Analysis of the Switched-Capacitor Dual-Slope Capacitance-to-Digital Converter
08 Jan 2010-IEEE Transactions on Instrumentation and Measurement (IEEE)-Vol. 59, Iss: 5, pp 997-1006
TL;DR: A dual-slope capacitance-to-digital converter that operates on the elements of a differential capacitive sensor and provides a digital output that is linearly proportional to the physical quantity being sensed by the sensor is presented and analyzed in this paper.
Abstract: A dual-slope capacitance-to-digital converter (CDC) that operates on the elements of a differential capacitive sensor and provides a digital output that is linearly proportional to the physical quantity being sensed by the sensor is presented and analyzed in this paper. The converter topology is so chosen that a linear digital output is obtained for not only a sensor possessing linear input-output characteristics but also a sensor possessing inverse characteristics. The digital output in the proposed converter is dependent only on, apart from the sensitivity of the sensor, a dc reference voltage. Hence, high accuracy and linearity are easily obtained by employing a precision dc reference. Since the proposed CDC is based on the popular dual-slope analog-to-digital converter structure, it possesses all the advantages (resolution, accuracy, and immunity to noise and component parameter variations) and limitations (requirement of auto-zero and low conversion speed) applicable to the dual-slope technique. A prototype built and tested for a typical differential capacitive sensor with a nominal capacitance value of 250 pF gave a worst-case error of less than 0.05%.
Citations
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TL;DR: Continuous measurement can be achieved with an implanted monitor to improve treatment regiments, assess patient compliance to medication schedules, and prevent unnecessary vision loss.
Abstract: Circuit blocks for a 1.5 mm3 microsystem enable continuous monitoring of intraocular pressure. Due to power and form-factor limitations, circuit blocks are designed at nanowatt power levels not completely explored before. The system includes a 75% efficient 90 nW DC-DC converter which is the most efficient reported sub- μW converter in literature. It also includes a novel 4.7 nJ/bit FSK radio that achieves 10 cm of transmission range at 10 -6 BER which is also the lowest number reported for short-range through-tissue wireless links for biomedical implants. A MEMS capacitive sensor and ΣΔ capacitance-to-digital converter measure IOP with 0.5 mmHg accuracy. A microcontroller processes and saves IOP data and stores it in a 2.4 fW/bitcell SRAM. The microsystem harvests a maximum power of 80 nW in sunlight with a light irradiance of 100 mW/cm2 AM 1.5 from an integrated 0.07 mm2 solar cell to recharge a 1 mm2 1 μAh thin-film battery and power the load circuits. The design achieves zero-net-energy operation with 1.5 hours of sunlight or 10 hours of bright indoor lighting daily.
272 citations
07 Apr 2011
TL;DR: Circuit blocks for a 1.5 mm3 microsystem enable continuous monitoring of intraocular pressure at nanowatt power levels not completely explored before and achieves zero-net-energy operation with 2.5 hours of sunlight or 10 hours of bright indoor lighting daily.
Abstract: Glaucoma is the leading cause of blindness, affecting 67 million people worldwide. The disease damages the optic nerve due to elevated intraocular pressure (IOP) and can cause complete vision loss if untreated. IOP is commonly assessed using a single tonometric measurement, which provides a limited view since IOP fluctuates with circadian rhythms and physical activity. Continuous measurement can be achieved with an implanted monitor to improve treatment regiments, assess patient compliance to medication schedules, and prevent unnecessary vision loss. The most suitable implantation location is the anterior chamber of the eye, which is surgically accessible and out of the field of vision. The desired IOP monitor (IOPM) volume is limited to 1.5mm3 (0.5x1.5x2mm3) by the size of a self-healing incision, curvature of the cornea, and dilation of the pupil.
268 citations
TL;DR: This paper examines how the geometry of the PCB-made interdigited electrode structure influences the characteristics of the sensor, and discusses a circuit that converts the changes in the sensor's capacitance into variations in the width of a pulse signal.
Abstract: Capacitors are used as sensors in several applications. Typically, the quantity to be measured changes either the geometric properties of the structure used as a capacitive sensor or the physical properties of the dielectric between the electrodes of that structure. On the other hand, if the sensor geometry maximizes the capacitance due to fringing, then the possibilities are totally different. In this case, the electric field can be projected onto the test sample. This paper covers the usage of fringing field sensors to measure the water present in the soil. We examine how the geometry of the PCB-made interdigited electrode structure influences the characteristics of the sensor. However, fringing field sensors are difficult to be analytically modeled due to its intrinsic nonlinear characteristic. Therefore, our conclusions were mainly based on results of simulations, but we also tried to reproduce the results of the simulations experimentally. Also, we discuss a circuit that converts the changes in the sensor's capacitance into variations in the width of a pulse signal. Finally, the proposed solution has been experimentally compared with the standard gravimetric method to quantify the water content in the soil. The results were encouraging since the proposed sensor's measurements fit to the experimental data with a coefficient of determination R
2
= 0.94.
