ACQUISITION DEVICE TO LIMIT LEAKAGE CURRENT IN ELECTROPHYSIOLOGICAL SIGNAL RECORDING DEVICES

Abstract

The device limits the leakage current in an electronic system for recording electrophysiological signals, where the transducer element is an active device, the device comprising an active transducer (1), intended to contact a human tissue, connected to a transimpedance amplifier (2), and a first resistor (6) connected parallel to the transimpedance amplifier (2), an alternate voltage source (7) and a direct voltage source (8), both connected to the active transducer (1), a first capacitor (3) connected between the alternate voltage source (7) and the active transducer (1), a second resistor (4) connected between the direct voltage source (8) and the active transducer (1), parallel with the first capacitor (3) and the alternate voltage source (7), and a second capacitor (5), connected between the active transducer (1) and the transimpedance amplifier (2).

Claims

1. An acquisition device to limit leakage current in electrophysiological signal recording devices, that comprises an active transducer connected to a transimpedance amplifier and intended to contact a body tissue, and an alternate voltage source, connected to the active transducer, opposite to the transimpedance amplifier, characterized in that the device additionally comprises: a direct voltage source, connected to the active transducer, parallel to the alternate voltage source, a first capacitor connected between the alternate voltage source and the active transducer, a second resistor connected between the direct voltage source and the active transducer, parallel to the first capacitor and the alternate voltage source, and a second capacitor, connected between the active transducer and the transimpedance amplifier.

2. The device according to claim 1, wherein the active transducer is a graphene transistor (gSGFET).

3. The device according to claim 1, wherein the resistance of the second resistor is higher than Vsupp/50 uA, wherein Vsupp is a maximum supply voltage of the direct voltage source.

4. A multiple active transducer acquisition device, that comprises an acquisition device according to claim 1, and that additionally comprises one or more acquisition modules, connected parallel with the active transducer and transimpedance amplifier of the acquisition device, and comprising each acquisition module an active transducer, connected to a transimpedance amplifier, and a first resistor connected parallel with the transimpedance amplifier.

5. The device according to claim 4, wherein the active transducers are graphene transistors (gSGFET).

6. A multiplexed array acquisition device, forming a matrix with m columns and n rows, wherein the device comprises mn acquisition devices according to claim 1, being the active transducers situated in each mn position of the matrix, and connected to: in each column, to a common first capacitor and a common second resistor parallel to the common first capacitor, being the common first capacitor connected to a common alternate voltage source and the common second resistor to a common direct voltage source, and in each row, to a common second capacitor, a common transimpedance amplifier following the common second capacitor and a common first resistor parallel to the common transimpedance amplifier.

7. The device according to claim 6, wherein the active transducers are graphene transistors (gSGFET).

Description

DESCRIPTION OF THE DRAWINGS

[0014] To complement the description being made and in order to aid towards a better understanding of the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description wherein, with illustrative and non-limiting character, the following has been represented:

[0015] FIG. 1Shows a schematic of an acquisition system based on electrodes from the state of the art.

[0016] FIG. 2Shows a schematic of an acquisition system based on active transducer (gSGFET) from the state of the art.

[0017] FIG. 3Shows a schematic of a DC coupling device from the state of the art.

[0018] FIG. 4Shows a schematic of an AC coupling device, which is the acquisition device.

[0019] FIG. 5Shows a block diagram of a product detector.

[0020] FIG. 6Shows a schematic representation of a graphene transistor (gSGFET) working as a mixer.

[0021] FIG. 7Shows the power spectral density (PSD) of a modulated and demodulated signal.

[0022] FIG. 8Shows a graphene transistor (gSGFET) equivalent circuit connected to AC coupled system.

[0023] FIG. 9Shows the leakage current through Cg-e during normal operation mode.

[0024] FIG. 10Shows the multiple active transducer acquisistion device n rows.

[0025] FIG. 11Shows the multiplexed array acquisition device with m columns and n rows.

PREFERRED EMBODIMENT OF THE INVENTION

[0026] With the help of FIGS. 1 to 11, a preferred embodiment of the present invention is described below.

