Capacitively-coupled Chopper Amplifiers

126 Pages·2017·3.98 MB·english

by  Qinwen Fan, Kofi A.A. Makinwa, Johan H. Huijsing

fi
Qinwen Fan Ko A.A. Makinwa
(cid:129)
Johan H. Huijsing
Capacitively-Coupled
fi
Chopper Ampli ers
123
Qinwen Fan JohanH.Huijsing
Mellanox Technologies Schipluiden
Delfgauw TheNetherlands
TheNetherlands
KofiA.A.Makinwa
Delft University of Technology
Delft, Zuid-Holland
TheNetherlands
ISSN 1872-082X ISSN 2197-1854 (electronic)
Analog CircuitsandSignal Processing
ISBN978-3-319-47390-1 ISBN978-3-319-47391-8 (eBook)
DOI 10.1007/978-3-319-47391-8
LibraryofCongressControlNumber:2016955322
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Contents
1 Introduction.... .... .... ..... .... .... .... .... .... ..... .... 1
1.1 Problem ... .... .... ..... .... .... .... .... .... ..... .... 1
1.2 Traditional Solutions.. ..... .... .... .... .... .... ..... .... 1
1.3 A Promising Solution: Capacitively Coupled Chopper
Amplifier .. .... .... ..... .... .... .... .... .... ..... .... 4
1.4 Challenging Issues ... ..... .... .... .... .... .... ..... .... 6
1.5 Organization.... .... ..... .... .... .... .... .... ..... .... 7
References.. .... .... .... ..... .... .... .... .... .... ..... .... 8
2 The Chopping Technique. ..... .... .... .... .... .... ..... .... 11
2.1 Basic Working Principle.... .... .... .... .... .... ..... .... 11
2.2 Basic Chopper Amplifier Topologies .. .... .... .... ..... .... 12
2.2.1 Basic Chopper Opamp and Instrumentation Amplifier
Topologies ... ..... .... .... .... .... .... ..... .... 12
2.2.2 Chopper Stabilization .... .... .... .... .... ..... .... 14
2.3 Ripple-Reduction Techniques.... .... .... .... .... ..... .... 16
2.3.1 The Switched-capacitor (SC) Notch Filter. .... ..... .... 16
2.3.2 AC-Coupled Ripple-Reduction Loop. .... .... ..... .... 17
2.3.3 Auto-Correction Feedback Loop.... .... .... ..... .... 19
2.3.4 Digitally Assisted Trimming... .... .... .... ..... .... 20
2.3.5 Chopping + Auto-Zeroing. .... .... .... .... ..... .... 20
2.4 Chopping Non-idealities.... .... .... .... .... .... ..... .... 25
2.5 Chopping Pros and Cons ... .... .... .... .... .... ..... .... 26
2.6 Conclusions .... .... ..... .... .... .... .... .... ..... .... 26
References.. .... .... .... ..... .... .... .... .... .... ..... .... 27
3 Capacitively Coupled Chopper Amplifiers. .... .... .... ..... .... 29
3.1 Capacitively Coupled Chopper Opamps (CCOPA).... ..... .... 29
3.1.1 Offset and 1/f Noise . .... .... .... .... .... ..... .... 30
3.1.2 Noise and Power Efficiency ... .... .... .... ..... .... 30
v
vi Contents
3.1.3 Common-mode Rejection Ratio (CMRR)
and Common-mode Voltage Range (CMVR) .. ..... .... 31
3.1.4 Input Impedance.... .... .... .... .... .... ..... .... 31
3.1.5 Settling and Transient Issues... .... .... .... ..... .... 32
3.2 Capacitively Coupled Chopper IAs (CCIA) . .... .... ..... .... 33
3.2.1 Offset and 1/f Noise . .... .... .... .... .... ..... .... 33
3.2.2 Noise and Power Efficiency ... .... .... .... ..... .... 33
3.2.3 CMRR and CMVR.. .... .... .... .... .... ..... .... 34
3.2.4 Gain Accuracy ..... .... .... .... .... .... ..... .... 34
3.2.5 Input Impedance.... .... .... .... .... .... ..... .... 35
3.2.6 Output Spikes. ..... .... .... .... .... .... ..... .... 35
3.2.7 Settling and Transient Issues... .... .... .... ..... .... 35
3.3 Conclusions .... .... ..... .... .... .... .... .... ..... .... 35
References.. .... .... .... ..... .... .... .... .... .... ..... .... 36
4 Choppers for High Input Common-Mode Voltages . .... ..... .... 37
4.1 Choice of Transistors . ..... .... .... .... .... .... ..... .... 37
4.2 High-voltage (HV) Chopper Topologies.... .... .... ..... .... 39
