Sensor Circuits - The Electronic Nose Artificial Olfaction Technology - page 192

Fig. 7.20 ADC process

Multiplexer (MUX)—a switching device that sequentially connects multiple
inputs or outputs in order to process several signal channels with a single A/D or
D/A converter.
Encoding: Assigning a unique digital code to each quantum, then allocating the
digital code to the input signal. ADC is also called encoding device.
Sample-and-Hold (S/H) Circuit—it acquires and stores an analog voltage on a
capacitor for subsequent processing.
Note that the ADC also works as a divider because ADC has an analog ref-
erence voltage or current next to which the analog input is compared.
A digital to analog converter (DAC) has a digital input that specifies an output
whose value changes in step. These step changes are in volts or amperes. An input
that can vary from a minimum to a maximum value of volts or amperes is the input
of the analog to digital converter.
The output is a digital number that represents the input value. Digital encoder
(8-line to 3-line) is shown in Fig. 7.21 .
A DAC takes an n- bit digital input and outputs a corresponding analog voltage.
DAC systems normally consist of three components: (a) A reference voltage, (b)
The DAC itself, and (c) An op-amp for output buffering. DAC is also called
decoding device.

Data Analysis Techniques

The sensor signals obtained are saved into the computer and then the data analysis
process is started. Signal preprocessing is the first computational stage, which
fulfills various purposes which include compensating for sensor drift, extracting