable ImageDisplay






I. INTRODUCTION










1D cMUT Array

Front-End Circuit

Portable ImageDisplay



DOI: 10.7763/IJIEE.2013.V3.256

International Journal of Information and Electronics Engineering, Vol. 3, No. 1, January 2013

20

mailto:[email protected]









an inverter can serve as even better candidate for achievinglow power budget while satisfying the performance requiredfor this application. The simulation result shows that thisdesign can achieve more than 100 dBΩ trans-impedance gain,1.44 nArms integrated input current noise over the frequencyband of 1.3 MHz to 3.9 MHz while consuming around 200μA current from a 1.8-V supply. The rest of the paper isorganized in the following manner. Section 2 justifies thesuitability of this topology as compared to traditionalcascaded common-source, source-follower structure throughsmall signal analysis. Section 3 reveals the circuit designfollowing discussion about specification. Section 4 showspost-layout simulation result covering relevant designmetrics. Section 5 concludes the paper.

II. CIRCUIT ANALYSIS

The core amplifier, that we are going to use as preamplifier,is essentially an inverter. The shunt-shunt feedback around itthrough a resistor makes it suitable to serve as a goodtrans-impedance amplifier. In order to judge the suitability ofthis topology as TIA we are going to compare key designparameters of this architecture with the traditional TIAtopology (cascaded common-source and source- followertopology) on the basis of small signal analysis. Fig. 3 showsthe two topologies stripped to their basics.

(a) TIA-1: Common source, source (b) TIA-2: Inverter basedfollower cascaded

Fig. 3.TIA Topologies

Equation (1) shows TIA-1 trans-impedance dc gain.

2121

12

111

1

vv

F

vv

vF

in

o

AA

R

AA

ARgm

I

V

(1)

1vA and 2vA are intrinsic voltage gains of common-source

stage and source follower stage respectively, as described in(2) and (3) below. Here gm and gds representtransconductance and output conductance of respective MOSrespectively. RF is the feedback resistor as shown in Fig. 3.

pn

nv gdsgds

gmA

1 (2)

242

22 gmgdsgds

gmAv

(3)

Trans-impedance from (1) shows that trans-conductance ofthe source follower transistor M2 should be large enough to

satisfy 11

2 vF ARgm . Also the source follower voltage

gain being less than unity the trans-impedance gain becomeslower than that of a single stage one. However, trans--impedance dc gain (4-5) of inverter based topology canachieve higher value owing to higher equivalent gm andhigher intrinsic voltage gain Av under same currentconsumption.

v

F

v

Feq

in

o

A

R

A

Rgm

I

V1111

1

(4)

eqeqv rogmA * , pneq gmgmgm ,

pneq gdsgds

ro

1

(5)

In terms of output impedance also this topology showsbetter promise because of higher voltage gain at same powerconsumption. Equations (6) and (7) show the outputresistance of TIA-1 and TIA-2 respectively.

21

422

1

)1||1||1(

vvout AA

gdsgdsgmR

(6)

v

eq

eqeqout A

ro

rogmR

1

11 (7)

For a given trans-impedance gain (decided by specification)only core amplifier noise can be reduced. In case of TIA-1noise from source follower stage also contributes to theoverall amplifier noise, thereby resulting in higher overallinput referred noise, as derived in (8), than that of TIA-2,shown in (9).

Fvn

np

F

nin

R

KTgmgm

gmAgmgmgm

R

KT

I

4)(

11)(

4

~

4222

21

22

2,

(8)

FpnF

eqnin R

KT

gmgmR

gmKTI

44~2

2,

(9)

Here is a parameter associated with thermal noise of

MOS device. The frequency response of TIA-1 also imposessome critical problems. Other than the two poles associatedwith input and output node the device capacitance associatedwith node X offers an extra pole. With three poles around thefeedback loop this TIA can even oscillate. On the other hand,in case of TIA-2 there are only two poles associated withinput and output node. As a result, stability can be easilyassured. The equation (10) captures the trans-impedancefrequency response for TIA-2.







