Sensors & Transducers - International Frequency Sensor Association

More documents

Recommendations

Info

Sensors & Transducers, Vol. 148, Issue 1, January 2013, pp. 1-10as to provide a stable reference voltage which isinsensitive to temperature and power supplyvariations. The final unit, the relaxation oscillator,consists of two parts: (a) a comparator operating incurrent mode and (b) a set of current sources thatcharge and discharge the sensor’s capacitance. Thisunit provides output period variations in form of apulse waveform which is linearly related to thesensor capacitance.To minimize various errors such asinterconnection parasitic, low frequency voltagevariations, and circuit temperature fluctuations, ratiometric measurements using reference capacitance isperformed by the supervising microcontroller unit,followed by a process of averaging via controllingsoftware to further minimize the errors. To determinethe unknown capacitance values, two sets of parasiticcapacitances in parallel to the unknown capacitancewere used, 343.2 pF and 15.8 pF and the sensitivityobtained were 0.89 µs/pF and 0.69 µs/pFrespectively.2.11. A CMOS Integrated Capacitance-to-Frequency Converter with DigitalCompensation Circuit Designedfor Sensor Interface ApplicationsCheng-Ta Chiang et al., has presented a low costCMOS integrated capacitance-to-frequency convertorwith digital compensation circuit for sensorinterfacing applications [13]. The uniquecharacteristic of the circuit proposed is that itproduces a digitized output signal without realizingan analog to digital convertor, thereby reducing thehardware cost along with increasing the timeresponse. A notable feature of the circuit is that, dueto pulse stream output the data can be transmittedover wide range of transmission media like radio,optical, Personal Satellite Network (PSN), Infra Red(IR), ultrasonic etc., hence making the circuit suitablefor remote sensing applications as well. The circuitworks on the principle of switched-capacitortechnique that was developed for integration ofdigital and analog signals on a single silicon chip.The circuit incorporates three stages to implementcapacitance to frequency convertor namelycapacitance-to-voltage converter (CVC), followed bya voltage-to-frequency converter (VFC), and thedigital compensation circuit as shown in Fig. 12. Thestage -I consists of a feedback capacitance ‘CF’ andreference capacitance ‘CR’ which are utilized toderive the voltage at the end of the 1st stage. Theoutput voltage of stage-I (Capacitance to voltageconverter) acts as the input to stage - II (the voltageto frequency convertor which consists of the switched-capacitor integrator, a comparator, and a D flipflop).After application of the switched capacitancetechnique the circuit applies charge redistributionmethod. Finally the circuit uses a unique and firstlyproposed digital compensation circuit whicheliminates the quantization error and provides adynamic output frequency range. The digitalcompensation circuit includes a detector, control, andfrequency circuits.The output frequency range achieved by thecircuit is from 0.5 to 500 kHz under the variablecapacitance of the detected sensor ranges from 4 to24 pF. The area of this chip is 940 × 1080 μm 2 andthe power consumption is 6.4 mW.Fig. 12. The Schematic of the Capacitance-to-Frequency Converter.3. SummaryIn this paper, we have discussed varioustechniques for measuring very small change incapacitance. The various techniques used aims atproviding a high resolution and linear output. Thecircuits been discussed are based on the principle tomeasure change in capacitance. The principles usedare double differential principle, RC phase delay,charging and discharging, oscillator and capacitanceto phase angle conversion. Following is a briefdescription of various papers that involves use ofdiscrete electronic devices rather than fabrication ofmicro-chips. A simple interfacing circuit usingdouble differential principle is that this can measure achange of 10 -18 F. The main feature of the design isthat the change in capacitance is independent of theinitial sensor’s capacitance. A low power capacitance8
