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1、 2025 IEEE International Solid-State Circuits ConferenceLow-noise current sensingJens Anders(jens.andersiis.uni-stuttgart.de)University of StuttgartFebruary 16,2025ISSCC 2025 Tutorial T1“Low-noise current sensing”Jens Anders1 of 88 2025 IEEE International Solid-State Circuits ConferenceOutlineMotiva

2、tion:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResistive TIAs and noise theoryDesign of TIAs with capacitive feedbackCurrent-input modulatorsAlternativ

3、e approaches:Current-to-X conversionSummary and conclusionJens Anders2 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceBiomedical sensing I Nanopore sensingNanopore sensing is used to analyze and characterize molecules by detecting them as

4、 they pass through a tiny pore,called a nanoporeThe nanopore is embedded in a thin membrane separatingtwo compartments filled with an electrolyte solutionAn applied voltage across themembrane drives and ionic current through the nanoporeThe molecule passing the pore alters the ionic currentChallenge

5、:Depending on the pore type need to detect current changes in the pA range for large channel countsJens Anders3 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”Reference 1 2025 IEEE International Solid-State Circuits ConferenceBiomedical sensing II Injectable sensorsInjectable,wirelessly power

6、ed CMOS chips for in-vivo electrochemistry measurements open up new ways formonitoring and diagnosticsChallenges:Small area,low power consumptionJens Anders4 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”Reference 2 2025 IEEE International Solid-State Circuits ConferenceNanochemistryMeasurin

7、g the electrochemical response from single entities,such as single nanoparticles,enzymes,or cells can provide fundamental insights into electrochemistry at the nanoscale as well as sample heterogeneity.Challenges:Very small sample volumesFast response times(180 and if 2=180,2,reduced BW,gain peaking

8、=,Butterworth ,reduced BW31 of 88Jens AndersISSCC 2025 Tutorial T1“Low-noise current sensing”FB,nom=50 M,BWCL=0.1 MHz,0=80 dB,p=1 pF,FB,opt=44 fF 2025 IEEE International Solid-State Circuits ConferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sens

9、or interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResistive TIAs StabilityBasic design equationsNoise fundamentalsNoise in resistive TIAsDesign of TIAs with capacitive feedbackCurrent-input modulatorsAlternative approaches:Current-to-X conv

10、ersionSummary and conclusionJens Anders32 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceNoise FundamentalsRecap:Deterministic inputs:=()()=()For random inputs,we do not want to relate individual realizations of the input random process(r

11、.p.)to individual realizations at the inputInstead,we want to relate the moments(time-domain)of the input r.p.to the moments of the output r.p.Assuming wide-sense stationarity,we obtain for the output autocorrelation:=d,where()=()()=(+)()dThe output power spectral density(PSD)(Wiener-Khintchine theo

12、rem)is given by:()=()2()Corresponding square-root spectra:=Jens Anders33 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”LTIsystemImpulse response()Frequency response()()()()()()()2025 IEEE International Solid-State Circuits ConferenceNoisy resistors and noisy diodesTo make use of the LTI syst

13、em theory from the previous slide,we have to replace our noisy circuit elements by noise-free elements and noise sources(Langevin theory)This slide shows the small-signal noise models of resistors and diodesNote:Ideal capacitors and inductors are noise-freeJens Anders34 of 88ISSCC 2025 Tutorial T1“L

14、ow-noise current sensing”R(noisy)R(noiseless)VnInR(noiseless)=4 (single-sided)=4 =4(single-sided)RdVnInGdnoisyID=4 2(single-sided)=1=+=4 2=2 (single-sided)2025 IEEE International Solid-State Circuits ConferenceNoisy transistors in saturationEquivalent model of a noisy MOS transistorThermal noise:2=4

15、 nD=4 nD m,nD=1.3 12=0.65 in weak inversion(WI)1.3 23 1 in strong inversion(SI)Flicker noise:2=4 (),()=,=4 Total input referred noise:nGtot2=4 nGtot,nGtot=+A large improves the input-referred voltage noiseJens Anders35 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”noisyIDnoiselessnoiselessVn

