InternationalJournalofNanoelectronicsandMaterialsVolume11,No.2,Apr2018[153‐166]

AnOptimizedLow‐NoiseLow‐PowerPreamplifierforCardiacImplants

SannaMairaj1,SuhaibAhmed1,*andVipanKakkar1

1DepartmentofElectronicsandCommunicationEngineering,ShriMataVaishnoDeviUniversity,Katra,J&K‐182320,India.

Received19October2017;Revised16December2017;Accepted2January2018

ABSTRACT

Amongvariousanalogcircuits,thepreamplifierhasaprominentroleintheanalogfrontend(AFE)circuitofbiomedicalimplantablemicrosystemsforareliableacquiringofweakbiophysiological signals. The AFE acts as an interface between the acquired analogbiosignals and the digital part in these implants. As the industry of the biomedicalimplantable devices develops, lowering the power consumption as much as possiblewithout sacrificing the performance is essential in improving the service time of thebattery,whichcannotbereplaced frequently.Hence,there isan increasingdemand foralownoise, lowpowerbio‐acquisitionsystemsoastoavoidbulkyconnectivityandreducepatientmobilityanddiscomfort.Thispaperthuspresentsanoperationaltransconductancebased preamplifier for the application in cardiac implants. The performance of theproposeddesignwasevaluatedbasedonvariousparameterssuchasgain,commonmoderejection ratio (CMRR), noise spectral density, and power consumption. Besides,optimizationbysupplyvoltagescalingoftheproposeddesignwasdonewiththefocusisonlowering the power consumption. The trade‐offs between various parameters like gain,speed,andpowerconsumptionwereobserved in theoptimizeddesign.Thepreamplifier,designedusinga180nmCMOStechnology,providesahighgainof50dBatasupplyvoltageof1.5Vandalowpowerdissipationof61.10µW.Basedontheperformanceevaluation,itwas observed that the proposed preamplifier is suitable for low power cardiacapplications.Keywords:CardiacImplants,Preamplifier,LowPowerElectronics,OperationalTransconductanceAmplifier.

1. INTRODUCTION

Thetermimplantrepresentsamedicaldevicethatactsasapartofthewholebiologicalsystemor can be used to provide support to a damaged biological structure [1,2]. Currentlybiomedical implants are used for various applications including cardiac pacemakers,defibrillators, and cardiovascular stents. The monitoring of biomedical signals provides usinformationaboutthevitalhealthofthebodyandthusthedatacanbeofprimeimportancetomedical practitioners [3].With the rapid development inmicroelectronics towardsmedicaltherapiesanddiagnosticaids, there isaneed for lowering thepowerconsumption inactiveimplantabledevicesthatarebatterypoweredso thatthedevice lifetimeincreases.Onesuchexample of active implantable device is the Cardiac Pacemaker in terms of its widespreadapplication [4]. A Cardiac Pacemaker is a device that uses electrical pulses to recover thenormalheartbeatofadiseasedheart.Themajorbuildingblocksofthepacemaker,asshowninFigure 1, are an analog front end (AFE) circuit, a microcontroller with ultra low power

*CorrespondingAuthor:sabatt@outlook.com

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consumption,abattery,andanoutputcircuitthatstimulatestheheart.TheAFEcomprisesofapreamplifier,alow‐passfilter,alevelshifter,andasynchronizingcircuit.Thecardiacsignalisgiventoalownoisepreamplifierforamplificationpurposesandisthenfiltered.Thisfilteredsignal is then given to the comparator that produces a pulse which depends upon thethresholdvoltagelevel.Theoutputstageofapacemakeriscalledachargepumpthatusesapulsegenerator to stimulate theactivityof theheart.Thus, a preamplifier is a criticalblockthatisusedforthedetectionofsmalllevelsignalsespeciallyforbiomedicalapplications[3].

Figure1.GeneralBlockDiagramofanImplantableCardiacPacemakerSystem.

