# Galvanically Decoupled Current Source Modules for Multi-Channel Bioimpedance Measurement Systems

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Problem Description

_{Bio}) and the electrode skin interface impedances (Z

_{ESI}). Therefore, the current sources and amplifiers are simplified as ideal components. In the equivalent circuit, it can be seen that the excitation current I

_{2}of source CS2 is divided into two parts (I

_{2a}and I

_{2b}). Since the GND electrodes of both channels are connected to the same electrical potential, I

_{2a}flows back to the correct electrode, but I

_{2b}does not. This behavior influences the voltage drop across the bioimpedance of interest (Z

_{Bio2}) and consequently the measurement results. Since the quantitative impact of this effect is not predictable and changes over time, it has to be prevented.

_{CS1}, GND

_{CS2}), which are galvanically isolated. Therefore, each current flows back to its intended electrode. Furthermore, the data interfaces, which control the configuration of the current source modules, have to be isolated, too. Because of its very high input impedances, the differential amplifiers can be fed by the same power supply. This work describes the implementation of this approach.

## 3. System Development

_{DC}from an external voltage supply or by a controlling bioimpedance measurement system. An isolating DC/DC converter (MTU2D0505MC (Murata Power Solutions, Nagaokakyō, Japan)) is used to implement the galvanic separation of the power supply and to generate a bipolar voltage supply of ±5 V

_{DC}, which is used for the analog components of the system. The output voltages of the DC/DC converter are filtered by passive LC-filters with a cut-off frequency of 23 kHz, as suggested in the data sheet. To generate 3.3 V

_{DC}for the digital components, a low dropout regulator (LDO, TPS73033 (Texas Instruments, Dallas, TX, USA)) is used.

_{c}= 400 kHz is realized by two operational amplifiers (LMH6646 from Texas Instruments) to smooth the sampled signal. Afterwards, the remaining DC offset voltage is removed by a passive 1st-order high-pass filter with a cut-off frequency of f

_{c}= 200 Hz. A voltage-controlled current source (VCCS), which is based on the AD8130 (Analog Devices Inc., Norwood, MA, USA) [9,20], converts the voltage signal into an AC current. This current flows via the electrodes through the bioimpedance (Z

_{Bio}) and a shunt resistor (R

_{S}= 69.8 Ω).

^{2}and have a weight of 23 g, when the 102 components are populated. A photograph of three modules connected in daisy chain mode is shown in Figure 5.

## 4. Results

#### 4.1. Quality of Generated Sinusoidal Signals

_{s}-f

_{signal}comes closer to the cut-off frequency of the low-pass filter after the DAC.

#### 4.2. Load Characteristic of the Current Source

_{rms}) is plotted over load resistance (R

_{L}) for these three frequencies. Up to a load resistance of about 1500 Ω the output current for each frequency is very stable, which represents a maximum compliance voltage range of ±3.2 V. The current changes over the entire range less than 2%. The attenuation in the 200 kHz plot of about 0.1 mA, which is constant over the load, is caused by the low-pass filter characteristic and does not have a negative influence on impedance measurements. It can easily be calibrated.

_{L}= 10 Ω and R

_{L}= 1.21 kΩ, are |Z

_{out, 30 kHz}| = 89 kΩ, |Z

_{out, 77 kHz}| = 68 kΩ, and |Z

_{out, 200 kHz}| = 49 kΩ.

#### 4.3. Efficiency of Decoupling

_{L1}= R

_{L2}= 1 kΩ each, as shown in Figure 8. Additionally, the outputs are connected with each other to model a simple ideal coupling. Both modules are powered by the same external voltage source and are just isolated by the DC/DC converters as described before.

_{11}and I

_{12}and the usage of Equation (2).

