Biomedical Sensor Systems Laboratory. Institute for Neural Engineering Graz University of Technology

Transcription

1 Biomedical Sensor Systems Laboratory Institute for Neural Engineering Graz University of Technology 2017

2 Bioinstrumentation Measurement of physiological variables Invasive or non-invasive Minimize disturbance of measured signal Comply with requirements of living system Maximize signal-to-noise ratio Signal processing (of electrical signal)

3 Sensors Physical sensors (resistive, capacitive, inductive, piezo, ultrasound, optical, ) Chemical sensors (chemical reaction with measured variable)

4 Data acquisition Sensors (transducers, electrodes) Biosignal (EEG) amplifier Analog/Digital Converter (ADC) Computer Recording software

5 Data acquisition Amplifier ADC card Electrodes Computer Amplifier

6 Transducers Converts signal to different energy form Why can t we measure electrical potentials on the scalp with metal wires, just like we do in normal electronics?

7 Electrode interface The current crosses it from left to right. The electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C + and anions A -. J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

8 Double layer models Simplified illustration of the potential development in the area and in the further course of a Helmholtz double layer.

9 Double layer models Helmholtz Model (parallel plate capacitor) Gouy and Chapman Model (diffuse layer) Stern Model (compact diffuse layer) Esin and Markov, Grahame, Devanathan Model (triple layer)

10 Half-cell potential J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

11 Polarizable/non-polarizable Perfectly polarizable: No charge crosses interface (capacitor) Perfectly non-polarizable: Current passes freely across interface

12 Ag/AgCl electrode J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

13 Sintered Ag/AgCl electrode Reusable many times Low noise J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

14 Electrodes

15 Equivalent circuit J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

16 Impedance J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

17 Equivalent circuit Can be shortened out (abrasive gel) J. G. Webster. Medical Instrumentation Application and Design. John Wiley & Sons, 1998.

18 Electrolyte Abrasive Adapts to skin surface Electrode gel

19 Motion artifacts Motion mechanically disturbs distribution of charge Minimal for non-polarizable electrodes

20 Electrode positions

21 Electrode caps

22 Homunculus

23 Amplifiers

24 Requirements Minimal loading of signal (high input impedance, >10MΩ) Protection of the organism (isolation circuit) Output impedance low Frequency-specific (high SNR) High gains (>1000) Differential amps must have high CMRR

25 Signal recording chain Preamplification High pass Amplification with galvanic isolation Low pass

26 Instrumentation amp Input buffer Differential amp

27 Instrumentation amp Low DC offset Low drift Low noise High gain High CMRR High input impedances

28 Filters Low pass High pass Notch Anti-Aliasing

29 Typical specifications Impedance: >100MΩ Frequency range: ,000Hz CMRR: >110dB Amplification: mv-v Input filter: Hz Sensitivity: mV Resolution: 12-22bit

30 Active electrodes Small preamplifier directly on electrode Minimizes noise from cable sway

31 Technical artifacts Noise sources Amplifier noise Electrode noise Saturation Electromagnetic fields (50/60Hz) Aliasing Quantization noise

32 Biological artifacts Eye movement (EOG) Heart (ECG) Muscles (EMG) Movement Sweat Noise sources

33 Signal-to-noise ratio: SNR SNR = P signal P noise SNR db = 10 log 10 P signal P noise

34 Artifacts Depending on the type, artifacts can be Avoided during the measurement Minimized after the measurement Identified after the measurement

35 Avoid artifacts Capacitive coupling of line noise: CMRR Inductive coupling: Twist leads Faraday cage Minimize movement Normal room temperature Correct filter settings