16、G VnG-tot InD 2025 IEEE International Solid-State Circuits ConferenceNoisy opampsTo model our TIA circuits,we do not just want to look at the noise from the individual transistors,resistors,etc.but rather use macro-noise-models of larger blocks such as opampsTo account for different source impedance

17、s and include the MOSFETs induced gate noise,we need the model shown on this slideHowever,in practice,we frequently only look at the equivalent input-referred voltage noise(will adopt this here)Jens Anders36 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”noisynoiselessIn+In-Vn 2025 IEEE Inter

18、national Solid-State Circuits ConferenceNoise in circuit analysisThe total output referred noise PSD can be calculated as(assuming uncorrelated noise sources and zero input signal)n,out()=1()2()The equivalent input-referred voltage noise produces the same output noise when applied to the inputThe in

19、put-referred noise,assuming a gain of()is given by:neq=nout 2Jens Anders37 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceExample calculationTotal output referred current noiseInout,tot=nM1+nM2+nM3+nM4Ini=4 mReferring this noise back to t

20、he input:neq=1m1/22 nout,totTotal input referred noise:neq=1m1/222 nM1/2+2 nM3/4=2 4 1m1/2neqM1/2 1+m3/4m1/2Design input devices in WI and load devices in SI for the best noise performanceJens Anders38 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits

21、 ConferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResistive TIAs StabilityBasic design equationsNoise fundamentalsNoise in resisti

22、ve TIAsDesign of TIAs with capacitive feedbackCurrent-input modulatorsAlternative approaches:Current-to-X conversionSummary and conclusionJens Anders39 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceResistive FB TIA Noise PerformanceModel

23、ing assumptions:Opamp current noise negligibleStandard Langevin noise modelingNoise from distinct components is uncorrelatedQuestions:Relative noise contributions/dominant noise source?Goal:Minimize the input-referred noiseJens Anders40 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE

24、 International Solid-State Circuits ConferenceNoise analysis output noiseApproach:Compute the output referred noise of each noise source and refer it back to the inputPerform a simple nodal analysis:0=in+p nRFB+x out FB+x out1FBout=op()nOPResulting noise TFs/voltage-noise spectraoutnRFB()=|TIA()|nRF

25、BoutnOP()=|TIA()|1+nOPFB,=1FB+FBNote:outnRFB()is the current noise of shaped by the TIA TFoutnOP()is the opamp voltage noise shaped by the TIA TF and an additional zeroJens Anders41 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceOutput vo

26、ltage noise PSDPoles of noise TFs are identical to TIA TF!With decreasing,noise peaking becomes more pronouncedOpamp noise has additional zeroZero in opamp noise TF at=1FB+FB1 FBWith increasing FB,the zero in the opamps noise TF moves to lower frequencies42 of 88Jens AndersISSCC 2025 Tutorial T1“Low

27、-noise current sensing”FB,nom=50 M,BWCL=0.1 MHz,0=80 dB,p=1 pF,FB,opt=44 fFvoutnOPvoutnRFBvoutntotvoutnOPvoutnRFBvoutntot 2025 IEEE International Solid-State Circuits ConferenceNoise analysis input noiseFrom slide 41:Output noise TFs/voltage-noise spectraoutnRFB()=|TIA()|nRFBoutnOP()=|TIA()|1+nOPFB,

28、=1FB+FBThe corresponding input noise spectra are:innRFB=nRFB=4 1innOP()=1+nOPFB,=1FB+FBNote:The current-noise of the feedback resistor appears unscaled at the inputThe voltage noise of the opamp is high-pass-filtered by and p|FBJens Anders43 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025