4.2.1 HV Chopper with HV Amplifier Level-Shifter. ..... .... 39
4.2.2 Capacitively Coupled HV Choppers. .... .... ..... .... 40
4.3 Transient Protection .. ..... .... .... .... .... .... ..... .... 43
4.4 Conclusions .... .... ..... .... .... .... .... .... ..... .... 45
References.. .... .... .... ..... .... .... .... .... .... ..... .... 45
5 Capacitively-Coupled Chopper Operational Amplifiers .. ..... .... 47
5.1 Introduction .... .... ..... .... .... .... .... .... ..... .... 47
5.2 Conventional Techniques to Expand the CMVR . .... ..... .... 48
5.3 The Single-Path Capacitively-coupled Operational
Amplifier (CCOPA) .. ..... .... .... .... .... .... ..... .... 49
5.3.1 Design of the Single-Path CCOPA.. .... .... ..... .... 49
5.3.2 Implementation of the Basic CCOPA.... .... ..... .... 53
5.3.3 Experimental Results. .... .... .... .... .... ..... .... 59
5.4 Multipath CCOPA ... ..... .... .... .... .... .... ..... .... 61
5.4.1 Design of the Multipath CCOPA (MCCOPA) . ..... .... 61
5.4.2 Implementation of the Multipath CCOPA. .... ..... .... 64
5.4.3 Experimental Results of the MCCOPA... .... ..... .... 66
5.5 Conclusions .... .... ..... .... .... .... .... .... ..... .... 68
References.. .... .... .... ..... .... .... .... .... .... ..... .... 69
6 Capacitively Coupled Chopper Instrumentation Amplifiers
for High-Side Current Sensing.. .... .... .... .... .... ..... .... 71
6.1 Introduction .... .... ..... .... .... .... .... .... ..... .... 71
6.2 Overview of the State of the Art . .... .... .... .... ..... .... 72
6.2.1 HV Chopper-Stabilized Current Feedback
Instrumentation Amplifier. .... .... .... .... ..... .... 72
Contents vii
6.2.2 HV Current-Mode Three-Opamp Instrumentation
Amplifier. .... ..... .... .... .... .... .... ..... .... 72
6.2.3 HV IA with Isolated Transformer... .... .... ..... .... 74
6.2.4 Conclusions... ..... .... .... .... .... .... ..... .... 74
6.3 Design of the CCIA for Current-Sensing Applications. ..... .... 75
6.3.1 Input Chopper. ..... .... .... .... .... .... ..... .... 76
6.3.2 Ripple-Reduction Loop (RRL) . .... .... .... ..... .... 76
6.3.3 CCIA Opamp . ..... .... .... .... .... .... ..... .... 78
6.3.4 Output Spikes. ..... .... .... .... .... .... ..... .... 78
6.4 Realization . .... .... ..... .... .... .... .... .... ..... .... 79
6.4.1 Global Parameters (Chopping Frequency and Capacitor
Bridge) .. .... ..... .... .... .... .... .... ..... .... 79
6.4.2 Implementation of the Input Chopper.... .... ..... .... 80
6.4.3 Implementation of the CM Biasing Circuit.... ..... .... 80
6.4.4 Implementation of the CCIA Opamp .... .... ..... .... 83
6.4.5 Implementation of the Output S&H Switch.... ..... .... 86
6.4.6 Implementation of the RRL.... .... .... .... ..... .... 86
6.5 Experimental Results . ..... .... .... .... .... .... ..... .... 88
6.6 Conclusions .... .... ..... .... .... .... .... .... ..... .... 91
References.. .... .... .... ..... .... .... .... .... .... ..... .... 92
7 Capacitively Coupled Chopper Instrumentation Amplifiers
for Low-Voltage Applications... .... .... .... .... .... ..... .... 93
7.1 Introduction .... .... ..... .... .... .... .... .... ..... .... 93
7.2 Overview of the State of the Art . .... .... .... .... ..... .... 95
7.2.1 State-of-the-Art Precision IAs for DC Sensing . ..... .... 95
7.2.2 State-of-the-Art IAs for AC Biomedical Sensing..... .... 97
7.2.3 Conclusions... ..... .... .... .... .... .... ..... .... 100
7.3 Design of a CCIA for Wireless Sensor Nodes ... .... ..... .... 100
7.3.1 Input Impedance Boosting Loop.... .... .... ..... .... 101
7.3.2 SC Ripple Reduction Loop (SC RRL) ... .... ..... .... 102
7.3.3 DC Servo Loop (DSL) ... .... .... .... .... ..... .... 103
