Amplifier and Filter Functions in Sleep Disorder Technology

Amplifier Function and Signal Filtering used
in Sleep Disorders Technology

An Overview

Amplifiers and Signal Filtering
You Need to know:
 What differential amplifier is
 What CMR is and how it works
 How TC filters work and TC relationship to LFF
 How LFF and HFFs work to reduce unwanted
frequencies
 How a 60Hz notch filter works
 Know basic polarity problems
 The difference between an AC and DC amplifier
 The difference between gain and sensitivity

2
Midwest Sleep and Neurodiagnostic Institute, Inc

Terms for Basic Electricity
Impedance: The total opposition to current flow
including resistance, inductance and capacitance.
Measured in ohms.
Resistance:The opposition to the flow of current.
Measured in ohms.
Volt: Measurement of the force pushing current
(electrons) through a conductor.
Polarity: Refers to the positive or negative poles of a
device

3

Alternating Current (AC)
Alternatively flows
between positive
and negative
(protons+ and
electrons-)
Alternates at 1/60
sec in USA
 1/50 sec in Europe

4

Ohms Law
Voltage is equal to current flowing in
the circuit, multiplied by the resistance.

Voltage = E (volts)
Current = I (amps)
Resistance = R (ohms)

5

V= I x R

E
Voltage

Current Resistance

I
I R R
R E I E

6

The Polygraphic Circuit
The function of the
polygraph is to
transform voltages
generated by body
into an interpretable
record

7

G2 G1

- - - - + + +
Chart
Drive
- - + ++

Paper / computer

8

Signal Processing
Electro conductive material
Electrode
 These act as a small capacitor

Wire to head box
Exploring electrode
 C3, C4

 O1, O2

Reference electrode
 A1, A2

9

10

Ground
Leakage current results when there is
either a break or a shunt in the circuit.
Patient grounding is essential in artifact
elimination and proper amplifier function.

Symbol for Ground

11

Differential Amplifier

Form of AC amplifier
Amplifies the
difference between
input (grid) 1 and
input (grid) 2
Rejects any similar
information between
G1 and G2 (CMR)

12

Common Mode Rejection
Common function of differential amplifier
Rejection of similar activity between input
1 and input 2
 Identical information rejected at near 100%

13

Polarity and Localization
Input 1
 When input one is negative, the pen goes
up, when input one is positive, the pen
goes down.
Input 2
 When input two is positive, the pen goes
up, when input two is negative, the pen
goes down.

14

Basic Polarity and Localization
Up rules Down rules

Input 1 (G1) Negative Positive Input 1 rules

Input 2 (G2) Positive Negative Input 2 rules

15

AC coupled Amplifier

Common Mode Rejection Ratio

Common Input Signal Should be high ie. 100,000 to 1
Output Signal

16

Polarity Calculation

26 uv
+ 41 uv
-15 uv

26
- 15
+ 41 uv

17

Galvanometer

Limited to a maximum response of 90 - 120hz
18

Amplifier/Machine
Calibration

All filters set the same

19

Montage Calibration

20

Time Axis
 The point (time) at which
each pen falls along the
same recording line
 The galvonometer or pen
mount may be adjusted to
correct problems
 Not a problem with digital
systems

21

Damping
Adjustment to
reduce overshoot good
Reduces erroneous
fast-wave artifact
Only adjusted on Under
polygraphs damping
 Not digital systems
Over
damping

22

Mechanical Baseline
Pen spacing without
amplifier input to
pens.

23

Electrical Baseline
 The “electrical zero” of a
recording pen position
Mechanical Baseline needs
 Found by turning the adjustment
individual chart amplifier
power off and on while
adjusting the baseline
knob.
 EZ if found when the
pen stops changing Electrical Zero
positions between
power-on, and power-off
24

1. Unequal sensitivity

2. Pen alignment—
time axis

3. Pen baseline off

4. LFF different
setting

25

Gain vs. Sensitivity
Gain is a measure of the ability to
change the magnitude of the input
voltage
Sensitivity is the amount of output
(pen deflection) to a given input
voltage

26

V= S x PD

V uV

Voltage

Sensitivity Pen Deflection

S
S PD
uV/mm
mm
PD
PD V S V

27

Sensitivity Calculations
Sensitivity is at 5uv/mm the voltage is 50 uv what is the
deflection?