BiosignalslikeEEG,ECG,EMG,EOGareweaksignalswithaninputamplitudetypicallyrangingfrom0.5µV to 5mV [5,6] and are highly susceptible to noise andpower line interference at50Hzto60Hz.Nowadaysthereisademandforalownoise,lowpowerbioacquisitionsystemso as to avoid bulky connectivity and reduce patients’ mobility and discomfort [7]. APreamplifierisoneoftheimportantcomponentsoftheanalogfrontendasitdeterminestheSNRoftheentirebiomedicalsignalacquisitionsystemandisrequiredforreliablemonitoringof the physiological signal [8]. These weak biosignals need to be amplified before beingcompatible for processing by the other components of the implant for measurement andtestingpurposes[9]. 2. DESIGNSPECIFICATIONS

Neuralimplantabledevices,EEG,andcochlearimplantsarethemajorapplicationareasforthepreamplifierdesign in the survey [10‐15]while less exploredapplicationsbeing the cardiacpacemaker, EOG, PCG, and retinal prostheses. Some of the key design parameters of anamplifier are its Gain, stability, power consumption, CMRR, PSRR,Noise, THD, andDynamicrange.Different applicationsdemanddifferentparameters that are critical to thatparticularapplication.SomeapplicationslikeNeuralImplantabledevicesrequirealownoise,lowpoweranalogfrontendwhileotherslikecochlearrequireahighPSRR,andgoodSNR.Basedontheliterature study [16‐18], the required design specifications for the preamplifier for CardiacImplants are formulated and summarized inTable 1.Ahigh gain of >50dB, a highCMRRofaround70dB,andahighPSRRof60dBarerequiredforthisparticularapplication.

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Table1DesignSpecificationsforPreamplifiersinCardiacImplants

Parameters TargetSpecifications

Gain >50dB

SlewRate 10V/µs

LoadCapacitance 10pF

MinandMaxOutputvoltages 2V

UnityGainBandwidth 7MEG

ICMR ‐1.5Vto2.5V

CMRR >70dB

PSRR >60dB

Technology 180nm

3. PROPOSEDLOWPOWERPREAMPLIFIER

ThefoldedcascodeOperationalTransconductanceAmplifier(OTA)waschosenforthedesignofthepreamplifierforcardiacapplications.AnOTAisactuallyanop‐ampwithouttheoutputbufferandiscapableofdrivingonlycapacitiveloads.OneoftheimportantfeaturesoftheOTAis that its transconductancecanbeadjustedbyabiascurrent.A foldedcascode isactuallyacompromisebetweenatwostageandatelescopiccascode.Thename“folded”comesfromthefactthatbecauseofthefoldingofP‐channelcascodeactiveloadsofadifferentialpairandthenconverting the MOSFETs to N channel. A folded cascode overcomes the drawbacks of atelescopicstructure,i.e.limitedoutputswingsanddifficultyinshortinginputandoutput,thusprovidesawideswing.AWideSwingmeansthataninputCommonmoderangeisveryclosetothe supply voltage [19]. TheOutput voltage swing is limited by supply voltages. In a foldedcascodeOTA a perfect balance of currents in the differential amplifier stage is not requiredbecause of the fact that excess dc currents can flow in or out of the current mirror. Thepractical version of an OTA and its small signalModel are shown in Figure 2 and Figure 3respectively.ThebiascurrentsI3,I4,andI5ofthetopologyshouldbedesignedsuchthattheDCcurrent in the cascodemirror never goes to zero, e.g. if the input voltageVin ismade largeenough such that M1 is turned on and M2 is turned off. Then, all the current will flowthroughM1 that results in and will be zero. If the currents and arenot greaterthan current , then will be zero. To avoid this problem, the values of currents and were kept between values of currents and 2 [20]. The resistances RA and RB as seenlookingintothesourceofM6andM7andaregivenas:

6 210

6 6 6

11

1A

rds Rgm

Rgm rds gm

(1)

7 9 9

7 7 7 71B

rds R RR

gm rds gm rds

(2)

Here, 9 9 9 11R gm rds rds

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Figure2.FoldedCascodeOTAschematic.