## 5. Summary and Outlook

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

- Grimnes, S.; Martinsen, O.G. Bioelectricity and Bioimpedance Basics, 2nd ed.; Academic Press: Cambridge, MA, USA, 2008. [Google Scholar]
- Lee, S.Y.; Gallagher, D. Assessment methods in human body composition. Curr. Opin. Clin. Nutr. Metab. Care
**2008**, 11. [Google Scholar] [CrossRef] [PubMed] - Matthie, J.R. Bioimpedance measurements of human body composition: Critical analysis and outlook. Expert Rev. Med. Devices
**2008**, 5. [Google Scholar] [CrossRef] [PubMed] - Grimnes, S.; Martinsen, O.G. Bioimpedance. In Wiley Encyclopedia of Biomedical Engineering; John Wiley and Sons, Inc.: New York, NY, USA, 2006; pp. 438–447. [Google Scholar]
- Koivumäki, T.; Vauhkonen, M.; Kuikka, J.T.; Hakulinen, M.A. Bioimpedance-based measurement method for simultaneous acquisition of respiratory and cardiac gating signals. Physiol. Meas.
**2012**, 33. [Google Scholar] [CrossRef] [PubMed] - Rutkove, S.B. Electrical impedance myography: Background, current state, and future directions. Muscle Nerve
**2009**, 40. [Google Scholar] [CrossRef] [PubMed] - Kusche, R.; Malhotra, A.; Ryschka, M.; Ardelt, G.; Klimach, P.; Kaufmann, S. A FPGA-Based Broadband EIT System for Complex Bioimpedance Measurements—Design and Performance Estimation. Electronics
**2015**, 4. [Google Scholar] [CrossRef] - Cherepenin, V.; Karpov, A.; Korjenevsky, A.; Kornienko, V.; Mazaletskaya, A.; Mazourov, D.; Meister, D. A 3D electrical impedance tomography (EIT) system for breast cancer detection. Physiol. Meas.
**2001**, 22, 9–18. [Google Scholar] [CrossRef] [PubMed] - Kaufmann, S.; Malhotra, A.; Ardelt, G.; Ryschka, M. A high accuracy broadband measurement system for time resolved complex bioimpedance measurements. Physiol. Meas.
**2014**, 35, 6. [Google Scholar] [CrossRef] [PubMed] - Luna-Lozano, P.S.; Pallàs-Areny, R. Heart rate detection from impedance plethysmography based on concealed capacitive electrodes. In Proceedings of the XIX IMEKO World Congress, Lisbon, Portugal, 6–11 September 2009. [Google Scholar]
- Kusche, R.; Adornetto, T.D.; Klimach, P.; Ryschka, M. A Bioimpedance Measurement System for Pulse Wave Analysis. In Proceedings of the 8th International Workshop on Impedance Spectroscopy, Chemnitz, Germany, 23–25 September 2015. [Google Scholar]
- Oh, T.I.; Wi, H.; Kim, D.Y.; Yoo, P.J.; Woo, E.J. A fully parallel multi-frequency EIT system with flexible electrode configuration: KHU Mark2. Physiol Meas.
**2011**, 32. [Google Scholar] [CrossRef] [PubMed] - Stanley, A.W.; Herald, J.W.; Athanasuleas, C.L.; Jacob, S.C.; Bartolucci, A.A.; Tsoglin, A.N. Multi-channel electrical bioimpedance: A non-invasive method to simultaneously measure cardiac output and individual arterial limb flow in patients with cardiovascular disease. J. Clin. Monit. Comput.
**2009**, 23. [Google Scholar] [CrossRef] [PubMed] - Halter, R.J.; Hartov, A.; Paulsen, K.D. A broadband high-frequency electrical impedance tomography system for breast imaging. Trans. Biomed. Eng.
**2008**, 55. [Google Scholar] [CrossRef] [PubMed] - Avery, J.; Dowrick, T.; Faulkner, M.; Goren, N.; Holder, D. A Versatile and Reproducible Multi-Frequency Electrical Impedance Tomography System. Sensors
**2017**, 17. [Google Scholar] [CrossRef] [PubMed] - Bera, T.K.; Jampana, N. A multifrequency constant current source suitable for Electrical Impedance Tomography (EIT). In Proceedings of the International Conference on Systems in Medicine and Biology (ICSMB), Kharagpur, India, 16–18 December 2010. [Google Scholar]
- Ross, A.S.; Saulnier, G.J.; Newell, J.C.; Isaacson, D. Current source design for electrical impedance tomography. Physiol. Meas.
**2003**, 24, 509–516. [Google Scholar] [CrossRef] [PubMed] - Tietze, U.; Schenk, C. Electronic Circuits: Handbook for Design and Application; Springer: Berlin/Heidelberg, Germany, 2008. [Google Scholar]
- Kaufmann, S.; Ardelt, G.; Ryschka, M. Measurements of Electrode Skin Impedances using Carbon Rubber Electrodes—First Results. In Proceedings of the XV International Conference on Electrical Bio-Impedance (ICEBI) & XIV Conference on Electrical Impedance Tomography (EIT), Heilbad Heiligenstadt, Germany, 22–25 April 2013. [Google Scholar]
- Zhao, X; Kaufmann, S.; Ryschka, M. A comparison of different multi-frequency Current Sources for Impedance Spectroscopy. In Proceedings of the 5th International Workshop on Impedance Spectroscopy, Chemnitz, Germany, 26–28 September 2012. [Google Scholar]
- Paavle, T.; Min, M.; Parve, T. Aspects of using chirp excitation for estimation of bioimpedance spectrum. In Fourier Transform—Signal Processing; Salih, S.M., Ed.; InTech: Rijeka, Croatia, 2012; pp. 237–256. [Google Scholar]

**Figure 1.**Simplified equivalent circuit of a simultaneous 2-channel bioimpedance measurement without (

**a**) and with (

**b**) galvanic decoupling of the current sources.

**Figure 6.**Normalized magnitude spectrum of a 1.5 mA output current signal with a frequency of 30 kHz, measured over a 1 kΩ resistor.

**Figure 7.**Output current of the current source depending on the load resistance and signal frequency. The output current was 1.5 mA.

Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
---|---|---|---|---|---|---|---|

Frequency | Frequency | Frequency | Activation | rms Value | rms Value | Address | Address |

**Table 2.**Signal-to-noise and distortion (SINAD) ratio of the generated 1.5 mA current signal over frequency, measured over a 1 kΩ resistor.

Frequency/kHz | 12 | 30 | 50 | 77 | 143 | 200 | 250 |
---|---|---|---|---|---|---|---|

SINAD/dB | 46.3 | 46.3 | 46.0 | 44.9 | 40.6 | 35.0 | 32.1 |

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Kusche, R.; Hauschild, S.; Ryschka, M.
Galvanically Decoupled Current Source Modules for Multi-Channel Bioimpedance Measurement Systems. *Electronics* **2017**, *6*, 90.
https://doi.org/10.3390/electronics6040090

**AMA Style**

Kusche R, Hauschild S, Ryschka M.
Galvanically Decoupled Current Source Modules for Multi-Channel Bioimpedance Measurement Systems. *Electronics*. 2017; 6(4):90.
https://doi.org/10.3390/electronics6040090

**Chicago/Turabian Style**

Kusche, Roman, Sebastian Hauschild, and Martin Ryschka.
2017. "Galvanically Decoupled Current Source Modules for Multi-Channel Bioimpedance Measurement Systems" *Electronics* 6, no. 4: 90.
https://doi.org/10.3390/electronics6040090