This document discusses electronic filters used in EEG machines, which perform signal processing to enhance desired frequency components while eliminating unwanted ones. It details the types of filters—low-frequency (high-pass), high-frequency (low-pass), and notch filters—along with their mechanisms, effects on EEG signals, and merits and demerits. Additionally, it highlights the significance of capacitance and resistance in filter design and their impact on signal quality in EEG recordings.

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
removeunwanted materialsfrom an object.
(For e.g. Air filter, water filter etc.)
• Asper 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:
Thecurrent flowing through aconductor between two pointsis
directly proportional to thepotential differenceacrossthetwo
points, and inversely proportional to theresistancebetween them.
V=IR
Filtersaremadeup of :
i)Capacitor ii) Resistance
1.Resistance
Any component which isused to provideresistanceiscalled a
resisto r.

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
iscalled acapacitor.
•The charge needed to produce a voltage of 1 volt across
thecapacitor iscalled itsCapacitance.

5. USAGE OF FILTERS
• Filtersareused to excludewaveformsof relatively high or low
frequency from theEEG so that thewaveform in themost
important range(1-30Hz) can berecorded clearly and without any
distortion.
• Thefilter receivetheEEG signal after it haspassed from the
differential amplifier to asecond, singleended amplifier. After
passing through each filter thesignal isamplified again by single
ended-amplifier.
(Reference46.1 fisch and spehlmann’s EEG primer)

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

7. TYPESOF 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 PASSFILTER:
• LFFsallow to passhigh frequency signalsand cut off low frequency
signals.
• It isalso known ashigh pass filter/ time constant.
• It ischaracterized by itseffect either on sinewaves, giving thelow filter
frequency’, or on squarepulses, giving thetimeconstant’.
(Reference3.5.1 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 specifiesthefrequency at which sinewavesarereduced in
amplitudeby afixed fraction.
• Thefilter reducetheamplitudeof sinewaveat
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
frequenciesby 30.3% or 3 db./octave.

14. MERITS OFLFF:
• LFF areused to attenuateunnecessary signals.
• It isused to cut off low frequenciesand to passhigh
frequenciesfor EEG convenience.
• It isused when you facesweating, ECG and voltage
fluctuations.
• It isvery usefulin demo nstrating fo calslo w wave if
simultaneously used with appropriatepaper 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 filtersallow passing low frequency signals
and cut off high frequency signals.
• It also known aslow passfiltersor high cut filter.
• It mainly effectsrisetimein acalibration signal.
• Thehigh frequency filter reducesthesizeof fast wavessuch
asnoiseand changeswavemorphology.

22. • It will not affect thelow frequencies
• In HFF thesignal first passthrough theresister and then
capacitor and finally giveactivity.
• It affect thefrequenciesby 30.3% (3 db/ octave)
• Rangesof 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 OFHFF
• It passeslow frequenciesand cut off high frequenciesto givea
perfect EEG record.
• It isused to filter out all musclesartifacts.

27. DEMERITS OFHFF
• It dropstheamplitudesby 30 % i.e. 3 db/octave.
• It causesrounding of peak which may change
themorphology of awaveform.
• It reducesamplitudeof awaveform which may
leadsto assymetry.
• It causesPhaseshift which will distort thefoci.

30. NOTCHFILTERS
• Any typeof 50 Hz or 60 Hz noisecomponentsthat arenot
originating from thebrain (non-cerebral activity) ishigh
frequency noises.
• Weuse50Hz or 60HZ filters, so thisisknown as50 or 60Hz
filters.
• Wecan useit when interferenceoccursfrom electronic devices
powered by AC.

31. MERITS OFNOTCHFILTERS
• Thenotch filter selectively reducestheamplitudeof wavesin
anarrow frequency range, in order to removeelectric line
interference(crossing of power cord or wires).
DEMERITS OFNOTCHFILTER
• It reducetheamplitudeof Sinewavesof 50Hz aswell asalso
reducetheamplitudesof neighboring frequenciesto some
extent.

33. The end