Analytical Chemistry
Signals and Noise
Signals
• A signal is an electromagnetic or electrical
current that carries data from one system or
network to another.
• In electronics, a signal is often a time-varying
voltage that is also an electromagnetic wave
carrying information, though it can take on
other forms, such as current.
• There are two main types of signals used in
electronics: analog and digital signals.
Analog Signal
• An analog signal is time-varying and generally bound to a
range (e.g. +12V to -12V), but there is an infinite number
of values within that continuous range.
• An analog signal uses a given property of the medium to
convey the signal’s information, such as electricity
moving through a wire.
• In an electrical signal, the voltage, current, or frequency of
the signal may be varied to represent the information.
• Analog signals are often calculated responses to changes
in light, sound, temperature, position, pressure, or other
physical phenomena.
Digital Signal
• A digital signal is a signal that represents data as a
sequence of discrete values.
• A digital signal can only take on one value from a finite set
of possible values at a given time.
• With digital signals, the physical quantity representing the
information can be many things:
Variable electric current or voltage
Phase or polarization of an electromagnetic field
Acoustic pressure
The magnetization of a magnetic storage media
• Digital signals are used in all digital electronics, including
computing equipment and data transmission devices.
Comparison of analog and digital signals
Comparison Chart
BASIS FOR
ANALOG SIGNAL DIGITAL SIGNAL
COMPARISON
Basic An analog signal is a continuous wave A digital signal is a discrete wave
that changes over a time period. that carries information in binary
form.
Representation An analog signal is represented by a A digital signal is represented by
sine wave. square waves.
Description An analog signal is described by the A digital signal is described by bit
amplitude, period or frequency, and rate and bit intervals.
phase.
Range Analog signal has no fixed range. Digital signal has a finite numbers
i.e. 0 and 1.
Distortion An analog signal is more prone to A digital signal is less prone to
distortion. distortion.
Transmit An analog signal transmit data in the A digital signal carries data in the
form of a wave. binary form i.e. 0 and 1.
Example The human voice is the best example Signals used for transmission in a
of an analog signal. computer are the digital signal.
Signals and Noise
Signal to Noise Ratio
Types of Noise
Signal to Noise Ratio Enhancement
Signal Averaging
Filtering
Signal-Noise Ratio
The signal is what you are measuring that is
the result of the presence of your analyte
Noise is extraneous information that can
interfere with or alter the signal.
It can not be completely eliminated, but
hopefully reduced!
Noise is considered random!
indeterminate
SIGNAL VS. NOISE
SIGNAL VS. NOISE
Since noise can not be eliminated (it is
random), we are more interested in the
S / N ratio than the intensity of the noise
Signal mean X
= =
Noise standard deviations
This is mathematically the inverse of the
RSD, or we can say that………
S= 1
N RSD
S/N = 4.3:1
S /N = 43:1
S / N Objective?
Reduce as much of the noise as possible by
carefully controlling conditions
Temperature, power supply variations, etc.
Increase the signal to noise ratio
More signal vs. noise means a lower STDEV!
More precise measurement
Lower STDEV means a better LOD
Lower limits of detection
A S / N ratio of 3 is usually the minimum that is
acceptable.
Types of Noise…..
Chemical Noise
Undesired chemical reactions
Reaction/technique/instrument specific
Instrumental Noise
Affects all types of instruments!
e.g., source, input transducer, the output
transducer, and all signal processing elements
Can often be controlled physically (e.g. temp) or
electronically (software averaging)
Instrumental Noise
Thermal (Johnson) Noise:
Thermal agitation of electrons affects their “smooth”
flow.
Due to different velocities and movement of
electrons in electrical components.
Dependent upon both temperature and the range of
frequencies (frequency bandwidths) being utilized.
Can be reduced by reducing temperature of electrical
components.
Eliminated at absolute zero.
Considered “white noise” occurs at all frequencies.
Shot Noise:
Occurs when electrons or charged particles
cross junctions (different materials, vacuums,
etc.)
Considered “white noise” occurs at all
frequencies.
Due to the statistical variation of the flow of
electrons (current) across some junction
Some of the electrons jump across the junction
right away
Some of the electrons take their time jumping
across the junction
Flicker Noise
Frequency dependent
Magnitude is inversely proportional to
frequency
Significant at frequencies less than 100 Hz
Results in long-term drift in electronic
components
Can be reduced by using resisters that are
metallic, wire wound
Environmental Noise: Room should be cold??