29、 IEEE International Solid-State Circuits ConferenceInput-referred current noise PSDnRFB=/FB Larger reduces current noiseLarger FBlowers low frequency input-referred opamp noise Larger FBalso lowers zero at which input-referred opamp noise rises does not alter the input referred noise(since FB p)44 o

30、f 88Jens AndersISSCC 2025 Tutorial T1“Low-noise current sensing”FB,nom=50 M,BWCL=0.1 MHz,0=80 dB,p=1 pF,FB,opt=44 fF,n,op=10nVHzineq,totineq,OPineq,RFBNo change w/CFB!ineq,OPineq,RFBineq,tot 2025 IEEE International Solid-State Circuits ConferenceIntegrated input-referred noiseLow BWs:Noise from domi

31、natesCorner BW exists beyond which opamp noise starts to dominateOpamp noise dominates for smalland very large values of Intermediate range in which noise from dominates can exist45 of 88Jens AndersISSCC 2025 Tutorial T1“Low-noise current sensing”FB,nom=50 M,BWCL=0.1 MHz,0=80 dB,p=1 pF,FB,nom=44 fF,

32、n,op=10nVHz324FBnOP2 1FB+pFBFB,nomFB=2BWCL FBBWint=20 BWCL 2025 IEEE International Solid-State Circuits ConferenceNoise analysis input noiseTIA TF:Designing for a maximally flat TF(good starting point):Input-referred noise of resistor and opamp:Design tradeoff:Increasing increases DC TIA gain,lowers

33、 input referred current noise due to and the noise floor of BUT increasing decreases the achievable bandwidth for a given and opamp GBWJens Anders46 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”TIA=outin=DC1+0+202DC=01+0 FB,0GBWFB+p FB,1+/FBGBW FB FBFB,opt2BWCL FB,GBWopt BWCL2+BWCL FBneq,RF

34、B=4FB,neq,OP=1+nOPFB,=1FB+FB 2025 IEEE International Solid-State Circuits ConferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResisti

35、ve TIAs and noise theoryDesign of TIAs with capacitive feedbackCurrent-input modulatorsAlternative approaches:Current-to-X conversionSummary and conclusionJens Anders47 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCapacitive FB TIA(C-TI

36、A)Basic idea:Reactive element in feedback is noise-freefundamental noise bandwidth tradeoff of resistive FB TIA is removedbut need to check the detailsBasic operation:Input current charges capacitor through the virtual ground at the opamp inputthe circuit works as an integrator Jens Anders48 of 88IS

37、SCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCapacitive TIA Transfer FunctionNeed FBto set the DC operating point and avoid saturation of the opamp outputCan view capacitive FB TIA as limiting case of resistive TIA with extremely large!For the

38、 TF becomes:TIA 00FB+111+1FB11+=GBW 0FB+0 FB+GBW FBFB+For very large the dominant pole moves towards DCIn the limit the circuit behaves like an integratorThe second pole is due to the role off of the opampJens Anders49 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Sol

39、id-State Circuits ConferenceFrom resistive to capacitive FB TIAIncreasing for a TIA initially designed for a maximally flat responsedominant pole moves towards lower frequenciesTIA TF looks more and more like an integratorJens Anders50 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE

40、International Solid-State Circuits ConferenceCapacitive TIA TF equalizationVery low-frequency pole(close to DC)most in-band frequencies are attenuated and need to be equalizedEqualizer circuit is a simple differentiator with TF:diff()=121+/p11+/p2,p1=111,p2=122Jens Anders51 of 88ISSCC 2025 Tutorial

41、T1“Low-noise current sensing”Assumptions:02 GBW2 2025 IEEE International Solid-State Circuits ConferenceCapacitive TIA Large signal behaviorIntegrator has very large low-frequency gain DC currents would saturate the opampneed to discharge FBto prevent this:continuously through periodically through a

42、 switchJens Anders52 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceFrequency-dependent dynamic rangeThe maximum DC current is given by max,DC=FS/DC(FS:full-scale of opamp)For AC signals,the dynamic range is either limited by the integrat

