Data Acquisition

• Using isolation amplifiers to isolate the input signals from the measurementsystem ground reference.• Permanently isolating the measurement system ground using isolationtransformers.• Temporarily isolating the measurement system ground reference with a digitalswitch whilst an input signal measurement is taken.3.5.1 Definition of noise and interferenceNoise, by definition, is the presence of an unwanted electrical signal in a circuit. Interferenceis the undesirable effect of noise. Where a noise voltage causes improperoperation of a circuit, or its relative magnitude is of the same order as the desired electricalsignal, then it is interference.Noise itself cannot be totally eliminated but only reduced in magnitude until it nolonger causes interference. This is especially true in data acquisition systems where theanalog signal levels from transducers measuring a physical quantity can be very small.Compounding this in many instances is the physical cable distance over which thesesignals must be transmitted and the effect that noise may have on this extended circuitry.3.5.2 Sources and types of noiseBefore considering the cabling and shielding requirements of data acquisition systems, itis important to understand the nature and source of interference caused by the coupling ofnoise into data acquisition systems.Figure 3.29 illustrates that there are three components involved in any noise-inducedproblem:• A noise source (AC power cables, high voltage or high current AC orswitching circuitry)• A coupling channel (common impedance, capacitance, mutual inductance)• A receiver (the circuitry that is susceptible to the induced noise)Figure 3.29Noise coupling between a noise source and a receiver