EEG sensor technology in brainwave, Thesis of Signal Processing and Analysis

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Analog Signal Chain considerations for Medical

applications

Bio-Potential signals (ECG/EEG)

Bio-potential signals

Types of bio-potential signals & other related measurements

Electro-Cardiogram Signal chain

 ECG Lead Derivation  Input Filtering and Defibrillation Protection  Typical ECG System Block Diagram  The INA front end  Right Leg Drive (RLD) Amplifier Selection and Design  The ECG Shield Drive  Lead Off Detection  PACE Detection  Gain, Resolution and Filtering

ADS129x Introduction, Features, and Advantages

Lowest power LMP9050x introduction

Questions

What is Electrocardiogram (ECG)?

A measure of the electrical activity of the heart

What is ElectroEncephaloGraphy (EEG)?

Electroencephalography (EEG) is the recording of electrical activity along the scalp, produced by the firing of neurons within the brain. In neurology, the main diagnostic application of EEG is in the case of Epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study.[

Other uses for EEG include: Bi-spectral index (BIS) monitor is a neuro-physiological monitoring device which continually analyses a patient's electroencephalograms during general anesthesia to assess the level of consciousness during anesthesia Evoked Potentials(EP) - EP involves averaging the EEG activity time- locked to the presentation of a stimulus of some sort (visual, somato- sensory, or auditory). Event-Related Potentials (ERPs) - Refers to averaged EEG responses that are time-locked to more complex processing of stimuli. Traumatic Brain Injury (TBI) – Software algorithms have been developed that can accurately determine the presence of damaged brain tissue. Stroke Detection – Software algorithms have been developed that can accurately determine Stroke occurrence.

Bandwidth of interest

ECG/EEG/EMG Signal Characteristics

1.5V

+/- 300mV

0V

Common mode 50 / 60Hz

Electrode offset

ECG : < +/- 2.5mV EEG : +/- 10uV to 100uV EMG : +/-50uV to 30mV

signal

signal

ECG : 0.05 – 150Hz EEG : 0.05 – 70Hz EMG : 7Hz – 20Hz

Noise specification over the specified bandwidth

ECG : 10uVpk-pk

EEG : 1uVpk-pk

EMG : 5uVpk-pk

ECG Measurement reality?

ECG irregular tracings due to external artifacts

Skin Electrode Interface Model

Electrical characteristics include a DYNAMIC resistance,

capacitance, and offset voltage

ECG Lead Derivation

3 Body Electrodes,

3 Derived Leads = I, II, III

LEAD I = VLA-RL – VRA-RL

LEAD II = VLL-RL – VRA-RL

LEAD III = VLL-RL – VLA-RL

Einthoven’s Law

In electrocardiogram at any given instant the potential of any wave in Lead II is equal to the sum of the potentials in Lead I and III.

Right Leg Reference, RL

ECG Einthoven Triangle, 1907

Standard Lead Configurations

Standards Electrodes Needed

1 Lead LA, RA 3 Lead LA, RA, LL 6 Leads LA, RA, LL 12 Leads LA, RA, LL, V1-

ECG Input Filtering and Protection

+Vs

+Vs

-Vs

Cdiff -Vs

CCM

CF CCM

CF

Rfilter

Rfilter

Rfilter

Rfilter

LA

RA

NE2H

NE2H

Protection Diodes

Zener Diode

Ne2H Lamps/TVS Protect Cdiff = 10 x Ccm

Against Defibrillation

Voltages

Series Resistance

Limits Input Current

CF, Ccm, and Cdiff +

Rfilter form LPF

RPatient

RPatient

Patient

Protection 10

-20k ohms

Example: LEAD I Protection with Input Filtering

The INA Front End

Key Features of the INA Front End

Average VCM is Inverted and Fed Back to RL; Cancels 50/60Hz noise

*Tapping off center of split gain resistor feeds the following voltage to the RL Drive Circuit [(Vcm+ ECGP )+ (Vcm+ ECGn )]/ 2

= Vcm + (ECGp + ECGn )/

The RL Drive Amplifier

The RL Drive Amplifier Serves 2 Purposes: (1) Common Mode Bias (2) Noise Cancellation

The ECG Shield Drive

• Capacitance of cable can be 500 pF to 1.5 nF
• Isolation resistor Necessary for improved EMI/RFI filtering - Shield is driven to

(VIN(+) — VIN( – )) /

- Eliminates Leakage

from CP1 and CP

ECGP

ECGN

CP

CP VCC/

Shield drive eliminates leakage to ECG Inputs