This document discusses noise and interference in biopotential recording. It begins by defining noise and interference, then classifies noise sources as either internal (thermal, contact, shot) or external (conductive coupling, electric and magnetic fields, power line interference). It describes strategies for measuring noise using SNR and noise factor. The document then reviews techniques for noise reduction, including using short, shielded wires, differential amplifiers, common mode rejection, and twisting or shielding wires. It concludes by listing some references on the topic.

1. PRESENTED BY
ROHIT VIJAY
M.TECH
PGICE1650127
NOISE AND INTERFERENCE

2. CONTENTS
 INTRODUCTION
 CLASSIFICATION OF NOISE
 NOISE MEASURING
 NOISE REDUCTION
 REFERENCE

3. INTRODUCTION
 NOISE AND INTERFERENCE:- Noise can be
characterized as any disturbance that tends to obscure a
desired signal. Noise can be generated within a circuit or
picked up from external natural or artificial sources.
 Interference is noise that tends to obscure the useful
signal. It is usually caused by electrical sources but can
be induced from other physical sources such as
mechanical vibration, acoustical feedback, or
electrochemical sources.
 As surface biopotential recording involves the
measurement of extremely small potential differences,
noise is likely to play an important role.

4. CLASSIFICATION OF NOISE
 INTERNAL NOISE 1.Thermal Noise
2.Contact Noise
3.Shot Noise
 EXTERNAL NOISE 1. Conductive coupling
2. Electric and magnetic fields
3. Power line interference

5.  Thermal noise:- Every resistor will produce a certain
amount of noise above 0° K. This is due to the fact that
heat is the random movement of elementary particles.
In theory the movements are in all directions, and
should cancel each other, however some imbalance
remain. The random movement of charged particles
causes thermal noise.
 Contact noise:-All resistors have noise voltages in
excess of the thermal noise due to other noise
generation mechanisms. This additional noise is called
contact noise; it is dependent on the quantity of current
and the type of resistor. Contact noise is also called
excess noise, flicker noise, or pink noise

6.  Shot noise:-The flow of current is not continuous
in a circuit but rather is associated with random
variations in the number of charge carriers passing
some voltage boundary. Charge is limited by the
smallest unit of charge available-that of the charge on
an electron. Shot noise, like thermal noise, has the
same power per unit of bandwidth; hence it is a
type of white noise. When amplified, it sounds
something like lead shot raining on a metal roof-
hence the term shot noise.

7.  Conductive coupling:- Conductive coupling of
noise requires at least two or more conductive
paths to the noise source. There cannot be a complete
circuit for the noise if the conductive path to reduce to
only one conductor.

8.  Electric and magnetic fields: When a current flows
through a conductor, electric and magnetic fields are
present. Electric field interference is also called
capacitive coupled interference since all the
configurations of conductors have capacitance
between them, allowing a coupling path. Magnetic
field interference can come from inductors,
transformers, conductors, or any low-impedance
source in a circuit.

9.  Capacitive coupling:- The patient, the electrodes and
the electrode cables are capacitively coupled to nearby
electrical fields, for instance power lines. In theory, the
voltages induced by this capacitive coupling will
appear as common mode voltages over the electrode
leads, assuming that both electrodes and electrode
leads are equally affected by the interference. When a
differential amplifier is used the common mode
voltage is rejected. A common-mode rejection ratio of
80-90 dB is customary in clinically used differential
amplifiers.

10.  Inductive coupling:- Because the measurement setup
forms a closed loop, a magnetic field can cause an
inductive current to flow in the loop. The simplest way
to reduce interference due to inductive coupling is
twisting of cable pairs. The sign of the induced voltage
is dependent on the orientation of the two cables.
When twisting the cables, multiple loops are created,
with opposing orientation. In every loop the induced
voltage opposes the voltage in the preceding loop, so
they cancel each other out. As twisting of the cables is
not always possible in complex ECG measurement
setups, shielding of the magnetic source may be
necessary

11.  Power line interference: Power line interference,
sometimes called hum in audio systems, can be caused
by an external source that introduces unwanted voltage
in the circuit or it can be internally generated from a
power supply. Both 50Hz and harmonics of the power
can be the sources of noise.

12. Other Source Of Noise
 Motion artifact:- Movement can cause changes in the
potentials that are created when an electrode is applied
to the skin. Normally, when the patient is relaxed, and
high quality electrodes are used, the recording is not
distorted by motion artifact.
 Noise from additional bioelectric events

13. NOISE MEASURING
 SNR:- The effects of noise on a signal are best analyzed as
a ratio of the signal compared to the noise. This ratio is
called the signal-to-noise ratio (S/N) and is often
expressed in decibels.
 Sensitivity
 Noise Factor:- The noise factor is a means of
specifying the added contribution of an amplifier to
the signal-to-noise ratio due to noise generated within
the amplifier. As such, it is a measure of the quality of
the amplifier and includes the overall effect of all noise
sources within the amplifier. Noise factor is frequently
expressed as a decibel ratio. As a decibel ratio, noise
factor is generally called noise figure:
FdB = 10logF

15. NOISE REDUCTION
 The type of connecting wires used between electrical
devices can have a significant impact on the noise
level of signal. Low-level signals of <100mV are
particularly susceptible to errors induced by noise.
Three simple rules will help to keep noise levels
low: (1) keep the connecting wires as short as
possible; (2) keep signal wires away from noise
sources; and (3) use a wire shield and proper ground.

16.  A ground is simply a return path to earth. A network
of wires that form the return path to earth would likely
act as antennae and pick up some voltage
potential relative to earth ground, therefore any
instrument grounded at the outlet would be referenced
back to this voltage potential, not earth ground. Thus if
the signal is grounded at 2 points, say at a power
source ground and earth ground, the grounds could be
at different voltage levels. This voltage is called the
common-mode voltage. This can lead to problems.

17.  Shields Long wires act as antennae and will pick up stray
signals from nearby electrical fields. The most common
problem is ac line noise. Electrical shields are effective
against such noise. A shield is a piece of metal foil or wire
braid wrapped around the signal wires and connected to
ground, it intercepts external electrical fields and return
them to ground. A shield ground loop is prevented by
grounding the shield at only one point, usually the
signal ground at the transducer (see figure)
 Twisting the lead wires together also tends to cancel any
induced voltage, as the currents through the two wires are
in opposite directions. A final resource is the use of a
magnetic shield made from a material having a high
ferromagnetic permeability.

19.  Common-mode voltages can be responsible for much
of the interference in biopotential amplifiers.
 Solution 1:amplifier with a very high common-mode
rejection
 Solution 2: • eliminate the source of interference
 Differential amplifiers (ideally) eliminate common-
mode noise. Real differential amplifiers always have
some small mismatch between the positive and
negative inputs, so CMRR is given as a figure of merit.

20.  A differential amplifier ideally amplifies only the
differential mode. In practice the common mode isn't
rejected entirely, the CMRR, or common-mode
rejection ratio, will specify the extent to which this is
true in the datasheet.

21. REFERENCE
 Noise in biopotential recording using surface
Electrodes E. Huigen
 Metting van Rijn, A.C., Peper, A. and Grimbergen,
C.A. (1990). High-quality recording of bioelectric
events. Part 1 Interference reduction, theory and
practice. Med. & Biol. Eng. & Comput., 28, 389-397.
 Google