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Microcircuitry of analog interfaces of electrochemical impedance spectroscopy systems  

 Samoilov L.K.
 Zhebrun E.A.
 Titov A.E.
Date of publication

 medicine is clinical diagnosis based on biological sensors (biosensors).
High sensitivity of modern biosensors makes them the optimal solution for many applications of qualitative and quantitative determination (concentration estimation) of organic and inorganic substances [1].
Biosensors are the basis of a measurement platform in the systems of electrochemical impedance spectroscopy (EIS). Such systems can be implemented on the basis of chips and systems-on-chip with the internal integration of the main functional blocks. This creates favorable conditions for their use in various fields of science and technology. EIS systems are used in diagnostic robots, complexes of ecological environmental monitoring, unmanned aerial vehicles with the functions of detecting explosives, nerve gases, chemical compounds, molecules of biological structures, etc.
In clinical tasks for making point of care (POC) diagnostics the high speed of analysis and the possibility of portable use is required. Comprehensive medical tests based on multiple biosensors with high information content and reliability becoming increasingly urgent. The method of impedance spectroscopy allows this through the parallel operation of several conversion channels.
From the viewpoint of hardware implementation in the most general case EIS systems are traditional control and test systems based on interface bus [15]. They have a device forming a test signal and a device for registering the reaction of the test object to this signal.
The task of EIS system is to represent biosensor signal using the analog interface in a convenient form to make a decision. Information from the analog output interface serves as the foundation for the user interface, presenting the final results.
The study of different biosensors parameters [2]-[8] shows that the dynamic range of the sensors signals can reach 100 dB (which corresponds to 16 digital bits). Additionally, the interrogation signal must be fed to the biosensor for the implementation of the impedance measurement. The frequency of this signal can vary in the range of 5mHz - 100MHz. The AC signal amplitude is in the range of 5 – 10 mV and DC amplitude is 0.15 - 0.3 V. The output levels of biosensor current signals are in the range from few microamperes to hundreds of femtoamperes [14].
Low-frequency signals (0.005 – 0.1 Hz) of biosensors can be fed into a computer for further processing using standard analog signals input modules at moderate speeds of operation of selected interface (e.g., RS-485 [16] [17] or USB [18]).
At higher interrogation signal frequencies affecting the biosensors the immediate input of the information from sensors outputs becomes difficult even when using a single biosensor system.
An acceptable solution can be obtained using the preliminary (prior to the computer input) information processing.
From a general perspective the output biosensor signal is an amplitude-modulated voltage with a low modulation frequency and small modulation factor. A signal with such characteristics has a bandpass spectrum with very narrow bandwidth. From the information point of view such signals are better transmitted and processed using quadrature representation [9] [10].
For EIS systems the use of quadrature components gives additional advantages: the quadrature components actually define the parameters of the complex conductivity of the biosensor and its equivalent electrical circuit. Changing these parameters corresponds to a certain concentration of the measured substance.
Narrowband high-frequency signal from biosensor output is represented in two low-frequency signals, which are obtained in the two operations of multiplication with following low-pass filtering. The implementation of these operations is possible in digital, analog [11] and mixed form.
The most simple, economical and precise solution is an intermediate treatment with a mixed representation of signals on the basis of dual-slope ADC (DS ADC) [12].
DS ADC can be perfectly combined with a biosensor. An integrator with current-voltage converter function is at the input of the considered ADC. This allows the DS ADC to work directly with a biosensor without a unifying converter.
Such ADC can additionally produce a multiplication of a digital sample to a digital value [13] [14] with simple circuit modification. The disadvantage is the low conversion speed.
For mass-market clinical diagnostics devices the effective solution is the creation of a multi-sensor system with multiplying DS ADCs within a single system-on-chip. From this viewpoint the researches of DS ADC possibilities [19] taking into account the latest achievements in the field of high speed operational amplifiers and analog switches based on them are topical.
 analog interface, biosensor, impedance spectroscopy, IO modules, quadrature sampling.
Library reference
 Samoilov L.K., Zhebrun E.A., Titov A.E. Microcircuitry of analog interfaces of electrochemical impedance spectroscopy systems // Problems of Perspective Micro- and Nanoelectronic Systems Development - 2016. Proceedings / edited by A. Stempkovsky, Moscow, IPPM RAS, 2016. Part 3. P. 79-86.
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