Performance evaluation of wideband bio-impedance spectroscopy using constant voltage source and constant current source

Abstract

Current sources are widely used in bio-impedance spectroscopy (BIS) measurement systems to maximize current injection for increased signal to noise while keeping within medical safety specifications. High-performance current sources based on the Howland current pump with optimized impedance converters are able to minimize stray capacitance of the cables and setup. This approach is limited at high frequencies primarily due to the deteriorated output impedance of the constant current source when situated in a real measurement system. For this reason, voltage sources have been suggested, but they require a current sensing resistor, and the SNR reduces at low impedance loads due to the lower current required to maintain constant voltage. In this paper, we compare the performance of a current source-based BIS and a voltage source-based BIS, which use common components. The current source BIS is based on a Howland current pump and generalized impedance converters to maintain a high output impedance of more than 1 MΩ at 2 MHz. The voltage source BIS is based on voltage division between an internal current sensing resistor (Rs) and an external sample. To maintain high SNR, Rs is varied so that the source voltage is divided more or less equally. In order to calibrate the systems, we measured the transfer function of the BIS systems with several known resistor and capacitor loads. From this we may estimate the resistance and capacitance of biological tissues using the least-squares method to minimize error between the measured transimpedance excluding the system transfer function and that from an impedance model. When tested on realistic loads including discrete resistors and capacitors, and saline and agar phantoms, the voltage source-based BIS system had a wider bandwidth of 10 Hz to 2.2 MHz with less than 1% deviation from the expected spectra compared to more than 10% with the current source. The voltage source also showed an SNR of at least 60 dB up to 2.2 MHz in comparison to the current source-based BIS system where the SNR drops below 40 dB for frequencies greater than 1 MHz.

Electrical Impedance Spectroscopy for Electro-Mechanical Characterization of Conductive Fabrics

“…Noting that the impedance analyzer produces erroneous results for very small and also very large values of the load impedance, we used the compensation method described in Mohamadou et al [43] to improve the measurement accuracy. In summary, we used a set of small and large loads with known impedance values and obtained their impedance spectra using the analyzer.…”

The differential Howland current source with high signal to noise ratio for bioimpedance measurement system

“…In Fig. 2, the operational amplifier has high input impedance, and the current inputting to operational amplifier A 2 is approximately equal to zero, so the output current of the current source can be expressed by (3).…”

“…However, in the voltage source circuit the input signal is needed to be adjusted according to different load down to the safety limit of the human injection current, which is relatively complicated. 2,3 Thus the current source has a wide range of applications especially in electrical impedance tomography (EIT). 4 The imaging system is also the measurement system, the accuracy and stability of the current source circuit is the key factor determining the quality of the reconstruction imaging.…”

A LabVIEW Based Data Acquisition System for Electrical Impedance Tomography (EIT)

“…EIT is a non linear inverse problem [12][13][14] which reconstructs the spatial distribution of the conductivity of the DUT from the voltage current data using the image reconstruction algorithm [12][13][14] which a computer program and developed with forward solver [12][13][14] and inverse solver [12][13][14]. An EIT system injects a constant current signal to the DUT such as a practical phantom [15][16][17][18][19][20][21][22][23][24][25][26][27][28] or human subject through electrodes or EIT sensors [1,11,25] using an EIT instrumentation [26][27][28][29][30][31][32][33] consisting of a current injecting device such as constant current source [34][35][36][37][38][39] and a voltage measuring device which may be a volt meter or other electronic voltage data measuring circuit (Fig. 1).…”