51 citations
Additional excerpts
...In fact, in this paper, one can find several other circuits for capacitance measurement, but most of them use techniques that are most appropriate for integrated circuit design (such as the use of switched capacitors) [20]–[23]....
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TL;DR: A novel signal conditioning scheme, which provides a linear-digital output directly from the resistive sensor(s) that are connected in such bridge configurations, and drastically reduces the effect on the output due to the lead wires that connect the Wheatstone bridge and the DSADC.
Abstract: Output of a typical Wheatstone bridge, when it is connected to measure from a single or a dual resistive element, possesses non-linear characteristic. This paper presents a novel signal conditioning scheme, which provides a linear-digital output directly from the resistive sensor(s) that are connected in such bridge configurations. In the present scheme, the input stage of a dual-slope analog-to-digital converter (DSADC) is suitably augmented to incorporate the quarter-bridge and (or) half-bridge containing the resistive sensor as an integral part of the DSADC. A combination of the current mode excitation and wisely selected integration and de-integration operations of the DSADC enable to achieve linearization in the digitization process itself, leading to an overall reduction in the complexity level and number of blocks used keeping the high accuracy unaltered. A detailed analysis has been conducted to quantify the effect of various sources of errors in the output of the DSADC. The details are presented in the paper. The proposed method not only provides a linear digital output but also drastically reduces the effect on the output due to the lead wires that connect the Wheatstone bridge and the DSADC. Thus, the proposed scheme is well suited for the situations where the sensor(s) is (are) remotely located at a distance. Simulation studies as well as results from a prototype developed and tested establish the practicality of the proposed scheme. The inherent non-linearity of the Wheatstone bridge is reduced by nearly two orders of magnitude.
49 citations
Cites methods from "Analysis of the Switched-Capacitor ..."
...As in a typical dual slope technique, here too, to initiate a conversion, the digitizer has to invoke an auto-zero phase to set the output to zero [26]....
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TL;DR: The proposed impedance-to-digital converter is based on a dual-slope technique, and hence provides high accuracy and immunity to noise and interference, and a prototype of the proposed IDC has been developed and tested in the laboratory.
Abstract: A new, simple and high-accuracy impedance-to-digital converter (IDC) is proposed in this paper. Conventionally, the parameters of sensors that can be modeled using a parallel combination of a capacitor ( ${C}$ ) and a resistor ( ${R}$ ) are measured using ac bridges, excited from a sinusoidal source. Recently, with the widespread use of digital systems in instrumentation, capacitance-to-digital converters and resistance-to-digital converters gained a lot of importance. An IDC that accepts sensors having ${C}$ and ${R}$ in parallel and provides digital outputs directly proportional to the ${C}$ and ${R}$ values is presented in this paper. This can be used not only for sensors whose ${C}$ and ${R}$ values vary with the measurand but also when ${C}$ or ${R}$ of a sensor needs to be measured keeping the output not affected by parasitic ${R}$ or ${C}$ present in parallel with the sensing element. Another application of the IDC is in the measurement of the dissipation factor of dielectric materials. The proposed IDC is based on a dual-slope technique, and hence provides high accuracy and immunity to noise and interference. A prototype of the proposed IDC has been developed and tested in the laboratory. Accuracy of the prototype IDC developed was 0.15% for the measurement of ${C}$ and 0.04% for the measurement of ${R}$ . The total conversion time of the prototype converter developed is 3 s, and its total power dissipation is 175.8 mW. The IDC was also interfaced with a polymer-based impedance humidity sensor, measured its ${C}$ and ${R}$ values for a range of humidity, computed the humidity, and compared its performance with another instrument, showing the practicality of the proposed IDC.
41 citations
Cites methods from "Analysis of the Switched-Capacitor ..."
...Capacitance-to-digital converters (CDCs) based on charge balancing [17], dual-slope [18], and sigma-delta [19] methods have been reported....
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References
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Abstract: Part 1 General Concepts 1 Types of Applications of Measurement Instrumentation 2 Generalized Configurations and Functional Descriptions of Measuring Instruments 3 Generalized Performance Characteristics of Instruments Part 2 Measuring Devices 4 Motion and Dimensional Measurement 5 Force, Torque, and Shaft Power Measurement 6 Pressure and Sound Measurement 7 Flow Measurement 8 Temperature and Heat-Flux Measurement 9 Miscellaneous Measurements Part 3 Manipulation, Transmission, and Recording of Data 10 Manipulating, Computing, and Compensating Devices 11 Data Transmission and Instrument Connectivity 12 Voltage-Indicating and -Recording Devices 13 Data-Acquisition Systems for Personal Computers 14 Measurement Systems Applied to Micro- and Nanotechnology
913 citations
"Analysis of the Switched-Capacitor ..." refers background or methods in this paper
...To obtain a measurable output relative to the parameter being sensed by a capacitive sensor, a signal-conditioning circuit that converts the variations in the sensor capacitances C1 and C2 to a proportional analog voltage or current is required [ 1 ], [2]....