[0027] FIG. 3 shows a DC coupling system, which comprises an active transducer (1), connected to a signal acquisition module, comprised of a transimpedance amplifier (2) with a gain fixed by a first resistor (RG) (6). The active transducer (1) is connected to a first direct voltage source (8), as well as the transimpedance amplifier (2). Represented with a dotted-line arrow is the active transducer (1) current and with a continuous-line arrow is the patient leakage current. The object of the present invention is to eliminate said patient leakage current.

[0028] FIG. 4 represents the device object of the present invention, an AC coupling system, which also comprises an active transducer (1), connected to a transimpedance amplifier (2) with a gain fixed by a first resistor (RG) (6). The active transducer (1) is connected to an alternate voltage source (7) and to a direct voltage source (8).

[0029] As shown in FIG. 4, the device comprises additionally a first capacitor (Cs) (3) connected between the alternate voltage source (7) and the active transducer (1), a second resistor (RDC) (4) connected between the direct voltage source (8) and the active transducer (1), in parallel with the first capacitor (Cs) (3) and the alternate voltage source (7), and a second capacitor (Cd) (5), connected between the active transducer (1) and the transimpedance amplifier (2).

[0030] In FIG. 4, Ids represents the active transducer (1) current, lined arrows indicate the patient leakage, which is limited by the second resistor (4) and dotted arrows indicate the patient leakage current, which blocked by both capacitors (3,5). All three circled elements are passive components that limit the leakage current. Particularly, the second resistor (RDC) (4) limits the maximum leakage current and allows to set the voltage bias point, and the first and second capacitors (3, 5) block any DC current while allowing the pass of AC current which contains a recorded signal.

[0031] The second resistor (4) and the capacitors (3, 5) limit the leakage currents, but at same time, limit the DC current through the active transducer (1) (Ids). Therefore, an AC signal coupling strategy must to be used for the neural signal acquisition instead of the DC signal coupling.

[0032] In an AC signal coupled system, the active transducer (1) behaves as a signal mixer between a carrier signal and an electrophysiological signal, allowing the use of amplitude modulation (AM) techniques for the signal amplification and processing.

[0033] The AM modulation consists on a mixer that multiplies a carrier signal and a modulator signal. The carrier wave (Wc) is a high frequency signal, where its amplitude will be a function of the modulator signal (Wm). The modulator signal can be recovered by a demodulation process, which consists on a product detector, as the one shown in FIG. 5. The input signal is multiplied by a carrier signal produced with a local oscillator to move the signal to the baseband, then a low pass filter (9) removes remaining high frequency components.

[0034] In FIG. 5, the block diagram of a product detector is presented. AM signal is multiplied by a local carrier signal of frequency w.sub.c with 0 phase (cosinus) to obtain real part and 90 phase (sinus) to obtain imaginary part. The low pass filter (9) is used to remove the high frequency components. The original signal (Wm) is recovered by computing the absolute value.

[0035] In a preferred embodiment of the present invention a graphene transistor can be used as the active transducer (1). The graphene transistor (1) can be used as a mixer to implement an AC coupled acquisition system. In this case, an electrical potential fluctuation applied on its gate (i.e neural signal) changes the conductivity of the channel through the gate capacitance. Therefore, applying a pure tone voltage signal on the graphene transistor (1) source (Vcarr), the current through the channel (Ids) results from the product of Vcarr(t).Math.Vsig(t), as shown in FIG. 6, where Vsig(t) is the brain signal applied in the gate of the graphene transistor (1) (Vgs).

[0036] The multiplication of those signals produces the folding of their frequencies, as can be seen in FIG. 7left, where a peak at the carrier frequency (fc) is observed, with an amplitude proportional to channel resistant Rds-DC, with two side bands at fc-fsig.sub.1 and fc+fsig.sub.1 which are proportional to Vsig/gm. The gate signal fsig.sub.1 is obtained after the demodulation process, as shown in FIG. 7right.

[0037] For an optimal device operation, it is needed to control the bias point. The gate source DC voltage coming from the DC voltage source (8) must be fixed at a certain value according with the environmental conditions and the graphene transistor (1) performance. For that, the second resistor (Rdc) (4) is placed in parallel to the first capacitor (Cs) (3), which is connected to the DC voltage source (8).

