Electrode and leakage current testing in an EEG monitor with an implantable part

1. A personal wearable EEG monitor comprising an implantable electrode part with at least two electrodes adapted for measuring an EEG signal of a person, said electrode part comprising an electronic circuit arranged in a housing with each electrode arranged external to said housing, said electronic circuit being adapted for receiving an analogue EEG signal from said electrodes and being provided with an analogue to digital converter for converting the analogue EEG signal into a digital signal, said electrode part comprising a testing circuit for testing for faults in said electrode part, wherein said testing circuit comprises

a capacitor coupled in serial connection to at least one of said electrodes, and

a test signal generator for providing a test signal, said test signal generator being adapted for being coupled between said electrodes and said electronic circuit during a testing period, wherein said EEG monitor is adapted for analyzing the test signal from said signal generator for identification of faults in the electrode part.

2. The EEG monitor according to claim 1 , wherein said test signal generator is arranged between said electrodes and an input of said analogue to digital converter, and wherein said EEG monitor is adapted for analyzing a signal at said input of the analogue to digital converter for identification of faults in the electrode part.

3. The EEG monitor according to claim 1 , wherein an output, signal from said analogue to digital converter is transferred to a digital signal processing unit arranged in the electrode part.

4. The EEG monitor according to claim 1 , wherein said capacitor is coupled in serial connection to the input of said analogue to digital converter.

5. The EEG monitor according to claim 1 , adapted for identifying at least one of a charging and a discharging function at an input of the analogue to digital converter and thereby detecting a current leak.

6. The EEG monitor according to claim 1 , wherein a resistor is arranged across an input of the analogue to digital converter.

7. The EEG monitor according to claim 1 , wherein the implantable electrode part comprises a coil for an inductive coupling to a non-implantable part, said inductive coupling being adapted for transfer of data and power.

8. The EEG monitor according to claim 1 , wherein said test signal generator provides a square wave test signal with frequency between 25 and 40 Hz.

9. A method for detecting a leak current in an implanted EEG monitor according to claim 1 , comprising the steps of

providing said test signal,

subtracting an estimate of said test signal from an output of the analogue to digital converter to obtain a resulting signal,

low-pass filtering the resulting signal in order to suppress EEG and noise components,

transforming the low-pass filtered signal into a logarithmic domain,

fitting the transformed signal to a straight line,

determining an interception with y-axis of the straight line, and

determining if a leak current is present based at least in part on the determined interception.

10. The method according to claim 9 , wherein said low-pass filtered signal is down-sampled in order to reduce the calculation complexity.

11. The method according to claim 9 , wherein said step of transforming the low-pass filtered signal comprises determining the power of the low-pass filtered signal and taking the natural logarithm of the power of the low-pass filtered signal.

12. The method according to claim 9 , wherein said step of transforming the low-pass filtered signal comprises determining the absolute value of the low-pass filtered signal and taking the natural logarithm of the absolute value.

13. The EEG monitor according to claim 2 , wherein said capacitor is coupled in series between said at least one electrode and an input of said analog-to-digital converter, and said test signal generator is coupled to supply said test signal to another input of said analog-to-digital converter.