Eeg machine filters

The filter setting of EEG, its merits, demerits and stranded setting for EEG to understand and not to misinterpreted.

1. EEG MACHINE FILTERS
&
THEIR MECHANISM
Shehzad Hussain
3-4 –Oct-2017

2. FILTERS
•Electronic filters are circuits which perform signal
processing functions, specifically to remove unwanted
frequency components from the signal, to enhance
wanted ones, or both. 
•The layman concept of filter is a device that is used to
remove unwanted materials from an object.
(For e.g. Air filter, water filter etc.)
•As per EEG concept;
Filters are bandwidth or circuits that allow alternating current
of some frequencies to pass through them more easily than
others.
Bandwidth:-
•Signals of a particular frequency ranges are called
Bandwidth.

3. Ohm’s Law:
The current flowing through a conductor between two points is
directly proportional to the potential difference across the two
points, and inversely proportional to the resistance between them.
V=IR
Filters are made up of :
i)Capacitor ii) Resistance

1.Resistance
Any component which is used to provide resistance is called a
resistor.

4. 2. Capacitor
•A capacitor is an electronic device which store the
electrical charge.
OR
•A combination of plates which can hold an electric charge
is called a capacitor.
•The charge needed to produce a voltage of 1 volt across
the capacitor is called its Capacitance.

5. USAGE OF FILTERS
•Filters are used to exclude waveforms of relatively high or low
frequency from the EEG so that the waveform in the most
important range (1-30Hz) can be recorded clearly and without any
distortion.
•The filter receive the EEG signal after it has passed from the
differential amplifier to a second, single ended amplifier. After
passing through each filter the signal is amplified again by single
ended-amplifier.
(Reference 46.1 fisch and spehlmann’s EEG primer)

6. Belton of EEG Machine:-
Diagram Ref.: Fisch and Spehlmann’s EEG Primer

7. TYPES OF FILTERS
•There are three basic types of filters we used in EEG machines.
•Low frequency filter/Time constant (high pass filters)
•High frequency filters (low pass filters)
•Notch filters (50Hz or 60Hz filters)

8. LOW FREQUENCY FILTERS/
TIME CONSTANT/ HIGH PASS FILTER:
•LFFs allow to pass high frequency signals and cut off low frequency
signals.
•It is also known as high pass filter / time constant.
•It is characterized by its effect either on sine waves, giving the low filter
frequency’, or on square pulses, giving the time constant’.
(Reference 3.5.1 fisch and spehlmann’s EEG
primer)

10. •EXPLANATION:
•If the output of RC circuit is taken across the
resistor then the maximal voltage drop occur only
when the frequency is high.
•If frequency is higher then capacitive reactance or Z
of capacitor is lower, result in little voltage drop.
•Resistor is not a frequency dependent.

12. TIME CONSTANT
•TC depend on the R & C circuit.
•It depends upon the size of a capacitor.
•It effects the decaying signal of a capacitor.
•In each TC period the capacitor charge further 63% and
discharge 37%
•So after 1 TC the Capacitor is 63% charge and 37% discharge.
•After 2 TC another 63% charge and 37% discharge.
•And after 5 TC capacitor is fully charged and discharged 99%.

13. •LFF specifies the frequency at which sine waves are reduced in
amplitude by a fixed fraction.
•The filter reduce the amplitude of sine wave at
0.16 Hz  1.00 sec
0.3 Hz  0.53 sec
0.5 Hz  0.30 sec
1.6 Hz  0.1 sec (standard LFF)
5.3 Hz  0.03 sec
10 Hz  0.0159 sec
15.9 Hz  0.0100 sec
53 Hz  0.003 sec
frequencies by 30.3% or 3 db./octave.

14. MERITS OF LFF :
•LFF are used to attenuate unnecessary signals.
•It is used to cut off low frequencies and to pass high
frequencies for EEG convenience.
•It is used when you face sweating, ECG and voltage
fluctuations.
•It is very useful in demonstrating focal slow wave if
simultaneously used with appropriate paper speed.

17. 0.1 sec
0.03 sec
1.0 sec
3.0 sec
5.0 sec
EFFECT OF LFF ON CALIBRATION SIGNALS

21. HIGH FREQUENCY FILTER
•High frequency filters allow passing low frequency signals
and cut off high frequency signals.
•It also known as low pass filters or high cut filter.
•It mainly effects rise time in a calibration signal.
•The high frequency filter reduces the size of fast waves such
as noise and changes wave morphology.

22. • It will not affect the low frequencies
•In HFF the signal first pass through the resister and then
capacitor and finally give activity.
•It affect the frequencies by 30.3% (3 db/ octave)
•Ranges of High Frequency Filters:
120 HZ
70 Hz is (standard)
50 Hz
35 Hz
15 Hz
EXPLAINATION:

24. EFFECT OF HFF ON CALIBRATION SIGNAL:-

26. MERITS OF HFF
•It passes low frequencies and cut off high frequencies to give a
perfect EEG record.
•It is used to filter out all muscles artifacts.

27. DEMERITS OF HFF
•It drops the amplitudes by 30 % i.e. 3 db/octave.
•It causes rounding of peak which may change
the morphology of a waveform.
•It reduces amplitude of a waveform which may
leads to assymetry.
•It causes Phase shift which will distort the foci.

30. NOTCH FILTERS
•Any type of 50 Hz or 60 Hz noise components that are not
originating from the brain (non-cerebral activity) is high
frequency noises.
•We use 50Hz or 60HZ filters, so this is known as 50 or 60Hz
filters.
•We can use it when interference occurs from electronic devices
powered by AC.

31. MERITS OF NOTCH FILTERS
•The notch filter selectively reduces the amplitude of waves in
a narrow frequency range, in order to remove electric line
interference (crossing of power cord or wires).
DEMERITS OF NOTCH FILTER
•It reduce the amplitude of Sine waves of 50Hz as well as also
reduce the amplitudes of neighboring frequencies to some
extent.

33. The end