V.J. Kumar

Use of Fourier Series Analysis for Motion Artifact Reduction and Data Compression of Photoplethysmographic Signals

Pulse oximeters require artifact-free clean photoplethysmograph (PPG) signals obtained at red and infrared (IR) wavelengths for the estimation of the level of oxygen saturation ( SpO2) in the arterial blood of a patient. Movement of a patient corrupts a PPG signal with motion artifacts and introduces large errors in the computation of SpO2. A novel method for removing motion artifacts from corrupted PPG signals by applying Fourier series analysis on a cycle-by-cycle basis is presented in this paper. Aside from artifact reduction, the proposed method also provides data compression. Experimental results indicate that the proposed method is insensitive to heart rate variation, introduces negligible error in the processed PPG signals due to the additional processing, preserves all the morphological features of the PPG, provides 35 dB reduction in motion artifacts, and achieves a data compression factor of 12.

A Novel Calibration-Free Method of Measurement of Oxygen Saturation in Arterial Blood

In present-day pulse oximeters, oxygen saturation in arterial blood (SpO2) is computed by utilizing an empirical relationship extracted from a calibration curve. The calibration curve is obtained by curve-fitting data acquired from volunteers. A novel method of computation of SpO2 that does not require the use of a calibration curve is presented in this paper. Based on a model for the attenuation of light through skin, tissue, bone, and blood, suitable processing steps are identified so that the analytical expression derived for the estimation of SpO2 becomes free of not only patient but sensor-dependent parameters as well. The experimental results presented in this paper establish the efficacy of the proposed method.

Motion Artifact Reduction in Photoplethysmographic Signals using Singular Value Decomposition

Artifact free photoplethysmographic (PPG) signals, obtained with red and infrared (IR) optical sources and detectors are necessary for non-invasive estimation of oxygen saturation (SpO2) in arterial blood. Movement of a patient corrupts the PPGs with motion artifacts, resulting in large errors in the computation of SpO2. This paper presents a novel singular value decomposition (SVD) based method to reduce motion artifacts from corrupted PPG signals. Test results on a prototype incorporating the proposed SVD technique show that stable and reliable SpO2 measurement is achieved even when PPGs are distorted by motion artifacts, thus establishing the efficacy of the proposed method.

Analysis of the Switched-Capacitor Dual-Slope Capacitance-to-Digital Converter

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%.

Switched Capacitor Signal Conditioning for Differential Capacitive Sensors

A novel switched capacitor signal-conditioning circuit for differential capacitive sensors is proposed. The main advantage of the proffered method lies in the fact that it accepts sensors possessing either linear or inverse characteristics and provides a linear output. Moreover, the output is dependent only on a pair of dc reference voltages and the transformation constant of the sensor. Hence, increased linearity and accuracy is easily achieved by employing precision dc reference voltages. Results from the tests on a prototype elucidate the practicality of the proposed method

A microcontroller-based quasi-balanced bridge for the measurement of L, C and R

A microcontroller-based quasi-balanced bridge for the measurement of parameters of an inductor or a capacitor is described. The unknown element (inductor or capacitor) in series with a resistor forms one-half of an ac bridge, while a multiplying digital-to-analog converter (MDAC) serves as the other half. The bridge is brought into two independent quasi-balanced conditions in succession by the microcontroller through the MDAC. The parameters of the unknown element are shown to be functions of the settings of the MDAC at the two quasi balanced conditions. The relevant expressions for these parameters are evaluated by the microcontroller and the results displayed in appropriate display fields. The proposed scheme was implemented using an Intel 8751 microcontroller and tested. The readings obtained on the prototype were compared to those obtained with a commercial LCR meter. Employing an MDAC of basic accuracy /spl plusmn/0.2%, over the frequency range of 100-1000 Hz, an overall uncertainty in measurement of /spl plusmn/0.7% for the prototype was achieved.

A Novel Dual-Slope Resistance-to-Digital Converter

A dual-slope resistance-to-digital (DSRDC) converter that accepts the resistance of a single-element resistive-type sensor as input and provides a direct digital output proportional to the parameter being sensed by the resistive-type sensor is presented in this paper. A high level of linearity and accuracy is achieved since the output of the DSRDC is dictated only by the magnitudes of a pair of dc reference voltages and the transformation constant of the sensor. Sensitivity analysis shows that the effect of circuit parameter variations on the output is minimal. Simulation studies and test results obtained on a prototype establish the efficacy of the proposed scheme.

A novel method of measurement of L and C

A novel method of measurement of component values of inductors and capacitors is described. The technique is independent of the voltage across or current through the unknown inductor or capacitor, as it involves only a set of phase measurements. The unknown capacitor/inductor is connected in series with a known standard resistance and this series circuit is excited by a source of required voltage and frequency. The resistive and reactive parts of the unknown component are measured by measuring the phase displacement between the three voltages, namely, voltage applied, voltage across the inductor/capacitor and the voltage across standard resistance. The proposed scheme is verified both by simulation as well as by building a prototype. The relationship between the range of the measurement and obtainable accuracy level is established. For the prototype built, with a one decade span in the measurement range, a commercially acceptable accuracy of /spl plusmn/2.0% was achieved. However, this accuracy can be further improved by suitable circuit modifications. >

Measurement of C and tan/spl delta/ of a capacitor employing PSDs and dual-slope DVMs

A scheme for measuring the C and tan/spl delta/ (dissipation factor D) values of a capacitor is described in this paper. Comparing the unknown capacitor to a standard capacitor and employing three-phase sensitive detectors and two digital voltmeters, the parameters of the unknown capacitor are determined. The method facilitates testing of a given capacitor at a desired frequency as well as voltage. The feasibility of the scheme is established by building a prototype. The accuracy of the prototype unit for the measurement of C is /spl plusmn/0.2% and that for tan/spl delta/ is /spl plusmn/2.5%.

Dual Slope Resistance to Digital Converter

A dual slope resistance to digital converter applicable to differential resistive sensors is presented. A dual slope digital converter is appropriately modified so that it accepts directly the resistive elements of a differential resistive sensor and provide a digital output that is linearly proportional to the physical quantity being sensed by the sensor. High accuracy is easily obtained since the output is decided only by a pair of dc reference voltages and the transformation constant of the sensor. Sensitivity analysis shows that the effect of circuit parameter variations on the output is minimal. The efficacy of the proposed scheme is clearly demonstrated by the test results obtained on a prototype.