A Battery-based Constant Current Source (Bb-CCS) for ... - IEEE Xplore

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A Battery-based Constant Current Source (Bb-CCS) for Biomedical Applications Tushar Kanti Bera, Manobjyoti Saikia, J. Nagaraju Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore Bangalore - 560012, Karnataka, INDIA Email: [email protected] Abstract-A constant current source is essential for impedance measurement techniques in several biomedical applications. The impedance measurement studies in biomedical applications such as bioelectrical impedance analysis (BIA), electrical impedance plethysmography (IPG), electrical impedance myography (EIM), electrical impedance cardiography (ICG), electrical impedance spectroscopy (EIS), electrical impedance tomography (EIT) all need a high precision impedance measurement circuit with high signal to noise ratio (SNR). Moreover the impedance measurement circuit applied to the biomedical, clinical or medical applications such as human subject studies should have a better patient safety. In this direction a Battery-based Constant Current Source (Bb-CCS) is developed for biomedical applications. The Bb-CCS is developed with a modified Howland current source (MHCS) fed by a battery based power supply (BBPS). The current amplitude, frequency and waveforms are set as the circuit variables which are found as adjustable as per the requirements. The frequency responses, load response, Fast Fourier Transform (FFT) of the Bb-CCS are studied and results show that the Bb-CCS can be suitably used for multifrequency impedance measurement methods in biomedical applications. Keywords - Bioelectrical Impedance, constant current source, Battery-based Constant Current Source (Bb-CCS), biomedical applications, modified Howland current source (MHCS), battery based power supply (BBPS), frequency response, load response, Fast Fourier Transform (FFT).

I. INTRODUCTION Electrical impedance of biological tissues [1-3] and their other electrical properties [4-9] such as resistivity [4-5], conductivity [6], permittivity [7-8] and admittivity [9] all have their own complex frequency responses and all are found as the function of tissue composition and signal frequency [4-10]. The frequency response of the electrical impedance of the biological tissues varies with physiological and physiochemical health status. Frequency response of the bioelectrical impedance varies from subject to subject [10-17] and it may also vary from one tissue part to another tissue part within a particular subject. Moreover, the bioelectrical impedance of biological tissues varies with the measurement directions for the same tissue called anisotropic tissues [9]. As the bio-impedance changes with the variations in tissue health status [1, 18-19], the frequency response studies of bioelectrical impedance can provide a lot of information about the anatomy and physiology of the tissue under test. Bioelectrical impedance methods measure the electrical

impedance of the object at a single frequency or at multiple frequencies from the set of voltage-current data. The voltagecurrent data are generally measured at the tissue surface by injecting a constant voltage signal or current signal using a constant voltage source [20-21] or constant current source [21-26] respectively, through the surface electrodes [27-28] with two electrode or four electrode methods (Fig. 1). In two electrode method the current injection and voltage measurements are conducted through the same electrodes pairs (Fig. 1a) and hence, it is known as two-electrode technique [16]. On the contrary, the four-probe method [16] injects the frequency dependent current signal (I(ω)) through two-electrodes (called current electrodes or driving electrodes) as shown by the red electrodes in Fig. 1b and the frequency dependent electric potential (V(ω)) developed, is measured [16] across other two electrodes (called voltage electrodes or sensing electrodes) blue electrodes (Fig. 1b). The frequency dependent electrical impedance Z(ω) is calculated by the Ohm’s law for alternating circuits which is given by: Zω 

Vω Iω

(1)

The Z (ω) is the transfer function of the object in which the applied current signal (I(ω)) and the voltage data (V(ω)) are input signal and output signals respectively.

Figure-1: Electrical impedance measurement schematic using two-probe or four-probe technique (a) impedance measurement using two-probe technique, (b) impedance measurement using four-probe technique.

A constant current source [21-26] is essential for impedance measurement techniques in several biomedical applications such as bioelectrical impedance analysis (BIA) [29-32], electrical impedance plethysmography (IPG) [33-34], electrical impedance myography (EIM) [35], electrical impedance cardiography (ICG) [36-38], electrical impedance spectroscopy (EIS) [39-45], electrical impedance tomography (EIT) [46-56] and so on. The BIA, IPG, EIM, ICG, EIS and