7.4 Realization . .... .... ..... .... .... .... .... .... ..... .... 108
7.4.1 Global Parameters (Chopping Frequency, Capacitive
Bridge, and Transistor Type)... .... .... .... ..... .... 108
7.4.2 Opamp of the CCIA. .... .... .... .... .... ..... .... 110
7.4.3 Biasing Resistor R .. .... .... .... .... .... ..... .... 111
b
7.4.4 Impedance Boosting Loop or Positive Feedback Loop
(PFL) ... .... ..... .... .... .... .... .... ..... .... 111
7.4.5 SC RRL . .... ..... .... .... .... .... .... ..... .... 113
7.4.6 DC Servo Loop (DSL) ... .... .... .... .... ..... .... 115
7.5 Experimental Results . ..... .... .... .... .... .... ..... .... 116
7.6 Conclusion. .... .... ..... .... .... .... .... .... ..... .... 119
References.. .... .... .... ..... .... .... .... .... .... ..... .... 120
viii Contents
8 Conclusions .... .... .... ..... .... .... .... .... .... ..... .... 123
8.1 Conclusions .... .... ..... .... .... .... .... .... ..... .... 123
8.2 Original Contributions ..... .... .... .... .... .... ..... .... 124
References.. .... .... .... ..... .... .... .... .... .... ..... .... 125
Chapter 1
Introduction
1.1 Problem
Inmanymeasurementsituations,thesignalofinterestissmall,possiblyintherange
of tens of microvolts, and is superimposed on a much larger DC common-mode
(CM) signal, possibly in the range of several volts. Coping with such a large CM
signal and at the same time accurately measuring such small signals is a big
challenge for interface circuits. A good example of such a measurement is in
high-side current sensing [1], as shown in Fig. 1.1, where the load current of a
battery is monitored by inserting a small sensing resistor R in series with the
sense
battery.Thus,thecurrentcanbedeterminedfromtheDCvoltagedropV across
sense
the resistor. To minimize its power consumption, R is usually very small
sense
(hundredsofmilliohm)andthus,V isalsosmall,typicallyrangingfromtensof
sense
microvolts to hundreds of millivolts. This requires a readout amplifier with low
offset and low 1/f noise. Moreover, V is accompanied by a large CM voltage,
sense
whichcanbeaslargeas30 Vinthecaseofalaptopbattery.Thisisfarbeyondthe
supplyvoltagesofnormalCMOScircuitry.Thus,novelcircuittechniquestoreject
this large CM voltage and accurately measure V must be found. This problem
sense
becomes more challenging as CMOS technology advances, since this has histori-
cally been accompanied by a steady decrease in supply voltages.
1.2 Traditional Solutions
When a large input CM voltage must be rejected, the best solution is to block it.
A first approach involves the use of a magnetic coupling [2–5]. The basic block
diagram[2]ofareadoutsystememployingmagneticcouplingisshowninFig. 1.2.
Itconsistsofaninputandoutputmodulator,atransformer,andareadoutamplifier.
In this case, the modulators are implemented as choppers, i.e., polarity-reversing
©SpringerInternationalPublishingAG2017 1
Q.Fanetal.,Capacitively-CoupledChopperAmplifiers,
AnalogCircuitsandSignalProcessing,DOI10.1007/978-3-319-47391-8_1
2 1 Introduction
Fig.1.1 Asimplified
schematicofahigh-side
current-sensingreadoutcircuit
Fig.1.2 Aschematicof
magneticisolationreadout
circuitforcurrent-sensing
application
switchesdrivenbyadigitalclocksignalwithafixedfrequency(f )[6–9].In[2],
chop
the choppers are also driven via an isolating transformer. The input chopper con-
verts the DC differential voltage V into a square wave, and the output chopper
sense
convertstheamplifiedsquarewavebacktoDC.Inthisway,thedifferentialsignalis
firstmodulatedtohighfrequencybytheinputchopperandsocanbecoupledtothe
inputofthereadoutamplifierviaatransformer.TheDCCMvoltage,however,will
not be modulated and thus will not be coupled to the readout amplifier.