43、or opampmax,AC1inint=FS max,AC=FS inintOr by the differentiator opamp(for 1)max,AC1inint =FS max,AC=FS/int Jens Anders53 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceC-TIA Integrated input-referred noiseTotal input referred current nois

44、e:neqOP=1+nOPFB,=1FB+FB 0Jens Anders54 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”Same results as for resistive FB TIA w/very large 0=80 dB,GBW=10 MHz,p=1 pF,FB=1 pF,nOP=10 nV/Hz 2025 IEEE International Solid-State Circuits ConferenceCapacitive TIAs with DC servo loopsTo avoid that the DC

45、 part of current saturates the opamp,a servo loop can be constructed which removes the DC part of Basic idea:has large gain at DC and high attenuation at higher frequenciesIdeally,the noise floor of the DC servo loop is determined by DC.Then the previous analysis is applicableJens Anders55 of 88ISSC

46、C 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceComparison R-TIA and C-TIA(1/2)Jens Anders56 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”R-TIAID-TIA w/DC servo loopIn-band gainFBint Bandwidthmin=0,max=GBWFB+p FBmin=12DCFB,max=GBW FBFB+Inp

47、ut-referred noise PSD4FB+1+2nopFB2,=1FB+FB4DC+1+2nopDC2,=1FB+DC 2025 IEEE International Solid-State Circuits ConferenceComparison R-TIA and C-TIA(2/2)Jens Anders57 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”R-TIAID-TIA w/DC servo loopDynamic rangemax=FS/FBmax,DC=FS/DC(slide 53)max,AC=FS i

48、nint(integrator limited)FS/int(differentiator limited)2025 IEEE International Solid-State Circuits ConferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory in

49、cluding basic TIAsResistive TIAs and noise theoryDesign of TIAs with capacitive feedbackCurrent-input modulatorsAlternative approaches:Current-to-X conversionSummary and conclusionJens Anders58 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits Confere

50、nceCurrent-input modulatorsMotivation:Perform I-V-conversion and digitization in one step to improve energy efficiencyJens Anders59 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”+Compact+Power and Area efficient-Unwanted DC offsets affectAC DR+Unwanted DC offsets do not affect AC DR-Power an

51、d area inefficient 2025 IEEE International Solid-State Circuits Conference modulator fundamentals OversamplingOversampling improves the SQNR by the oversampling ratio OSROSR definition:OSR=(/2)/In-band quantization noise(IBN):IBN=bb2/12sd=2bs212=1OSR2/12SQNR of oversampled A/D converter for a signal

52、 power of maxSQNRdB=10log max 10log212+10log OSRJens Anders60 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”=2121:Quantizer step size 2025 IEEE International Solid-State Circuits Conference modulator fundamentals Noise shapingThe modulator(SDM)further improves SQNR by shaping the quantizatio

53、n noise,i.e.pushing it outside the band of interestTheoretical SQNR for modulatorsSNRmax=322 122mod+1OSR2mod+1Can improve resolution by increasing the modulator order(stability tradeoff)Jens Anders61 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits C

54、onferenceBasic current-input modulatorContinuous-time architecture to avoid anti-aliasing filterCurrent input by removing the input resistor of a voltage-input SDMSingle-bit internal quantizer for high intrinsic linearityTwo ways of looking at the circuit:A current-input modulator A capacitive TIA w

55、ith a feedback loop established by FB,whose average feedback current equals the average current of inJens Anders62 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceSystem-level design to circuit parametersTo find the optimum coefficients fo

56、r the DSM,one can resort to https:/www.sigma-delta.de/or the famous“Schreier toolbox”https:/ mapping of the coefficients is e.g.explained in M.Ortmanns,F.Gerfers,Continuous-Time Sigma-Delta A/D Conversion,SpringerJens Anders63 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE Internati