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IEEE - 31661

number of passive circuit elements [68]. The MAX038 is a high-frequency, precision function generator and generates high quality, high frequency triangle, saw-tooth, sine, square, and pulse waveforms with a minimum number of external components. The frequency of the VCO is made adjustable by changing the values of frequency controller capacitance (Cf) and resistance (Rin). The VCO is developed to generate a pure sinusoidal signal with an adjustable frequency from 10 Hz to 10 MHz. The VCCCS is basically a modified Howland based voltage controlled constant current source (Fig. 3) developed with two AD829 Op-Amps. The AD829 [69] is a low noise (1.7 nV/√Hz), high speed op amp with custom compensation that provides the user with gains of ± 1 to ± 20 while maintaining a bandwidth greater than 50 MHz [69]. The current amplitude of VCCCS is controlled by different values of resistors connected through a resistor bank (Fig. 3). An 8pin DIP switch based resistor bank (Fig. 3) is developed with eight resistors (RS1, RS2, RS3, …, RS7, RS8) and is used to control the current amplitude in VCCCS. The resistors RS1, RS2, RS3 , …, RS7, RS8 are individually connected with the eight switches S1, S2, S3, …, S7, S8 which connect these resistors to provide the eight different current amplitudes (I1, I2, I3, …, I7, I8) in the VCCCS. When any one of the switches (say, S3) is operated, it connects the corresponding resistor (RS3) in the VCCCS circuit which provides a particular amplitude of current signal (I3) (Fig. 3). The current amplitude of the VCCCS is simultaneously sensed and measured by putting a current sensing resistor (RSENCE) in series with the load (RL) as shown in Fig. 3.

EIT all are studied either with a constant current injector circuit or readymade impedance measuring instruments. Being the established and proven methods, the BIA and EIS instrument are available in market. But for available ICG, IPG the instruments are less in number. Being a bioimpedance tomographic technique EIT has several advantages over other impedance methods and other tomographic methods and hence it is being studied by a number of research groups. As the number of available EIT instruments is very few, the constant current injectors are usually developed, studied and evaluated in EIT phantom studies [57-67]. To obtain a higher measurement accuracy BIA, IPG, EIM, ICG, EIS, EIT all need a high precision impedance measurement circuit or impedance measurement instrument such as impedance analyzer (Fig. 2) with high signal to noise ratio (SNR). As the biological tissues have variable impedance response over a wide range of signal frequency, the current source with multifrequency application is always advantageous. Moreover the impedance measurement circuit applied to the clinical and medical applications such as measurement studies on human subject should have a better patient safety. In this direction a multifrequency Battery-based Constant Current Source (BbCCS) is developed and studied in biomedical applications. The Bb-CCS is developed with a modified Howland current source (MHCS) powered by a battery based power supply (BBPS). The amplitude, frequency and waveforms of the output current signal are set as the circuit variables which are made adjustable as per the practical applications. The frequency responses, load response, Fast Fourier Transform (FFT) of the output current signal of the Bb-CCS are studied to evaluate the Bb-CCS. Bb-CCS is also found suitable for the impedance measurement processes in biomedical applications.

Figure-3: Schematic of the VCCCS with the resistor bank for setting the different current amplitudes.

B. Battery Based Power Supply (BBPS) A battery based power supply (BBPS) is developed (Fig. 4) for increasing the signal to noise ratio (SNR). The BBPS is developed with two 12 V sealed lead acid rechargeable batteries and a battery charging circuit (BCC) developed to generate a pure ±12 V dc supply required for charging the batteries. The BCC (Fig. 4) is developed with a filterregulator-filter (FRF) block developed as per the battery power consumption. The BCC is fed by the power supply obtained from the bridge rectifier circuit (BRC). The BBPS is charged with a transformer based power supply (TBPS) as shown in Fig. 4 and can be used for feeding the MHCS directly if requires. In TBPS a 240: (24:0:24) volts, 1 A, 1 ϕ, step down, center tapped (secondary), iron core transformer is used to step down the ac main supply (240 V, ac, 50 Hz).

Figure-2: Schematic of the electrical impedance spectroscopy (EIS) studies on biological materials with four-probe technique using an impedance analyzer [current signal is injected through the outer (shown in red) electrodes and the voltage is measured across the inner (shown in blue) electrodes].