Sensors & Transducers, Vol. 148, Issue 1, January 2013, pp. 1-10circuit is to measure capacitance change usingdifferential structure. It uses TSMC 0.35 μm bulkCMOS process for fabrication. It has very highresolution and high degree of linearity. It is useful forbiomedical sensor applications and used a RC phasedelay concept to measure capacitance change. It usesMOSFET switch to avoid nonlinearity. The output ofthis is stable and linear with very high resolution. Thedesign is simple and requires a small number ofcomponents. Interfacing with a microcontrollermeasures charging and discharging time of unknowncapacitor through known capacitance. It’s simple andlow cost technique. It is mainly used for interfacingcircuits. Used novel oscillator concept to measurecapacitor change, it gives very high resolution, highaccuracy and high stability. It has very less sensitivityto noise. The output has high stability and highaccuracy. It has negligible influence of parasiticcapacitance and a capacitance to phase angleconversion for measurement. It has a high resolutionand linear output. The system has an extremely highstray immunity. It is used to measure very smalldisplacement.Table 1 shows comparing the various parametersof the design for capacitance measurement isprovided. It provides the details regardingmethodology, measurement range, sensitivity, errorand application of the design. The application ofthese Capacitance Measurement techniques could beextended to the MEMS based sensors as well [14].MethodologyTable 1. Comparison summary of various design techniques and their applicationsMinimummeasurementvalueDouble Differential Principle aF 2.5 ppmSensitivity Error Applications0.92 pF for 1 µFcapacitorDifferential Principle fF 1.32 mV/fF -Measure very smallcapacitance changewhere initial can beunknown mainly inindustries andinterfacing circuits.Bio-medical sensorapplicationRC Phase Delay pF 246 mV/fF 2.6 % Micro sensorsCharging and Discharging usingBelow 4 % FSRpF -Interfacing circuitsmicrocontrollerfor 1 pF to 10 pF100 aF for 2 pFOscillator pF -Interfacing circuitscapacitorCapacitance to phase angleconversionA CMOS Integrated Capacitanceto-FrequencyConverter withDigital Compensation CircuitDesigned for Sensor InterfaceApplicationsA 16-channel capacitance-toperiodconverter for capacitivesensor applicationsCapacitance to FrequencyConverter Suitable for SensorApplications Using TelemetryUniversal Capacitive Sensors andTransducers Interface (USTI)pF 32 ppm Less than 0.5%Very small displacementmeasurements.4-24 pF NA NA Sensor interfaceNA NA NA Sensor interfaceNA NA NA50 pF to100 µFNA ±0.036 %Suitable design forpassive telemetrysystemsVaried applications.Suitable for wide rangeC/F convertersReferences[1]. D. M. G. Preethichandra, Katsunori Shida,A Simple Interface Circuit to Measure Very SmallCapacitance Changes in Capacitive Sensors, IEEETrans. Instrum. Meas., Vol. 50, Dec. 2001,pp. 1583-1586.[2]. M. R. Haider+, M. R. Mahfouz, S. K. Islam, S. A.Eliza, W. Qu and E. Pritchard, A Low-powerCapacitance Measurement Circuit with HighResolution and High Degree of Linearity, IEEECircuits and Systems, Sep. 2008, pp. 261-264.[3]. Robert N. Dean and Aditi Rane, An ImprovedCapacitance Measurement Technique Based of RCPhase Delay, in Proceedings of the IEEE Instrum.Meas. Tech. Conf., 2010, pp. 367-370.[4]. F. Reverter, M. Gasulla, and R. Pallàs-Areny,A low-cost microcontroller interface for low-valuecapacitive sensors, in Proceedings of the IEEE IMTC,Como, Italy, May 2004, pp. 1771–1775.[5]. Frank M. L. van der Goes and Gerard C. M. Meijer,A Novel Low-Cost Capacitive-Sensor Interface, IEEETrans. Instrum. Meas., Vol. 45, Apr. 1996, pp. 536–540.9
Page 2 and 3: Copyright 2013 IFSA Publishing. Al
Page 5: A Portable Colloidal Gold Strip Sen
Page 10 and 11: Sensors & Transducers, Vol. 148, Is
Page 12 and 13: Sensors & Transducers, Vol. 148, Is
Page 16: Sensors & Transducers, Vol. 148, Is

Sensors & Transducers - International Frequency Sensor Association

Create successful ePaper yourself

Delete template?

Save as template?