57、onal Solid-State Circuits ConferenceCurrent-input SDM STF and GBWSignal transfer function(STF)inside the band of interestSTFIB=outin=FBGBW requirements of the opampRC integrator:GBW 10 1FBintSDM:1FBint=1Including:GBW 10 1int+intJens Anders64 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”=DC1

58、+GBWDCM.Rajabzadeh et al.,Comparison Study of Integrated Potentiostats:Resistive-TIA,Capacitive-TIA,CT Modulator,2018 IEEE International Symposium on Circuits and Systems(ISCAS),Florence,Italy,2018,pp.1-5:sampling frequency1:system-level DAC feedback coefficient of the DSM 2025 IEEE International So

59、lid-State Circuits ConferenceCurrent-input SDM Usable frequency rangeThe SDM can operate over a frequency range from min=0(DC)to a maximum frequency limited by the sampling frequency and the OSR,i.e.max=(/2)/OSRFor a comparison of all TIA architectures,we can express the usable frequency range asmin

60、=0,max=2 OSRGBW201OSRintint+Jens Anders65 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceJitter in CT modulators The single-bit feedback is susceptible to clock jitter,which can greatly degrade the overall modulator performance Jens Ander

61、s66 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceJitter in CT modulators The single-bit feedback is susceptible to clock jitter,which can greatly degrade the overall modulator performance FIR feedback for improved jitter immunityJens An

62、ders67 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceJitter in CT modulators The single-bit feedback is susceptible to clock jitter,which can greatly degrade the overall modulator performance FIR feedback for improved jitter immunityJens

63、 Anders68 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceDynamic range of current-input SDMsIn many applications,the signal current is accompanied by a(large)DC offset current that reduces the SDMs usable dynamic rangeCan introduce a DC s

64、ervo loop to process the offset current separately Jens Anders69 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”+Compact+Power&Area efficient+Unwanted DC offsets do not affect SDM DR 2025 IEEE International Solid-State Circuits ConferenceA real-world implementationThe single-bit feedback is s

65、usceptible to clock jitter,which can greatly degrade the overall modulator performance FIR feedback for improved jitter immunityActive LPF to implement DC servo loop for DR enhancementJens Anders70 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits Con

66、ferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResistive TIAs and noise theoryDesign of TIAs with capacitive feedbackCurrent-input

67、modulatorsAlternative approaches:Current-to-X conversionCurrent conveyorsCurrent-to-frequency conversionSummary and conclusionJens Anders71 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceAlternative techniques 1:Current conveyorsCurrent-m

68、ode circuits can directly process the currentNo need for a transimpedance to go to the voltage domainAn example was the current-input modulatorA very useful type of a current-mode circuit for current sensing are so-called current conveyors(CCs)Advantages of current conveyorsNo need for a high-perfor

69、mance closed-loop amplifier Scale benignly into advanced CMOS nodes due to the small voltage signals as a result of the current inputs and outputsNo need for precision passive componentsProvide a good tradeoff in terms of bandwidth and accuracyFun fact:It can be shown that all active devices can be

70、implemented using one or two CCsJens Anders72 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCurrent conveyor II-Working principleJens Anders73 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”A second-generation current conveyor(CCI

71、I)canbe represented by the following transfer matrix=000100010Voltage follows voltage Node Y has an infinite(input)impedanceNode Z has infinite(output)impedance and carries the same(or a 180-phase shifted version)of This makes CCIIs attractive for current sensingLow input impedance(not loading the s

72、ensor)High output impedance(good for driving successivecurrent-mode circuits)Well-defined bias voltage for the sensor1:1 2025 IEEE International Solid-State Circuits ConferenceCurrent conveyor II Circuit implementation Transistors Mn1-Mn2-Mp1-Mp2 form a translinearloop(TL)TL for MOS transistors in w

73、eak inversion impliesccw,GS,=cw,GS,ccw,=cw,Here,assuming equal aspect ratios and in=0,the TL ensuresMn1=Mp1=Moreover,the TL ensures Class-AB push-pull output stageCurrent gain of Jens Anders74 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits Conferen