Figure3.SmallsignalmodeloftheOTA.Current isgivenas:

1 1 4 110

1 42 2in in

A

gm rds rds V gm Vi

R rds rds

(3)

Current canbewrittenas:

2 2

79 2 5

7 7

2 12 1

in ingm V gm Vi

KR gds gds

gm rds

(4)

TypicallyK 1 9 2 5

7 7

R gds gdsK

gm rds

(5)

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TheOutputVoltagecanbeexpressedasthesumof and flowingthrough

1 2

1 11

2

2 2 1 2 2out

in

V gm gm Kgm R

V K K

(6)

whereR11,theoutputresistanceisgivenas 11 9 9 11 7 7 2 5R gm rds rds gm rds rds rds

3.1AnalyticalModeling

Thedesignof theProposedFoldedCascodeOTAwasdoneusing level49BSIM3v3MOSFETand TSMC 0.18µm. T‐Spice parameters were used, which are summarized in Table 2. Thedesignconsistsofdeterminingthespecifications,selectingdevicesizesandbiasingconditions,andsimulatingandcharacterizingvariousperformanceparameterssuchastheopenloopgain,CMRR,slewrate,andoutputswing.

Table2ParametersfromTspiceModelFile(TSMC180nm)

Parameter Value

kn’ 172.1μA/

kp’ ‐36.4µA/

LowFieldMobilityμn 398.72( )/V*s

LowFieldMobilityμp 84.33( )/V*s

Vthn 0.37965V

ToxforNMOS 4.1e‐9m

Vthp ‐0.4215V

ToxforPMOS 4.1e‐9m

The main goal is to design a preamplifier based on the above design specifications and tochooseproperW/Lsizesfortheamplifierdesign.Thedesignstepstocomputethetransistorsizesareasfollows:i. Smallestdevicelengthischosensuchthatthechannellengthmodulationparameterλ

iskeptconstant,andalsothereisgoodmatchingforcurrentmirrors.(HereL=1µm)ii. FromtheSlewrateexpression,thetailcurrent isobtainediii.

3 LI SR C (7)

WhereSRistheslewrateand istheloadcapacitance.

iv. Biascurrentsintheoutputcascodewhileavoidingzerocurrentsinthecascodearegivenas:

3 4 5 31.2 1.5I I I I (8)

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v. Designfor and isobtainedfromthemaximumoutputVoltage.LetS4=S14=S5=S13andS6=S7

5

5 2'5 5

2

p

W IS

L k Vds

(9)

77 2'

7 7

2

p

W IS

L k Vds

(10)

Here 5 7

(max)(sat) (sat)

2DD outV V

Vds Vds

vi. Designfor and isobtainedusingminimumoutputvoltage.LetS10=S11andS8=S9

1111 2'

11 11

2

n

W IS

L k Vds

(11)

9

9 2'9 9

2

n

W IS

L k Vds

(12)

Here 9 11

(min)(sat) (sat)

2out SSV V

Vds Vds

vii. Calculationofresistancevalues.

13 81 2

12 6

(sat) (sat),

Vds VdsR R

I I (13)

viii. Designof and isobtainedfromthevalueofUnitygainbandwidthandload

capacitance .

2 2

1 2 3'3

L

n

GB CS S I

k I

(14)

Where l

L

gmGB

C

ix. designisobtainedfromtheminimumInputcommonmodevoltage

33

3 ' 31 '

1

2

(min)n in SS tn

W IS

L Ik V V V

k S

(15)

x. and designisobtainedfromthemaximumInputcommonmodevoltage.

4

4 5 2'1

2

(max)p D in t

IS S

k V V V

(16)

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Also and valuesshouldbelargeenoughtosatisfythemaximumInputcommonmodevoltage.

xi. ThedifferentialVoltagegainisgivenas

1 11 3

2

2 2v

KA gm R I

K

(17)

xii. Powerdissipationgivenas

3 12 10 11 3diss DD SSP V V I I I I I (18)

xiii. PositiveCMRgivenas:

53 1max

3

(max) (min)in DD TO T

IV V V V

(19)

xiv. NegativeCMRxv.

