DEE 514�ELECTRONIC INSTRUMENTATION�

Input Output Configuration of Measuring Instruments & Instrument Systems

• Input quantities are classified into three categories
• Desired inputs
• Interfering inputs and
• Modifying inputs

As shown in the figure below

• Desired inputs (i_D) represent the quantities that the instrument is specifically intended to measure
• Interfering inputs(i_I) represent quantities to which the instrument is unintentionally sensitive

• A desired input produces a component of output according to an input-output relation symbolized by F_D Which denote the mathematical operations necessary to obtain the output from the input.
• F_I serve a similar concept for interfering inputs

• Modifying inputs (i_M) are the quantities that cause a change in the input-output relations for the desired and interfering inputs, that is they cause a change in F_D and/or F_I

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Example of above concept

Methods of correction for Interfering and Modifying Inputs

• A number of methods for nullifying the effects of spurious inputs are available But the most widely used are given below
• Method of inherent insensitivity,
• Method of high gain feedback,
• Method of calculated output corrections,
• Method of signal filtering, and
• Method of opposing inputs.

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Method of high gain feedback

Closed-Loop Feedback System

K_AMK_MoK_spK_fb >>1 then,

Method of signal filtering�a) Input Filtering

Output Filtering

Examples

Method of Opposing Inputs

Lecture-3

Noise

• A spurious current or voltage extraneous to the current or voltage of interest in an electrical or electronic circuit is called Noise
• With reference to an electrical system, noise may be defined as any unwanted form of energy/signal which tends to interfere/superimpose with the energy/signal of interest thereby causing a deviation of the output from its expected value/Signal. (OR)
• Noise is random, undesirable electrical energy that enters the communications system via the communicating medium and interferes with the transmitted message. However, some noise is also produced in the receiver.
• Noise is a signal that does not convey any useful information

• The effect of noise in a measurement system may be
• an annoying “hum” in the speaker of a radio receiver
• owing to 50Hz power line frequency noise,
• to the transmission of incorrect data in telemetering systems or
• data communication systems,
• to life threatening situations caused by incorrect interpretation of waveforms related to vital human organ in biomedical instrumentation where the expected (desired) signals are intrinsically weak.

signal to noise ratio (S/N ratio)

Classification of Noise

• Noise may be put into following two categories.

1. External noises

2. Internal noise in communication

Generated noise.

• From the above fig., one of the possible sources of noise is on account of internal components of the amplifier like resistor, capacitors and transistors etc. therefore the noise in this case is generated inside the amplifier and is called Generated noise.

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Conducted Noise

• The power supply to the amplifier could be the source of noise since it may have spikes, ripples or random deviations that are conducted to the amplifier circuit through power wiring. This type of noise is called Conducted Noise

Radiated Noise. �

• There may be electric or magnetic fields or disturbances in the environments around the amplifier because of which unwanted signals are radiated into the interior of the amplifier and this is called Radiated Noise.

thermal noise or white or Johnson noise.

• Electrons and "ions" strongly bound by molecular forces.
• The ions vibrate randomly about their normal (average) positions, however, this vibration being a function of the temperature.
• Continuous collisions between the electrons and the vibrating ions take place
• Thus there is a continuous transfer of energy between the ions and electrons.
• This is the source of resistance in a conductor.

• The movement of free electrons constitutes a current which is purely random in nature and over a long time averages zero.
• There is a random motion of the electrons which give rise to noise voltage called thermal noise.
• Thus noise generated in any resistance due to random motion of electrons is called thermal noise or white or Johnson noise.
• the noise power(Pn) generated by a resistor to be proportional to its absolute temperature.
• Noise power(Pn) is also proportional to the bandwidth over which it is measured.

• Pn ∝ T(BW)
• Pn = KT(BW) ------ (1)

Where,

• Pn = Maximum noise power output of a resistor.
• K = Boltzmann’s constant = 1.38 x10-23 joules I Kelvin.
• T = Absolute temperature.
• BW or Δf = Bandwidth over which noise is measured.

1. External noises,

• External Noises are the noise whose sources are external.
• External noise may be classified into the following three types:
• 1. Atmospheric noises
• 2. Extraterrestrial noises
• 3. Man-made noises

or industrial noises.

• 2. Internal noise in communication, i.e. noises which get, generated within the receiver or communication system
• Internal noise may be put into the following four categories.
• 1. Thermal noise or white noise or Johnson noise
• 2. Shot noise.
• 3. Transit time noise
• 4. Miscellaneous internal noise.

• External noise cannot be reduced except by changing the location of the receiver or the entire system.
• Internal noise on the other hand can be easily evaluated Mathematically and can be reduced to a great extent by proper design.