D = V/S

D = 5uv/mm 50uv
D = 10 mm

28

Sensitivity is set at 7 uv/mm and the
deflection is 12 mm. What is the
voltage?
V=SXD

V = 7uv/mm X 11 mm

V = 77 uv

29

Voltage is 75uv and the deflection is 15
mm. What is the Sensitivity setting?
S = V/D
S = 15mm 75 uv

S = 5 uv/mm

30

60 Hertz Notch Filter

Also known as an AC Filter
In North America, all AC electrical current oscillates
at 60 Hz.
Uses CMR by tapping into line current and
measuring it against output.

31

60 Hertz Notch Filter
100
90
80
70
60
50
40
30
20
10
0
40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80
32

Units of Measure Used in Filtering
Hertz (Hz)
 Most common unit in electroneurodiagnostics
Second (sec)
 Used in Time Constant (TC) filter calculations
Decibels (dbl)
 Most basic unit of measure in frequency filtering
 Rarely referred to in sleep diagnostics

33

Time Constant (TC)
Units = seconds (s)
The time it takes in seconds 100%
for a waveform to drop to
37% of its calculated
amplitude 37%

Faster TC – Reduces .003 .3 1
amp. of slow frequencies Time (s)
Low cut – high pass
filter

34

TC (sec) and LFF (Hz) Relationship

TC and LFF work with same goal
 Reduction of slow frequencies
 Different units of measure to describe the same
action
Conversions:

1 1
LFF = TC =
2π x TC 2π x LFF

35

Basics of LFF and HFFs
Reduces amplitude of listed frequency
at a fixed value
 20% (Nihon Kohden)
 30% (Grass)
Reduces amplitude of frequencies
above (HFF), or below (LFF) at linear
levels

36

Low Frequency Filters

Designed to reduce the amplitude of
frequencies at and below the selected
frequency
“Low cut, high pass filters”.

37

Expected Amplitude Decay: LFF = 10Hz
Based on Nihon Kohden filters

100
90
Reduces amplitude of a 10 Hz signal by 20%
80
 Reduces a 5 Hz signal by 50%
Amplitude

70

60  Reduces a 2.5Hz signal by 75%
50  Reduces a 1.25 Hz signal by 87.5%
40
30

20
10

0
1.25Hz 2.5Hz 5Hz 10Hz
38

39

Low Frequency Response Curve

40

High Frequency Filters
Designed to reduce the amplitude of
frequencies at and above the selected
frequency
High cut – low pass filters

41

Expected Amplitude Decay: HFF = 70Hz
(based on Nihon Kohden filters)

100

90
Reduces amplitude of a 70Hz signal by 20%
80

Reduces a 140 Hz signal by 50%
Amplitude

 70

60
 Reduces a 280 Hz signal by 75%
50 
Reduces a 560 Hz signal by 87.5%
40

30

20
10

0
70Hz 140Hz 280Hz 560Hz

42

High Frequency Response Curve

43

44

Filters Effect Amplitude & Phase

High Filter

Low Filter

Note: The Higher the filter the lower the amplitude

Digital Filters will not cause a Phase Shift
45

Direct Current Amplifiers
Used to amplify signals which represent
either all negative, or all positive
voltage
 Not alternating between – and + (AC)
Used most often with slow trending
data (SaO2 and heart rate trending)
No LFF or HFF filters utilized
 TC infinite (No frequencies to filter)

46

Wheatstone Bridge Circuit
A four-arm bridge used to measure
resistance in a circuit
Commonly used in DC amplifiers when
resistance devices are in use
 Strain gauges
 NPT
 Effort gauges

47

Analog to Digital Conversion
Brainwaves and other PSG signals are
analog or continuous
Digital is based on values of 0 or 1
ADC samples the signal and assigns a
digital value

48

Sampling Rate
The rate at which the computer
program samples the analog
information to convert it to digital
signal.
 The faster the sampling rate the more
accurate the tracing is
 The faster the sampling rate the larger the
patient data file will be

49

Sampling Rate
Nyquist principle
 The sampling rate must be at least twice
as fast as the fastest frequency that will be
recorded.
 ASET Standard is three times faster than
the HFF setting
 Can vary by channel
 200+/sec for EEG
 10/sec for respiratory

50

Sampling Rate
Aliasing
 Sampling rate too low

51

Amplifier and Filter Functions in Sleep Disorder Technology

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Amplifier and Filter Functions in Sleep Disorder Technology