II. MATERIALS AND METHODS A. Modified Howland current source (MHCS) Modified Howland current source (MHCS) is developed with a voltage Controlled Oscillator (VCO) and a voltage controlled constant current source (VCCCS). The VCO is developed with MAX038 IC connected with a minimum

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IEEE - 31661

The 240 volts (r.m.s.) mains is stepped down to 24 volt (r.m.s.) and is fed to the BRC connected to a the filterregulator-filter (FRF) block (Fig. 4). The filter-regulator-filter (FRF) block is developed with standard regulator ICs (78XX and 79XX series) as shown in Fig. 4. As per the requirement, the different dc voltage levels (in ±5 V dc or ±9 V or ±12 V dc) are obtained from the battery output (±12V dc supply) using voltage divider circuits. All the circuit blocks are developed in such a way that they can operate from 5 V to 18 V range except VCO as MAX038 IC only works at 5 V. But for low power consumption the whole instrumentation is operated at 5 V dc supply and the instrumentation is found suitable for biomedical applications. An isolation transformer (1:1) is also used to isolate the imaging object from the electric power lines for safety. Isolation transformer provides the isolation between the subject under test (SUT) and the measuring instruments and it is generally used to protect the SUT against electric shock, to suppress electrical noise, or to transfer power between current source and the SUT.

BIA, IPG, EIM, ICG, EIS, EIT etc. The output signal of the battery based constant current source (Bb-CCS) is also studied at different frequencies (Fig. 6) and the Bb-CCS is found suitable for multifrequency bioimpedance methods.

Figure-5: Output signal waveform studies and its frequency responses of BBPS based constant current source (Bb-CCS) obtained in digital storage oscilloscope (a) output signal, (b) FFT of the output signal.

Figure-4: Schematic of the Battery Based Power Supply (BBPS).

III. RESULTS AND DISCUSSION Frequency response of the VCO show that the VCO generates pure sinusoidal, triangular and square waveforms from 10 Hz to 10 MHz. Frequency response of the VCCCS show that the VCCCS generates pure current signal from 10 Hz to 1 MHz with sinusoidal, triangular and square waveforms. The output signal of the VCCCS is studied at 50 kHz in DSO and the signal is saved to obtain the FFT of the signal. FFT shows that the signal is generated at 50 kHz with negligible noise and hence it is found very suitable for biomedical application. Fig. 5a and Fig. 5b show the output current signal of the Bb-CCS and its FFT obtained for 1 kΩ resistive load at 50 kHz using DSO. FFT shows that the current signal is generated at 50 kHz almost without noise and hence it is found very suitable for biomedical applications like

Figure-6: Waveform studies and the frequency response studies of output signal of the Bb-CCS obtained in digital storage oscilloscope (a) signal at 25 kHz, (b) FFT of the signal a), (c) signal at 50 kHz, (d) FFT of the signal c), (e) signal at 100 kHz, (f) FFT of the signal e), (g) signal at 250 kHz, (h) FFT of the signal g), (i) signal at 500 kHz, (j) FFT of the signal i).