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...However, a large CGQ may influence the performance of opamp OA. Highaccuracy differential capacitive sensors are always provided with a guard ring to avoid effects due to fringing [ 1 ], [2]....
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01 Aug 1996
TL;DR: Capacitive sensors in Silicon Technology as discussed by the authors have been shown to have high dielectric properties of various materials, including electret microphones, acceleration sensors, and sensors with different types of connectors.
Abstract: Preface Introduction Electrostatics Capacitive Sensor Basics Circuit Basics APPLICATIONS Capacitive Micrometers Proximity Detectors Motion Encoders Multiple Plate Systems Miscellaneous Sensors DESIGN Circuits and Components Switched Capacitor Techniques Noise and Stability Hazards PRODUCTS Electret Microphone Accelerometer StudSensor Proximity Detector Vernier Caliper Graphic Input Tablet Camera Positioner Digital Level References Appendix 1--Capacitive Sensors in Silicon Technology Appendix 2--Dielectric Properties of Various Materials Index
695 citations
"Analysis of the Switched-Capacitor ..." refers methods in this paper
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TL;DR: A tutorial description of the physical phenomena taking place in an SC circuit while it processes noise is provided and some specialized but highly efficient algorithms for estimating the resulting sampled noise in SC circuits are proposed, which need only simple calculations.
Abstract: Thermal noise represents a major limitation on the performance of most electronic circuits. It is particularly important in switched circuits, such as the switched-capacitor (SC) filters widely used in mixed-mode CMOS integrated circuits. In these circuits, switching introduces a boost in the power spectral density of the thermal noise due to aliasing. Unfortunately, even though the theory of noise in SC circuits is discussed in the literature, it is very intricate. The numerical calculation of noise in switched circuits is very tedious, and requires highly sophisticated and not widely available software. The purpose of this paper is twofold. It provides a tutorial description of the physical phenomena taking place in an SC circuit while it processes noise (Sections II-III). It also proposes some specialized but highly efficient algorithms for estimating the resulting sampled noise in SC circuits, which need only simple calculations (Sections IV-VI ). A practical design procedure, which follows directly from the estimate, is also described. The accuracy of the proposed estimation algorithms is verified by simulation using SpectreRF. As an example, it is applied to the estimation of the total thermal noise in a second-order low-distortion delta-sigma converter.
262 citations
"Analysis of the Switched-Capacitor ..." refers background in this paper
...The value of (v̄(2) c1,1 + v̄ 2 c1,2) is 2kT/C1 [17], where k is the Boltzmann constant (k = 1....
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TL;DR: An analog-to-digital converter is developed based on the charge-balancing principle that consists of a switched-capacitor integrator, comparator, and digital logic circuit.
Abstract: An analog-to-digital converter is developed based on the charge-balancing principle. It consists of a switched-capacitor integrator, comparator, and digital logic circuit. Driven by the two phase clock, the integrator accumulates consecutively the incremental signal charge while extracting the quantized reference charge from the accumulated signal charge each time its output becomes positive, to keep their charge balance. The ratio between the accumulated and extracted frequencies for a given period of time then provides the digital representation of an input analog signal. A conversion accuracy higher than 14 bits can be expected from its integrated realization because the offset voltage and the finite open-loop gain of an op-amp and the parasitic capacitance have no effect upon the conversion process. It also features a small device-count integrate onto a very small chip area. Some applications are also presented to demonstrate its validity.
90 citations
"Analysis of the Switched-Capacitor ..." refers background in this paper
...A charge-balancing-type capacitance-to-digital converter (CDC) reported earlier employs a very complicated switching arrangement [8]....
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04 Jun 1996
TL;DR: An interface circuit based on the idea that the ratio of one of the transducer capacitances to its total capacitance represents the offset binary equivalent of the physical quantity under measurement for high-accuracy signal processing of differential-capacitance transducers is developed.
Abstract: For high-accuracy signal processing of differential-capacitance transducers, an interface circuit is developed. The architecture is based on the idea that the ratio of one of the transducer capacitances to its total capacitance represents the offset binary equivalent of the physical quantity under measurement. An op amp-based capacitance-to-voltage converter is commonly used for capacitance detection and an analog-to-digital converter is used for the ratiometric operation. A circuit analysis shows that the interface can detect the capacitance change as small as 0.01% of the total capacitance. Experimental results are also given to confirm the analysis.
64 citations