2121

12

111

1

vv

F

vv

vF

in

o

AA

R

AA

ARgm

I

V

(1)



pn

nv gdsgds

gmA

1 (2)

242

22 gmgdsgds

gmAv

(3)


satisfy 11



v

F

v

Feq

in

o

A

R

A

Rgm

I

V1111

1

(4)


pneq gdsgds

ro

1

(5)


21

422

1

)1||1||1(

vvout AA

gdsgdsgmR

(6)

v

eq

eqeqout A

ro

rogmR

1

11 (7)


Fvn

np

F

nin

R

KTgmgm

gmAgmgmgm

R

KT

I

4)(

11)(

4

~

4222

21

22

2,

(8)

FpnF

eqnin R

KT

gmgmR

gmKTI

44~2

2,

(9)









2121

12

111

1

vv

F

vv

vF

in

o

AA

R

AA

ARgm

I

V

(1)



pn

nv gdsgds

gmA

1 (2)

242

22 gmgdsgds

gmAv

(3)


satisfy 11



v

F

v

Feq

in

o

A

R

A

Rgm

I

V1111

1

(4)


pneq gdsgds

ro

1

(5)


21

422

1

)1||1||1(

vvout AA

gdsgdsgmR

(6)

v

eq

eqeqout A

ro

rogmR

1

11 (7)


Fvn

np

F

nin

R

KTgmgm

gmAgmgmgm

R

KT

I

4)(

11)(

4

~

4222

21

22

2,

(8)

FpnF

eqnin R

KT

gmgmR

gmKTI

44~2

2,

(9)




21

)1(])1()1([

)1(2

vinvinLeqFvFeqF

eqFeqFF

in

o

AsCARCroCARsroR

roRgmsCR

I

V

(10)

where

v

eqFin A

roRR

1

, inFinLLF CCCCCC (11)

CF accounts for gate to drain capacitance of two MOSFETsalong with parasitic cap associated with the feedback resistor.Interestingly, this structure introduces one RHP zero at

F

eqz C

gm . However, this zero falls at a very higher

frequecy compared to our frequency of interest.

III. CIRCUIT DESIGN

First the specification was derived through careful systemdesign. The required trans-impedance gain is 100 dBΩ over afrequency band 1.3-3.9 MHz, while keeping the integratedinput referred current noise within 3 nArms. The design hasto provide a wide dynamic range in order to support inputcurrent signal amplitude in the range 14.5 nA to 4.55 μA. Theload offered by next stage is RC parallel load - 250KΩ||150fF.The desired power consumption has to keep within 400 μwattfrom a 1.8V supply. The CMUT device at the input to theTIA is modeled as shown in Fig.4. It is an AC current sourcein parallel with the intrinsic source impedance of the CMUT.Fig.5 shows the preamplifier circuit. In order to make theamplifier insensitive to power supply it is wise to bias it witha current bias. However, in the whole system the TIA powerwill be supplied from a LDO. This will obviate the need tobias it through a current source.

Fig. 4. CMUT model

As per discussion in section II, feedback resistor RF needsto be more than 100 KΩ to satisfy trans-impedance gain of100 dBΩ. Ignoring flicker noise, we figured out gm neededfor the input transistors to meet the noise criteria from (9).This allows us to choose a bias current for the amplifier stage.Approaching this way, the device sizes are optimized to meetother design metrics. Special care is taken during layout toreduce parasitic cap associated with feedback resistor RF. Inorder to relax dynamic range requirement of later stages we

add 20 dB of coarse grain-tunability through switches in theresistive feedback path. At lower gain settings the inverterbias current is reduced accordingly, since at higher inputsignal the amplifier can afford to have larger noise. This waysome power can be saved at lower gain mode. The capacitorCb is used to bypass the noise from bias current. The T/Rswitch is implemented with 30-V NMOS device so that it cansustain 10-V pulses during transmit mode. RX_EN is used toturn off the preamplifier during transmit mode.

