Blocks of a data acquisition system

A data acquisition system (DAS) is the part of a digital control system that collects analog signals from sensors at many points of a plant, conditions them, and converts them into digital numbers that the computer can read.

Block diagram

 Sensor1 -> Amp -> LPF --+
 Sensor2 -> Amp -> LPF --+--> Analog -> S/H -> A/D -> CPU
 Sensor3 -> Amp -> LPF --+     MUX      ^      ^
                                |       |      |
                                +-------+------+
                                |
                         Control logic / clock

Blocks

1. Transducers (sensors): convert physical quantities (temperature, pressure, flow, speed, position) into electrical signals. Examples: thermocouple, strain gauge, tachogenerator, encoder.
2. Signal conditioning amplifier: amplifies the small sensor signal (millivolts) to the full-scale range of the ADC, gives high input impedance, isolation and common-mode rejection (instrumentation amplifier).
3. Low-pass (anti-aliasing) filter: removes high-frequency noise and limits the signal bandwidth to below $\omega_s/2$ so that aliasing does not occur.
4. Analog multiplexer: a set of electronic switches that connects the channels one at a time to the single S/H and ADC. This time-sharing lowers cost.
5. Sample-and-hold circuit: samples the selected channel at the sampling instant and holds the value constant while it is converted.
6. A/D converter: quantizes the held voltage into one of $2^n$ levels and codes it as an n-bit binary word (successive approximation, dual slope, flash).
7. Control logic/clock: selects the multiplexer channel, issues sample/hold and start-of-conversion commands, and signals the computer at end of conversion.
8. Computer interface: the digital word is read through a port or bus and stored for processing.

Operation

The controller selects channel 1, the S/H samples it, the ADC converts it, and the result is stored; then channel 2 is selected, and so on. All channels are scanned within one sampling period $T$.

Example: in a power substation, voltage, current and temperature transducers feed a multiplexed 12-bit ADC whose output is sent to a SCADA computer.