74、ceCurrent conveyor II Small signal propertiesThe TF vs.frequency is given by(inis the input capacitance)outin=1+ininThe input resistance is given byin1mn+mpGm can be small for small bias currents,can increase(power and noise tradeoff)or use the feedback topology on the right Jens Anders75 of 88ISSCC

75、 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCurrent conveyor II NoiseThe input-referred noise of the CCII is given byneq,CC2=4+24+21+in22Can use chopping to eliminate Flicker noiseThe input-referred noise of the CCII with feedback OTA is given b

76、yneq,FB2=neq,CC21+022+,21+2where 0is the OTA gain and is the outputresistance of the sensorThe CC input-referred noise scales with and needs to be minimized at the cost of an increased power consumptionJens Anders76 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-

77、State Circuits ConferenceOutlineMotivation:Applications of low-noise current interface circuitsRequirements for current sensor interfacesFundamentals of current-to-voltage conversionReview of basic feedback theory including basic TIAsResistive TIAs and noise theoryDesign of TIAs with capacitive feed

78、backCurrent-input modulatorsAlternative approaches:Current-to-X conversionCurrent conveyorsCurrent-to-frequency conversionSummary and conclusionJens Anders77 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceAlternative Techniques:Current-to

79、-FrequencyAdvantage of this readout?Direct quantization of the current No amplifier+ADC needed Saves area+powerUse the timing infoSuited for technologies with scaled voltage headroomMostly used in area-critical applications(implantable circuits,IoT)Pulse/Time-based output is compatible with certain

80、low-power TX protocols(e.g.load shift keying),which are used in IoT applicationsTwo main circuits can be used:Current-Starved Ring Oscillator,current controlled oscillator(CCO)Duty-Cycle ModulationJens Anders78 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State

81、 Circuits ConferenceCurrent-to-Frequency:CCO Readout Current-Controlled OscillatoroOften a current-starved inverter-based ring-oscillator is usedoEasy to implement in CMOSoLow area Readout is performed by counting the edges of the oscillator with a counterCCO acts as an integratoroFirst-order noise

82、shaping for freeoCan be used in combinationwith other SDM techniques(see next slides)Readout can be open or closedloopJens Anders79 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:CCO based SDMThe CCO provides inherent

83、 first-order noise shapingFor higher accuracy,the CCO can be placed in a higher-order SDM modulator topologyFor example:Many variations are possible:True higher-order SDM(Park,ASSCC 2022)MASH DSM:Forward the quantization error to a second stage(Sacco,JSSC 2020)Jens Anders80 of 88ISSCC 2025 Tutorial

84、T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:CCO ReadoutOpen loop readout Directly feed the current to the oscillatorOscillator has to work over full input-current rangeMain challenges with CCOCCO is usually PVT sensitive,but can be compens

85、atedwith reference oscillator The non-linear I-F characteristic of the CCO limits the linearity of the readout Jens Anders81 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”Y.Liao et al.,A 3-W CMOS Glucose Sensor for Wireless Contact-Lens Tear Glucose Monitoring,in IEEE Journal of Solid-State

86、Circuits,vol.47,no.1,pp.335-344,Nov.2012 2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:CCO ReadoutThe intrinsic CCO linearity limits the useful DR of the readoutIncreasing the linearity requires closed-loop readoutFeedback is requiredHowever Feedback is only as accurate

87、 as the feedback factorHence,a sufficiently linear DAC requires DEM,DWA High OSR requiredThis limits the BW and/or increases the power consumption of the readout!Jens Anders82 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”P.Prabha et al.,A Highly Digital VCO-Based ADC Architecture for Curren

88、t Sensing Applications,in IEEE Journal of Solid-State Circuits,vol.50,no.8,pp.1785-1795,Aug.2015 2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:Duty-Cycle ModulationThe current from the sensor is provided by a current mirror By the modulation of the duty cycle of the osc