Unlimited possible sources
Can often be eliminated by eliminating the source
Other noise sources can not be eliminated!!!!!!
Methods of eliminating it…
Moving the instrument somewhere else
Isolating /conditioning the instruments power source
Controlling temperature in the room
Control expansion/contraction of components in instrument
Eliminating interferences
Stray light from open windows, panels on instrument
Turning off radios, TV’s, other instruments
NOISE SPECRUM
Has the property of Flicker noise
Signal Averaging
(one way of controlling noise)
Ensemble Averaging
Collect multiple signals over the same time or
wavelength (for example) domain
EASILY DONE WITH COMPUTERS!
Calculate the mean signal at each point in the
domain
Re-plot the averaged signal
Since noise is random (some + / some -), this helps
reduce the overall noise by cancellation!
Si, individual measurements of the signal including noise
If we sum n measurements to obtain the ensemble
average, the signal Si adds for each repetition. The total
signal Sn is given by
S n = i=1 S i = n
n
S i
S / N is good KEEP ADDING!
Boxcar Averaging
Boxcar averaging is a data treatment method that
enhances the signal-to-noise of an analytical signal
by replacing a group of consecutive data points
with its average
Take an average of 2 or more signals in some
domain
Plot these points as the average signal in the same
domain
Can be done with just one set of data
You lose some detail in the overall signal
Hardware device for noise reduction
Grounding and shielding
• One way to reduce environmental noise is to prevent it from
entering into the instrument's electronic circuitry.
• One approach is to use a Faraday cage in which the instrument
sits within a room or space covered with a conductive
material.
• Electromagnetic radiation from the environment is absorbed
by the conductive material and then shunted away to the
ground.
• Rather than encasing the entire instrument in a Faraday cage,
particularly sensitive portions of the circuitry can be shielded.
• Making sure all circuits have the same common earth ground;
surrounding a circuit or instrument with a conducting material
that is attached to earth ground; and reducing the lengths of
conducting wires.
Differences and instrumentation amplifiers
• A difference amplifier is an electrical circuit used to determine the
difference between two input voltages or currents and to return
that difference as a larger voltage or current.
• As the magnitude of the noise in the two input signals is generally
similar in value—that is, it is in phase—while the signal of interest
is not, much of the noise's contribution to the signal is subtracted
out.
Analog filtering
• When the frequency of the noise is quite different from the
frequency of the signal, a simple electrical circuit can be used to
remove the high frequency noise and pass the low frequency
signal; this is called a low-pass filter.
Modulation
• When the signal of interest has a low frequency, the effect of
flicker noise becomes significant because a technique that removes
low frequency noise will remove the signal as well.
• Modulation is a process of increasing the frequency of the signal.
When complete, a high-pass filter is used to remove the noise.
Reversing the modulation returns the original signal, but with
much of the noise removed.
Modulate signal to a frequency region that is more noise
free! Must first modulate the signal by adding it to a
carrier frequency, and the demodulate it after
measurement to remove the carrier frequency.
Hardware device for noise reduction
Modulating chopper for modulating a light beam. (a) rotating disk chopper (b)
rotaing vane chopper (c) oscillating tuning fork design
A modulating chopper can reduce noise in a circuit by effectively controlling the flow
of current and voltage in a way that minimizes interference:
1. Switching Action: The chopper rapidly switches the current on and off. This process
can help in averaging out noise over time, making it less impactful on the signal being
processed.
2. Frequency Modulation: By modulating the switching frequency, the chopper can
shift noise components away from the signal frequency. This helps to separate the
desired signal from unwanted noise, allowing for better signal integrity.
3. Filtering: The rapid switching creates harmonics that can be filtered out with
appropriate circuitry, such as low-pass filters, further reducing noise levels in the
output.
4. Power Supply Regulation: In power applications, choppers can stabilize voltage
and current, reducing fluctuations that can introduce noise.
5. Improved S/N Ratio: By maintaining a more consistent signal level, the modulating
chopper enhances the S/N ratio, making it easier to distinguish the desired signal
among the noise.
Hardware device for noise reduction
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