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IEEE - 31661 [10] Bera TK, Seo JK, Kwon H and Nagaraju J, A LabVIEW Based Electrical Bio-Impedance Spectroscopic Data Interpreter (LEBISDI) for Studying The Equivalent Circuit Parameters of Biological Tissues, 15th International Conference on Electrical Bio-Impedance (ICEBI) and 14th Conference on Electrical Impedance Tomography (EIT), Germany, 2013, pp 77. [11] Ackmann JJ, Complex Bioelectric Impedance Measurement System for the Frequency Range from 5 Hz to 1 MHz, Annals of Biomedical Engineering. 1993; 21:135-146. [12] Bera TK and Nagaraju J, Electrical Impedance Spectroscopic Study of Broiler Chicken Tissues Suitable for The Development of Practical Phantoms in Multifrequency EIT, J Electr Bioimp, vol. 2, pp. 48–63, 2011. [13] B. Bodakian, The Dielectric Properties of Meat, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 2, April 1994, 181-187. [14] Ackmann JJ, Seitz MA., Methods of complex impedance measurements in biologic tissue, Crit Rev Biomed Eng. 1984; 11(4): 281-311. [15] Cha K, Chertow GM, Gonzalez J, Lazarus JM, and Wilmore DW, Multifrequency bioelectrical impedance estimates the distribution of body water, J Appl Physiol. 1995; 79:1316-1319. [16] Bera TK and Nagaraju J, Electrical Impedance Spectroscopic Study of Broiler Chicken Tissues Suitable for The Development of Practical Phantoms in Multifrequency EIT, J Electr Bioimp, vol. 2, pp. 48–63, 2011. DOI:10.5617/jeb.174 [17] Bauchot AD, Harker FR, Arnold WM, The use of electrical impedance spectroscopy to assess the physiological condition of kiwifruit, Volume 18, Issue 1, January 2000, Pages 9–18 [18] Bera TK and Nagaraju J, A Multifrequency Constant Current Source for Medical Electrical Impedance Tomography, Proceedings of the IEEE International Conference on Systems in Medicine and Biology 2010 (IEEE ICSMB 2010), India, Dec’2010, India, pp 278-283. [19] P. Bertemes-Filho, A. S. Paterno, R. M. Pereira, Multichannel Bipolar Current Source Used in Electrical Impedance Spectroscopy: Preliminary Results, World Congress on Medical Physics and Biomedical Engineering, September 7 - 12, 2009, Munich, Germany, IFMBE Proceedings Volume 25/7, 2009, pp 657-660. [20] T R Qureshi, C R Chatwin and W Wang, Design of Wideband Voltage Source Having Low Output Impedance, Flexible Gain and Controllable Feedback Current for EIT Systems, 2012 2nd International Conference on Biomedical Engineering and Technology, IPCBEE vol. 34 (2012), IACSIT Press, Singapore, 45-50. [21] Youssoufa Mohamadou, Tong In Oh, HunWi, Harsh Sohal, Adnan Farooq, Eung Je Woo and Alistair Lee McEwan, Performance evaluation of wideband bio-impedance spectroscopy using constant voltage source and constant current source, Meas. Sci. Technol. 23 (2012) 105703 (10pp). [22] Alexander S Ross, G J Saulnier, J C Newell and D Isaacson, Current source design for electrical impedance tomography, Physiol. Meas. 24 (2003) 509–516 PII: S0967-3334(03)54203-4. [23] Jeong Whan Lee, Tong In Oh, Sang Min Paek, Jae Sang Lee, and Eung Je Woo, Precision Constant Current Source for Electrical Impedance Tomography, Proceedings of the 25" Annual International Conference of the IEEE EMBS, Cancun, Mexico, 2003, 1066-1069. [24] José Antonio Gutiérrez Gnecchi, Voltage Controlled Current Source (VCCCS) for Electrical Impedance Tomography (EIT) Measurements in the α and β Dispersion Frequency Ranges, 2010 Electronics, Robotics and Automotive Mechanics Conference, 677-681. [25] Bera TK and Nagaraju J, A Multifrequency Constant Current Source Suitable for Electrical Impedance Tomography (EIT), Proceedings of 2010 International Conference on Systems in Medicine and Biology, 1618 December 2010, IIT Kharagpur, India, pp 278-283. [26] Engr. Angelito A. Silverio, M.S., Engr. Angelina A. Silverio, M.A., M.S., A High Output Impedance Current Source for Wideband Bioimpedance Spectroscopy Using 0.35μm Tsmc Cmos Technology, International Journal of Engineering and Applied Sciences, 2012, December 2012. Vol. 1, No.2. 68-75. [27] Webster JG, Electrical impedance tomography, first edition. Adam Hilger Series of Biomedical Engineering Adam Hilger, New York, USA. 1990. [28] J. G. Webster, Medical Instrumentation: Principle and design, John Wiley & Sons, Hoboken, NJ, USA, 1995

The output signal of the battery based constant current source (Bb-CCS) is studied at different frequencies in DSO and the signals are saved to obtain the FFT of the signal. FFTs of the current signals are studied at all the frequencies. Results show that the Bb-CCS generates the current signal from 10 Hz -1 MHz with negligible noise and hence it is found very suitable for multifrequency bioimpedance methods like EIS, EIT etc. Results show that the Bb-CCS generates high accuracy current signals with an amplitude of 200 µA – 7.5 mA within a frequency range of 10 Hz to 1 MHz. Load response of Bb-CCS showed that Bb-CCS can drive up to 7.8 kΩ load with a supply voltage of 9 V for 1 mA supply. Bioimpedance studies like EIT and EIS show that Bb-CCS can suitably drive biological loads and works well with high accuracy and high SNR. Thus Bb-CCS is found suitable for impedance studies in biomedical applications studies with variable current amplitude (200 µA – 7.5 mA), multifrequency (10 Hz – 1 MHz), different current waveforms. IV. CONCLUSIONS Individual and combined frequency responses of VCO and VCCCS are studied. Results show that the output of the VCO remains constant up to 10 MHz. Frequency response of VCCCS showed that the current output of the VCCCS remains constant up to 1 MHz. The frequency response of the Bb-CCS is also studied and it is found suitable for multifrequency bioimpedance application from 10 Hz to 1 MHz. Thus Bb-CCS is found suitable for impedance studies in biomedical applications with variable current amplitude (200 µA – 7.5 mA), multifrequency (10 Hz – 1 MHz), different current waveforms. V. REFERENCES [1] [2]

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