AC and balanced detection methods

All continuous constant-amplitude (DC) and periodic-variable amplitude (AC up to a few hundred Hertz) signals can be amplified by standard, low-cost electronics and displayed by common analogue or digital meters. Here, we summarize the basic principles of the methodology and highlight points of importance that need to be considered in the design or choice of appropriate signal detection apparatus. 

Simple discrete-component or integrated-circuit amplifiers are commonly used to establish a proportional link between the incoming signal (normally a voltage or current signal, V ig or I ig) and the meter display (normally in the form of Fout from the amplifier), i.e. 

The ratio references the desired absorption signal to the behaviour of the illumination source as such, it eliminates any fluctuation of the source, as long as the relationship between activation and reaction is linear. 

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