Calibration of an EEG machine involves checking various parameters to ensure accurate measurements. It is important as it allows correct interpretation of recordings and comparison to previous studies. Parameters checked include paper speed, pen alignment, centering and damping, time constant, high frequency filter, sensitivity, amplitude linearity, gain, noise level and more. Verifying these helps identify any issues needing adjustment and confirms the machine is functioning properly.

1. Shehzad Hussain
Technologist

2.  Calibration is a procedure to ensure through the checks
that the EEG machine is fit for the purpose.
 It is set of sequential steps to test, compare and adjust the
measuring instrument before recording.

3.  Ensure that the instrument is fit for the purpose
 Helps interpreter in correct measurements
 Lets interpreter know the minor deviations
while reporting
 Helps to compare the recording with previous.

4.  Bio calibration assesses the response of the
amplifiers, filters and the recording apparatus on a
complex biological signal. Electrode FP1 and O2
are connected to all amplifier inputs. The
recordings for all channels should be identical.
 SIGNIFICANCE OF BIOLOGICAL CALIBRATION:
 It is a montage because a fixed pattern is set in
which throughout activity of 18 channels is
obtained.
 It reassures the machine calibration through
amplifiers.

5. Then we can interpretate
wrong and the result will
be wrong.
Either we make a normal
EEG abnormal.

6. Conventional EEG machine

7. Schematic Diagram

8. 1. Sufficient ink
2. Sufficient paper
3. Paper speed
4. Pen alignment
5. Pen centering
6. Damping

9. 7) Time constant or LFF
8) High frequency filter (HFF)
9) Sensitivity
10) Step gain
11) Amplitude linearity
12) Noise level

10. Digital Machine

11. Analog Machine

12.  Ensure that as per plan of the recording their is
more than enough paper/Disc space.
 Before taking printout sufficient paper is
present in the printer.
What’s wrong if not checked?

13.  Insufficient paper will lead cessation of live
recording
 Cessation may lead to miss:
1. any paroxysmal event
2. a brief Interictal abnormality
3. onset of a seizure.

14. Sufficient ink should be present in the tube
of writer/ printer cartridge should not be
empty.
Consequences:
1. any paroxysmal event
2. a brief Interictal abnormality
3. onset of a seizure.
4. Loss of paper

15.  It is the speed at which the paper roles off through
the pen writer.
 Standard paper speed is 3cm/sec or
30millimeter/second
 Standard paper size is 30 cm/fold
 Slow Paper Speed:
 Used with time constant to make the slowing
prominent.
 Like 15mm/sec.
 Fast paper Speed:
 It is used to check the onset of an event.
 60mm/sec

16.  Paper speed could be checked in two ways:
1. Manually by;
by measuring the distance simultaneously
running the paper and monitoring the time
lapsed.
Distance/time lapsed= paper speed.
2. Automatically by;
By using the time marker of the machine.

17. Distance=27cm, Time =30seconds P.S=27cm/30sec= 0.9cm/sec or 9mm/sec

18.  Misinterpretations of frequency
 Misinterpretation of waveform duration
 Wave morphology could change like spike
could appear as sharp wave and sharp
wave as slow wave.

19.  In digital EEG machine internal setting of
machine is difficult, interfere can cause other
consequences. So….
 If paper speed is wrong then inform the
biomedical engineer to correct it.

20.  All pens should start and stop simultaneously
in a perpendicular line.
 Could be checked by drawing a straight line
perpendicular to the paper drive by using set
squares
 This can be done with or without calibration
signal

21. Misalignment of Pens

22.  If not aligned then:
1. Will cause distortion of focal activity
2. Misinterpretation of EEG

23.  Causes of misalignment:
1. More/ less Bent of stylus nib
2. Worn out stylus nib (Nib can be bent inward
or outward).
 Corrections of Pen alignment:
Simply change the stylus, all pens should be
change simultaneously.

24.  All pens should be in the center ( at Zero line)
of the pen writer/galvano meter.
 How it is checked
 There should be no deflection off the base line
while starting the machine or switch on and off
the amplifier with no calibration signal.

25. Out Pen centering in conventional EEG

26. Mechanically Out Pen centering

27. Electrical out Pen centering

28.  Types of problems:
1. Mechanical pen centering
Apparently pens are not in the center of the
galvano meter.
1. 2. Electrical pen centering
While giving the electrical signal to the pens
and simultaneously on and off the switch of
galvano meter. Ideally there should no
deflection.

29.  Mechanical pen centering problem means that
the pen are not staying at zero line apparently
when the machine is switched on.
 Electrical pen centering problem means that the
pens are deviating from base (zero) line when
amplifier switched on or off without any
calibration signal while the machine is
switched on.

30. Potentio meter

32.  Effect on EEG recording:
1. Decrease in the high amplitude activities
2. Pseudo asymmetry of amplitudes
3. Decrease in the dynamic range
4. Distortion in the channel layout

33.  1. Correct the mechanical pen centering by
Allen Wrench-key, pull out the pen from its
base form the galvanometer and fix it on the
zero line.
 2. Correct the electrical pen centering by switch
on and off the amplifier and adjusting the
potentiometer. There should be no deflection
off the zero line.
 3. Again adjusting the mechanical pen
centering

34.  It is the pressure of the pens on the paper
 This could be
 Mechanical pressure on pens called mechanical
damping
 Electrical damping undesirable resistance in
the GVM circuit called electrical damping.
 Standard damping is 2.5 gram.

35. Over Damping
Under Damping

36.  There should be no rounding or overshoot of
peak while the TC is set to 0.1second and HFF
is set to 70 Hz as an standard parameters.

