PerkinElmer 200 UV/VIS Series User Manual page 165

Application Notes Principles of Operation
There are various forms of noise. Electronic noise can arise from line voltage fluctuations,
the detector's circuitry, and radio frequency interference. Short term noise has frequencies
that range from 1 cycle/1 sec to 1 cycle/5 min. The very wide peaks and valleys observed in
long term noise can arise from system variations such as flow rate and pressure variations,
teaks, temperature variations, aging lamps, and gas in the detector cell.
Response Time Versus Time Constant
The time constant value describes the degree of filtering of frequencies. For a single pole fil-
ter, the time constant (τ) is the time it takes the output signal to reach 63.2% of the value of
an input signal change. In 2τ, 86.5% of the final value is reached, and 95% is reached in 3τ. It
is important to realize that time constant is a valid mathematical description for only single
pole filters.
Figure 8-4. Definition of rise time on a response versus time curve.
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