89、illator,the sensor current is digitizedCombining and according to+=1+refsensor,the effect of DDand intcan be removedJens Anders83 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”M.Ahmadi and G.A.Jullien,“A wireless-implantable microsystem for continuous blood glucose monitoring,”IEEE Trans.Bio

90、med.Circuits Syst.,vol.3,no.3,pp.169180,Jun.2009.H.Jiang,X.Zhou,S.Kulkarni,M.Uranian,R.Seenivasan and D.A.Hall,A Sub-1 W multiparameter injectable BioMote for continuous alcohol monitoring,2018 IEEE Custom Integrated Circuits Conference(CICC),San Diego,CA,USA,2018,pp.1-4=(DD int)/(2 sensor)=(DD int)

91、/(2 ref)2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:Duty-Cycle ModulationReadout accuracy is determined by the reference current,without dependence on DDand int:=(DD int)/(2 sensor),=(DD int)/(2 ref)+=1+refsensorWhat limits this accuracy?Jens Anders84 of 88ISSCC 2025

92、Tutorial T1“Low-noise current sensing”M.Ahmadi and G.A.Jullien,“A wireless-implantable microsystem for continuous blood glucose monitoring,”IEEE Trans.Biomed.Circuits Syst.,vol.3,no.3,pp.169180,Jun.2009.H.Jiang,X.Zhou,S.Kulkarni,M.Uranian,R.Seenivasan and D.A.Hall,A Sub-1 W multiparameter injectable

93、 BioMote for continuous alcohol monitoring,2018 IEEE Custom Integrated Circuits Conference(CICC),San Diego,CA,USA,2018,pp.1-4 2025 IEEE International Solid-State Circuits ConferenceCurrent-to-Frequency:Duty-Cycle ModulationCircuit non-idealitiesNoise of the comparators cause variations in duty-cycle

94、Offset variations of the two comparators introduce non-linearityDelay variations of the comparators lead to distortionOverall only offer limited DR and BWJens Anders85 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceLow leakage PCB design

95、guidelinesSensor and AFE are typically not on the same dieHave to make sure that no current is lost in the interconnectUse guard-ring at correct potential to avoid leakage currentsJens Anders86 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits Confere

96、nceSummary and conclusionMany existing and upcoming applications require the readout and digitization of currentsEspecially upcoming applications in nanochemistry and quantum technologies require the readout of tiny currents with high accuracy and a high temporal resolutionSuch applications require

97、high-performance feedback TIAsFeedback TIAs can be implemented using resistive or capacitive feedback with different tradeoffs in terms of noise,requirements on the used opamp,and circuit complexityCurrent-input DSMs are an attractive solution to perform the current-to-voltage conversion and digitiz

98、ation in one blockCurrent-to-frequency converters are an attractive solution for medium-resolution readouts(around 10 bits)and low energy and area consumptionJens Anders87 of 88ISSCC 2025 Tutorial T1“Low-noise current sensing”2025 IEEE International Solid-State Circuits ConferenceKey References1 Ros

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102、iver,”ISSCC 2003,paper 4.2.Expanded version in JSSC,Dec.2003,pp.2094-2100.7 N.Nedovic et al.,“A 40-to-44Gb/s 3x Oversampling CDR 1:16/DEMUX,”ISSCC 2007,paper 12.2.Expanded version in JSSC,Dec.2007,pp.2726-2735.8 M.van Ierssel et al.,“A 3.2Gb/s Semi-Blind-Oversampling CDR”,ISSCC 2006,paper 18.5.Expanded version in JSSC,Oct.2007,pp.2224-2234.9 T.H.Lee et al.,“A 155-MHz Clock Recover-and Phase-Locked Loop,”JSSC,Dec.1992,pp.1736-1746.10 J.McNeill,“Jitter in Ring Oscillators,”JSSC,June 1997,pp.870-879.88 of 88Jens AndersISSCC 2025 Tutorial T1“Low-noise current sensing”