Furthermore, theoffsetand1/fnoiseofthereadoutamplifierwillbeup-modulated
by the output chopper and so can be filtered out. A big disadvantage of this
approach, however, is that transformers are difficult to integrate on chip. Although
integrated microtransformers [2] can be realized in some processes, they tend to
occupy a lot of chip area.
AsecondapproachinvolvesisolatingtheDCCMvoltageoptically,e.g.,withan
opto-isolator [10–13]. Although there are many types of opto-isolator, the most
common type simply consists of an LED and a photodiode as shown in Fig. 1.3.
The LED is connected to the input signal and converts it into an optical signal,
which is then picked up and converted back to an electrical signal by the photo-
diode and a readout amplifier. In this way, the input CM voltage is completely
isolated from the readout amplifier. The main disadvantage of this approach,
however,isitslackofaccuracy.ThesignaltransferfunctionbetweentheLEDand
the photodiode depends on several parameters such as the voltage-to-light transfer
1.2 TraditionalSolutions 3
Fig.1.3 SchematicofCM
isolationwithopto-isolator
Fig.1.4 CMisolationwith
basicdifferentialpair
functionoftheLED,theintensityofthelightpickedupbythephotodiode,andthe
light-to-voltage transfer function of the photodiode. These parameters can be dif-
ficult to control and reproduce accurately in large-scale production. Thus, the
accuracy of the measurement, especially the system’s overall gain, is not well
defined. Moreover, the linearity of the signal is often low, which requires an extra
feedback circuit [4].
Athirdapproach,whichisthemostcommonlyused,istoisolatetheCMvoltage
electrically. A simple way to do this is to use the basic differential pair shown in
Fig. 1.4.Neglectingcircuitnon-idealities,theinputdifferentialpairactsasanideal
voltage to current converter, which floats between the tail current source and a
potential close to ground. Since it is only sensitive to the input differential signal,
theCMsignaliscompletelyisolatedfromtherestofthecircuit.Inreality,however,
circuitnon-idealitiessuchasmismatchwillsignificantlydecreasethemeasurement
accuracy.Toimprovethis,thechoppingtechniquecanagainbeappliedasshownin
Fig. 1.5. Since the input chopper only modulates the differential input signal, its
contribution to the output voltage is separated from that of the CM signal in the
frequency domain. The offset and 1/f noise are also up-modulated and so are
removed from the base band. However, the maximum CM voltage that can be
handled is determined by the differential pair’s supply. So handling a large CM
voltage requires a large supply voltage and, consequently, high-voltage input
transistors in the input stage. This increases the power consumption considerably
[14,15].Moreover,thelimitedoutputimpedanceoftheinputtransistorswillreduce
the circuit’s CM immunity, while their offset and 1/f noise will reduce its DC
4 1 Introduction
Fig.1.5 CMisolationwith
chopperdifferentialpair
Fig.1.6 SchematicofCM
isolationwiththeflying
capacitor
precision. Finally, the common-mode voltage range (CMVR) of the circuit shown
in Fig. 1.5 will not cover both the negative and positive rails [14, 15] (Fig. 1.3).
Another approach to isolate the input CM voltage is to use a so-called flying
capacitor [16] to sample and hold the input signal, as shown in Fig. 1.6. The CM
voltageattheinputofthesucceedingamplifierissetbyafeedbackresistornetwork
and so it can be realized with low-voltage circuitry. However, the kT/C noise
associatedwiththesample-and-holdactionoftheflyingcapacitorincreasesthetotal
input-referred noise. Moreover, continuous-timeoperation isnotpossible. Last but
notleast,theinputswitchesmustoncemorebeabletohandlethelargeCMsignal.
From the above introduction, it seems that none of these approaches is very
satisfactory. Thus, a new approach is required.
1.3 A Promising Solution: Capacitively Coupled
Chopper Amplifier
An intuitively appealing solution to the problem of CM isolation is the use of
capacitive coupling. Capacitors are widely available in most standard CMOS
process and exhibit a natural ability to block DC signals without any extra power
consumption. Thus, a capacitively coupled amplifier will perfectly reject DC CM
voltages, as long as they are smaller than the breakdown voltage of the coupling
capacitors (Fig. 1.7). Although the breakdown voltages of on-chip capacitors is
usuallylessthan100 V,thisisstillsufficientformanyapplications.However,itis