0.5

55 1

1

(min) (sat) (max)in SS T

IV V Vds V

(20)

14.Thesecondstageresistanceis

1 9 7 37 2 5

1 1 1R R gm I

gds gds gds

(21)

Where 9 9 9 11 3R gm rds rds I

15.Theparameterscanbeevaluatedfromthefollowingequations

'3 , ,2 n p n p Dgm I k S I (22)

Dgds I (23)

1 1

D

rdsgds I

(24)

3

2(sat) DSIVds I

(25)

Fromtheabovemathematicalequations,thebiascurrentsandthetransistorsizes( )fortheamplifierwerecalculatedandsummarizedinTable3and4respectively.

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Table3ParameterValues

Parameter Value 100µA= 125µA 10pF

= 2KΩ

Table4Calculated oftransistors(AssumingL=1µm)

W/Lratiosoftransistors TransistorWidths(

=109.80 =109.8065.93 =65.93

=47.44 =47.44= =11.24 = =11.24=8.421 =8.421=10.5 =10.5

3.2CircuitSchematic

Theproposedpreamplifierschematicisshowninfigure4.Thetopologyusescascodecurrentmirrors as active biasing elements. Current mirrors are more economical than passiveresistorsintermsofthediearearequiredtoprovidebiascurrentofacertainvalue.

Figure4.SchematicoftheproposedfoldedcascodeOTAbasedpreamplifier.

A cascode current mirror provides a wider swing compared to a Wilson current mirror.Becauseofthiscurrentmirror,foldedcascodeop‐amphasawideroutputswing.The3main

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stagesof theOTA include thedifferential inputstage, the foldedcascode loadstage, and theoutput stage. The Cascode topology increases the gain of the input differential stage. ThefoldedcascodewasusedtoimprovetheICMRandalsotoreducetherequiredsupplyvoltage.TheCurrentmirrorsinkusedinthisfoldedcascodetopologyhasthefollowingadvantages:

i. HighOutputresistanceii. LowPowerdissipationiii. Self‐biasingiv. Highswingv. SmallsaturationVoltage

4.RESULTSANDDISCUSSIONS

LowVoltageandlowpowerareimportantattributesforcardiacimplantstoachievealongerbattery life time.Theproposedpreamplifierwasfirstdesignedatasupplyof2.5Vandthenoptimizedbyscalingitdownto1.5Vtoachievealoweraveragepowerconsumption.TofindtheACresponseoftheOTA,a100mVinputwasgiventothedifferentialamplifieralongwithDC voltages; the ac analysis was done from 1Hz to 10MHz. Figure 5 shows the frequencyresponseoftheproposeddesigns.

(a)

(b)

Figure5.Frequencyresponseoftheproposedpreamplifierat(a)2.5Vand(b)1.5Vsupplyvoltage.

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TofindtheCommonModegain,bothVin+andVin‐inputswereshortedandthenACandDCvoltageswereapplied.ThesimulatedresultsinFigure6showthecommonmodegain.

(a)

(b)

Figure6.Commonmodegainat(a)2.5Vand(b)1.5Vsupplyvoltage.

Asquarewavewasgiventotheinputsofthedifferentialstage,andthetransientanalysisgavethesimulationresults.TheSlewrateactuallyrepresentsthecurrentavailabletodrivetheloadcapacitance.TheslewratefortheproposeddesignscanbeevaluatedfromFigure7.

Figure7.Slewrateat2.5V.

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(a)

(b)

Figure8.Outputswingat(a)2.5Vand(b)1.5Vsupplyvoltage.