37.  Causes of mechanical damping
1. Excessive / loose tightening of pen holding
screw.
2. Worn out pens
 Causes of electrical damping
1. Excessive / low resistance in the GVM circuit

38. How these can be corrected;
 Mechanical damping should be considered
first.
 By using pen pressure tool and adjusting the
central pen holding screw to get critical
damping
 By changing pen if worn out
 By adjusting pen damping potentiometer to get
critical damping

39. 1. Over Damping (under shoot)
Excessive pressure on pens will result in
over damping or Under shoot.
Decrease in voltages measurement b/c of
rounding of peak.
2. Under Damping (over shoot)
low pressure on pens will result under
damping or over shoot.
Increase in voltage measurement b/c of
overshoot at peak.

40.  Undershoot result in rounding of peak of
waveform
 pseudo amplitude reduction.
 Overshoot will result in pseudo voltage
increase.
 Pens can collide with each other and can break.
 Both will result in waveform distortion.

42.  It is the low frequency filter/High pass filter
 One of the most important component of the circuit
which can make EEG normal and abnormal.
 Defined as the time content is the time required for
the pen to fall 63% from the peak of the deflection
produced.
 T.C depends upon the size of the capacitor.
 T.C effects the decaying signal of the Calibration
signal.

44.  It is measured in second but can be converted
into frequency
 Formula for conversion is:
f = 1/ 2 T.C

45.  Record a known calibration signal
 Ideally at standard parameters setting of:
Sen = 7 V/ mm
Input voltage = 50 V
Paper speed = 30 mm / sec
TC = ?

46.  Measure 1/3 ( 37%) of the total pen deflection
from the base line and mark it
 Draw a base line crossing the initial deflection
till is merge back to the baseline
 Draw a parallel line to the baseline at the mark
mentioned above to get crossing at decaying
curve.

47.  Draw a perpendicular using set square from
this point to the baseline
 Measure the distance from the initial deflection
to this bisecting perpendicular
 Divide this distance with paper speed to get
TC.

48.  Effects on EEG / usage
 It attenuates the low frequencies.
 Could be used with appropriate paper speed to
show focal abnormalities prominent.
 This can be used with extreme care as in the
case of sweating artifacts b/c it can make an
EEG normal or abnormal.

49.  Also called Low pass filter.
 It mainly effects the rise time in calibration
signal.
 In EEG we are interested in the high frequency
of 70 Hz.
 It allows low frequencies to pass and stop
higher frequencies.

52.  If not calibrated, then contact to Bio-Medical
engineer to fix it as this can be the resistor
dysfunction in HFF circuit

53. Effects of the HFF on EEG:
1. It drops the amplitude by 30% i.e. 3db/octave.
2. Causes rounding of peak which may the
morphology of the waveforms.
3. Amplitude reduction which may lead to
asymmetry
4. Phase Shift will distort the focus

54. Voltage attenuation

55.  This is the magnitude of input voltage requires to
produce a standard pen deflection i.e. (known input
voltage to produce known pen deflection)
 Can be checked by inducing a known input
calibration voltage (50 V) and measuring the pen
deflection under standard parameters i.e. HFF 70Hz,
LFF 0.1 Sec.
 Sensitivity=input voltage/Pen deflection=50uV/7.1mm
= _____V / mm

57.  All channels should have the same sensitivity
as standard
 If input voltage is change the output pen
deflection should be change in direct
proportion.
 sensitivity is adjusted to avoid cutting of the
waveforms.

58.  Effects on the EEG:
1. Misinterpretation of amplitudes.
2. Wrong sensitivity setting in individual
channels will lead to pseudo asymmetry
3. Blocking of channels will cause difficulty in
measurement of voltage and recognition of
spike or sharp activity

59.  A recording channel is said to be linear if
the pen deflection is proportional to the
amplitude of the input signal applied.
 If input voltage is doubled the pen
deflection should be doubled in all
channels.

60.  How it can be checked
1. Keeping sensitivity constant e.g 100uV/cm
2. By inducing calibrator signals of 20,50 & 100uV.
3. Measuring the calibration signal.
 P.D=100 micro volt/10 micro volt
 =1mm
 P.D=100 micro volt/20 micro volt
 =5mm
 P.D=100 micro volt/50 micro volt
 =2mm
1. If not controlled, then call Bio-Medical engineer

61.  Characteristics:
 Every system has an amplitude level beyond
which non-linear distortion occurs.
 The range over which the system is linear is
called dynamic range.
 Non-linearity can occur at very low pen
deflection because of pressure or default in the
writer system causing small amplitude
calibration signal (1 or 2 mm) to be rounded.

62.  This is the ratio of signal voltage obtained at the
output of amplifier to signal voltage applied at the
input.
 For example if an amplifier is set to give a voltage of
10V for an input voltage of 10uV then the gain is said
to be 1 million.
 In contrast to sensitivity, gain is predefined
 it increases with increasing amplification.
 Gain is not as important than sensitivity because gain
is not measured. It is prebuilt in the system.

63.  Effect on the EEG
1. Misinterpretations of voltages
2. Pseudo Asymmetry if this happen in
individual channels.
3. For correction call the bio-medical engineer

64.  All electrical circuit has small vibration in the
components of the circuit if current pass
through them because of electron movements
 This give a small output even if there is no
external signal
 This is called noise level of the measuring unit.

65.  How to check it
1. Switch on the machine
2. Disconnect the junction box to avoid any input
signal
3. Set the sensitivity to 2uV/mm
4. Record the trace
5. A peak to peak 2 mm deflection is acceptable.

66.  Effect on EEG
1. Very low voltage EEG like ECS recording
would be misinterpreted because of noise level
2. How it is corrected
If there is a problem call bio-medical engineer

67. What is this????
Square wave
Calibration
Sine wave
Calibration

68.  Basic principles of digital and analog
EEG,Fisch and Spehlmann’s EEG Primer