CHM 302

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CHM 302

• CHAPTER 6
• Electronics and Noise

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Overall process of an instrument measurement
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• Some terms often used
• Detector refers to a mechanical, electrical
• or chemical device that identifies, records or
• indicates a change in one of the variables in
  its
• environment
• Transducer refers specifically to devices that
• convert information in non-electrical domains
• to information in electrical domains and vice
• versa

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Current, Voltage and Resistance

• Measurements of these parameters are
• done through appropriate circuit
• - a closed pathway that may be followed by
• an electric current
• Convention current flows from more positive
• to more negative potential
• Current magnitude depends on the electric
• potential and the resistance to the flow

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Ohms Law V IR for a series circuit

• Electrons may move spontaneously through a
• resistor or under the force of a voltage

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Power

• The power P in watts in a resistive element
• is given by the product of the current (in
• amperes and the potential difference in volts
• P IV
• From Ohms Law
• P I2R V2/R

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• Kirchhoffs Laws
• Two laws the current law and the voltage
• law
• Current law - the algebraic sum of currents
• around any point in a circuit is zero
• Voltage law is similar

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• Two types of basic dc circuits
• - series resistive and parallel resistive
• circuits

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• The previous series circuit is called a
• voltage divider
• In parallel circuits, resistors are placed with
  end
• connected to each other so they all have the same
  
• voltage across them

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• A Wheatstone bridge allows the
• measurement of very small differences in
• voltage or resistance

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Time-dependent circuit responses

• When current in an electrical circuit is
• increased or decreased energy is required to
• change the electric and magnetic field
• associated with the flow of charge
• An inductor resist the changes in the current
• - inductors are characterized by their
• inductance

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• A capacitor resists changes in voltage
• - characterized by the capacitance
• A typical capacitor consists of a pair of
• conductors separated by a thin layer of a
• dielectric substance
• - a capacitor may be thought of as a time-
• dependent resistor that lowers its resistance
• as the frequency rises

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Noise

• Every analytical measurement is made up
• of two components
• - the signal (carries information about the
• analyte)
• - noise made up of extraneous
• information that degrades accuracy and
• precision
• - also lower detection limits

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• The term noise is used to describe the
• random fluctuations observed whenever
• replicate measurements are made on
• signals that are measured continuously
• In most measurements, the average strength of
• the noise is constant and independent of the
• signal

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• For a typical dc signal
• - noise ? standard deviation of
• numerous measurements of signal
• strength
• - signal ? mean of the measurements
• Then

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• Two types of noise
• - chemical and instrumental noise
• Chemical noise arise from uncontrollable
• variables that affect the chemistry of the system
• being analyzed
• - changes in light intensity that affect
• photosensitive materials

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• Instrumental Noise
• Noise associated with each component
• of an instrument
• Certain kinds of instrumental noise are
• recognizable
• thermal or Johnson noise
• shot noise
• flicker or 1/f noise
• environmental noise

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• Thermal noise
• - caused by the thermal agitation of
• electrons or other charge carriers
• - the agitation is random
• Thermal noise disappears only at absolute zero

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• Shot Noise
• Shot noise is encountered whenever
• electrons or other charge particles crosses
• a junction
• Currents are composed of a series of quantized
• events
• - these events occur randomly and the rate
• at which they occur is subject to statistical
• fluctuations

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Like thermal noise, shot noise is white
noise 1/f noise - characterized by having a
magnitude that is inversely proportional to the
frequency of the signal being measured
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• Environmental noise
• This type is a composite of different forms
• of noise that arise from the surroundings
• So there are numerous sources
• - ac power lines, radio and TV stations, etc.
• Can be reduced by
• - shielding
• - grounding
• - minimizing conductor lengths

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• The electrical signals generated from a
• stimulus of an analyte are usually quite
• small
• They are usually amplified
• Operational amplifiers are ubiquitous
• - they perform mathematical operations
• - allow for precise measurement of I, V, R
• Amplifiers have one or more inputs and an
• output connection

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• A signal arriving at an input is increased by a
• certain amount before leaving the output
• When the output is a multiple of the input, the
• amplifier is a linear amplifier

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• In fact the linearity of an instrument is
• directly dependent on the linearity of the
• amplifiers
• The amount of amplification is usually referred
• to as the gain
• Gain Poutput/Pinput
• Another important type of amplifier is the
• difference amplifier

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• Noise appears in phase at both the
• inverting and non-inverting inputs
• - it is subtracted out by the circuit
• - so at the output noise is substantially
• reduced