[0038] FIG. 8 shows the graphene transistor (1) equivalent circuit connected to the transimpedance amplifier (2) together with the protective elements second resistor (4) and capacitors (3, 5). It can be observed how the DC current circulating through the graphene transistor (1) is approximately zero in a stationary regime, when the gate capacitance (Cg-e) of the graphene transistor (1) and protective capacitances (3, 5) are charged at the bias voltage (Vsource) coming from the DC voltage source (8).

[0039] If any electronics or device failure occurs, the maximum allowed leakage current will be limited by the second resistor (4). To accomplish with the regulation, the second resistor's (4) value should be high enough to limit the current through itself to a maximum value of 50 A. For that, the worst-case scenario is chosen, which is the case where any of the circuit nodes have the maximum voltage (the electronics supply voltage Vsupp). Thus, the value of the second resistor (4) have to be chosen higher than Vsupp/50 uA.

[0040] FIG. 9 shows the leakage current through the gate capacitance (Cg-e) of the graphene transistor (1), during normal operation mode, which is lower than 10 A, meeting the IEC60601-1.

[0041] To achieve a correct functionality of the device, the value of the second capacitor (Cd) (5) and first capacitor (Cs) (3) should be chosen according with the frequency of the carrier signal (Fc) that the AC voltage source (7) introduces to avoid signal attenuation.

[0042] For the first capacitor Cs (3) capacitance value, it has to be considered that it is in series with a channel resistance (Rds) (10) of the graphene transistor (1), as shown in FIG. 8. Accordingly, the first capacitor (3) value has to accomplish the following expression:

[00001]$f_c>\frac{1}{2.Math..Math.C_s.Math.\left(R_{\mathrm{ds}}//R_{\mathrm{DC}}\right)}$

[0043] For Cd capacitance value, it has to be considered the channel resistance Rds (10).

[00002]$f_c>\frac{1}{2.Math..Math.C_d.Math.R_{\mathrm{ds}}}$

[0044] Typically, electrophysiological applications require several recording sites, and at the same time minimizing the number of connection wires. For this reason, the graphene transistors (1) can be arranged in arrays where the source terminal is shared by all the graphene transistors (1) inside the array.

[0045] FIG. 10 shows how to implement the leakage current limitation in this kind of arrays. In this case, a unique first capacitor (3), second resistor (4) group is necessary because all the graphene transistors (1) use the same carrier signal, introduced by the AC voltage source (7), and are biased at the same working point. On the other hand, each graphene transistor (1) acquires a different electrophysiological signal, then different second capacitors (Cd) (5) need to be placed between each graphene transistor (1) and each transimpedance amplifier (2).

[0046] As commented, graphene transistors (1) can be used in multiplexed arrays to minimize the wiring connectivity. In FIG. 11, the arrays are arranged in m columns and n rows, implementing mn recording sites while the wires needed for the connections is just n+m.

[0047] To implement the proposed protection in this kind of arrays, as shown in FIG. 11, the first capacitor (Cs) (3), second resistor (RDC) (4) group should be placed at each column, to provide the carrier signal and bias voltage to each one of the graphene transistors (1), and the second capacitor (Cd) (5) should be placed between each row and the transimpedance amplifier (2), as in the case of the non-multiplexed arrays.

[0048] The work leading to this patent application has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreements No. 649953 (Graphene Flagship) and No. 732032 (BrainCom). We also acknowledge funding the 2DTecBio project (FIS2017-85787-R) funded by the Spanish Ministry of Science, Innovation and Universities, the Spanish Research Agency (AEI) and the European Regional Development Fund (FEDER/UE); and the European Regional Development Funds (ERDF) allocated to the Programa operatiu FEDER de Catalunya 2014-2020, with the support of the Secretaria d'Universitats i Recerca of the Departament d'Empresa i Coneixement of the Generalitat de Catalunya for emerging technology clusters devoted to the valorization and transfer of research results (GraphCAT 001-P-001702).