Fig. 5. Inverter based TIA

IV. POST LAY-OUT SIMULATION RESULT

The design has been carried out using Global foundry0.18-μm CMOS/DMOS 1P6M PDK. All circuit simulationsare done with Cadence-Spectre simulator. Fig. 6 shows thefinal layout of the proposed circuit. During simulation theboard and package parasitic have been considered.

Fig. 6. Preamplifier layout

Fig. 7. Trans-impedance frequency response

Fig.7 captures trans-impedance gain frequency responsefor both pre-layout and post-layout simulation at maximum

)1(])1()1([

)1(2

vinvinLeqFvFeqF

eqFeqFF

in

o

AsCARCroCARsroR

roRgmsCR

I

V

(10)

where

v

eqFin A

roRR

1



F

eqz C



III. CIRCUIT DESIGN


Fig. 4. CMUT model









)1(])1()1([

)1(2

vinvinLeqFvFeqF

eqFeqFF

in

o

AsCARCroCARsroR

roRgmsCR

I

V

(10)

where

v

eqFin A

roRR

1



F

eqz C



III. CIRCUIT DESIGN


Fig. 4. CMUT model










22

gain setting. Around 104 dBΩ trans-impedance gain has beenachieved over the bandwidth of 1.3 MHz to 3.9 MHz. Fig.8shows the input referred noise power density plot for bothpre-layout and post-layout simulation at maximum gainsetting. The integrated input referred noise calculated overfrequency band of interest is 1.44 nArms for post-layoutsimulation.


Fig. 9. Transient simulation under 9.1 μA peak-to-peak input current swing

Fig. 9 demonstrates the TIA output for maximum inputcurrent swing i.e. 9.1 μA peak-to-peak. For maximum inputswing case the circuit is operated at minimum gain mode.

This simulation with largest signal swing ensures that there isno visible distortion or any sign of ringing. The circuitconsumes only 200 μA current from 1.8-V supply qualifyingit an ideal candidate for needle application.

V. CONCLUSION

We present the design and implementation of a fullyintegrated front-end preamplifier dedicated to interface 1DCMUT array for ultrasonic needle applications, using0.18-µm CMOS/DMOS 30-V HV fabrication technologyfrom Global Foundries. New fully integrated low power lownoise preamplifier has been described. Simulation resultsvalidate its ability to meet the specifications of ultrasonicapplication. The proposed circuit has been sent forfabrication and measurements will be done when theprototypes are ready.

REFERENCES

[1] L. Dolkart, M. Harter, and M. Snyder, “Four-dimensionalultrasonographic guidance for invasive obstetric procedures,” Journalof Ultrasound in Medicine, vol. 24, pp. 1261-1266, 2005.

[2] I. Wygant et al., “An integrated circuit with transmit beamformingflip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging,”IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, vol.55, no.2 pp. 327-342, Feb. 2008.

[3] Gurun G, P. Hasler, and F. L. Degertekin, “Front-end Receiverelectronics for High Frequency monolithic CMUT-on-CMOS Imagingarrays,” IEEE Trans. on Ultrasonics, Ferroelectrics, and FrequencyControl, vol. 58, no. 8 pp. 1658-1668, Aug. 2011.

[4] I. Wygant et al., “An integrated circuit with transmit beamformingflip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging,”IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control,vol. 56, no.10, pp. 2145-2156, 2009.

[5] P. Behnamfarn and S. Mirabbasi, “Design of a High-Voltage AnalogFront-End Circuit for Integration with CMUT Arrays,” BiomedicalCircuits and Systems Conference (BioCAS), pp. 298-301, 2010.

[6] K. Insoo et al., “CMOS Ultrasound Transceiver Chip forHigh-Resolution Ultrasonic Imaging Systems,” IEEE Trans. onBiomedical Circuits and Systems, vol. 3, no. 5, pp. 293-303, 2009.






V. CONCLUSION


REFERENCES












V. CONCLUSION


REFERENCES








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dipankar nag, jia hao cheong, and minkyu jehere gm and gds represent transconductance and output...

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