Theoutputswingrepresentsthevaluesofthemaximumpositiveandnegativevoltagesabovewhichtransistorwillremainoutofsaturationandthusspecifiestherangeoverwhichoutputvoltagecanvarywithoutexcessivedistortion.ThegraphsinFigure8showsthevoltageswingfrom‐2.5V to+2.5V.TheOTAprovidesawideswing,meaning the inputCMR isclose to thesupplyvoltages.TheoutputswingislimitedbythesupplyvoltagesasisevidentfromFigure8.

(a)

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(b)

Figure9.PSRRat(a)2.5Vand(b)1.5Vsupplyvoltage.

IfthereisachangeXinthesupplyvoltagethatproducesanoutputvoltagechangeofYvolts,thenthePSRRisexpressedasaratioofX/Y.ThisquantityisexpressedindB.ThePSRRcanbeapositiveornegativequantity.TodeterminethePSRR,DCbiaswasappliedtotheinputsofthedifferentialstageandanACVoltagewasappliedtothepositiveVDDterminaltodeterminethePSRR+.Fortheproposeddesigns,thePSRRcanbecalculatedfromthegraphshowninFigure9.Thispaperfirstpresentsthedesignofapreamplifierforcardiacimplantsdesignedatasupplyvoltageof2.5V,whereahighgainofaround60dBwasobtainedbutthepowerconsumptionisslightlyhigh.Therefore, furtheroptimizationwasdonewherea supplyvoltageof 1.5Vwasobtained.Theaveragepowerconsumption reduced to61.10µWat theexpenseof a reducedgain of 50dB. Also, there is an improvement in the noise spectral density at 1.5V. TheperformanceparametersoftheproposedandoptimizeddesignsaresummarizedinTable5.

Table5ComparsionofProposedandOptimizedPreamplifierDesigns

Specification ProposedDesign OptimizedDesign

SupplyVoltage 2.5V 1.5V

OpenLoopGain 62dB 50dB

PhaseMargin ‐95.83º ‐92.02º

CMRR 102dB 114dB

PSRR+ ‐50dB ‐60dB

PowerConsumption 0.131mW 61.10µW

OutputSwing ‐2.5Vto2.5V ‐1.4Vto1.4V

NoiseSpectralDensity250µV/√ at100mVinput

3.5µV/√ at100mVinput

The state of the art comparison of the proposed low power preamplifier with some otherrelatedworkisgiveninTable6.Againof50dB,alowpowerconsumptionof61.10µW,andahigh CMRR of 94dBwere achieved,which are suitable for the cardiac applications. Also, animprovementinthenoisespectraldensityisquiteevidentfromthetable.

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Table6PerformanceComparisonofProposedPreamplifier

Parameter [21] [22] [23] [24] ThisWork

Technology(µm)

0.6 0.35 0.18 0.18 0.18

Gain(dB) 39.4 20.4 40 63.06 50

PowerConsumption

(W)

2.4µ 23.1µ ‐ 0.0154m 61.10µ

CMRR(dB) 66 ‐ 82 ‐ 114

PSRR(dB) 80 ‐ ‐ ‐ 60

SupplyVoltage(V)

2.8 3.3 1.8 1.8 1.5

5.CONCLUSION

Parameterssuchassize,weight,andbatteryareofprimeimportanceincardiacimplants.Astheindustryofthemedicalimplantabledevicesdevelops,loweringthepowerconsumptionasmuch as possible is essential in improving the service time of the battery,which cannot bereplaced frequently.Hence,a lowvoltageoperationbecomescritical toachievea lowpowerconsumption.ThispaperpresentsthedesignandoptimizationofanOTAbasedpreamplifierforcardiacapplication.Thepreamplifierprovidesagainof50dBatasupplyvoltageof1.5Vand a low power dissipation of 61.10µW, thereby making it suitable for this particularapplication.Thetrade‐offsbetweenvariousparameterssuchasgainandpowerconsumptionare also evident in the optimized design. In the future, further optimization to an ultra‐lowpowerlevelcanbedone.Also,differentapplicationssuchasretinalprostheses,EOG,andPCGcanbeconsideredsincemuchoftheworkisdoneonneuralimplantabledevices.

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