--- title: "Blocks of a data acquisition system" subject: "Digital Control System" chapter: 1 key: "semester_6/digital_control_system:data-acquisition-system-blocks" source: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-system-blocks site: BCT Engineering --- # Blocks of a data acquisition system Digital Control System · Chapter 1: Introduction to Discrete Time Control System - Asked in: 2068 Jestha, 8 marks; 2068 Magh, 4 marks (asked 2 times) - Video: [Data Acquisition Systems | Block Diagram | Part-1/2 | Electrical Instruments ( EIM ) | Lec - 85](https://www.youtube.com/watch?v=jRbw2n2PA0E) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-system-blocks - Whole chapter: https://www.bctengineer.com/md/semester_6/digital_control_system/chapter-1 ## Question What is data acquisition system? Describe different blocks of data acquisition system. ## Short answer A data acquisition system collect analog sensor signals, condition them, multiplex, sample/hold, and convert to digital for the CPU. Its block include transducers, signal conditioning (amplification, anti-alias filtering), analog multiplexer, sample-and-hhold circuit, A/D converter, and control logic. ## Answer 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the job of a data acquisition system in a digital control system? A. Collect analog signals from sensors, condition them and convert them to digital numbers for the computer B. Convert computer outputs into actuator signals C. Store the control program D. Generate the reference signal Answer: A. The DAS is the input side that conditions and digitizes plant signals. (Key point: DAS definition) 2. Which is a function of the signal-conditioning (instrumentation) amplifier? A. Hold the voltage during conversion B. Scale millivolt signals to the ADC full-scale range, with isolation and common-mode rejection C. Select the multiplexer channel D. Quantize the signal Answer: B. It raises the small sensor signal to the ADC range and provides high input impedance, isolation and CMRR. (Key point: Signal conditioning amplifier) 3. Below what frequency should the LPF limit the signal bandwidth? A. ωs B. 2ωs C. ωs/2 D. ωs/10 only if noise is low Answer: C. Limiting bandwidth below ωs/2 avoids aliasing. (Key point: Anti-aliasing filter cutoff) 4. What does the control logic/clock do in the DAS? A. Quantizes the held voltage B. Amplifies signals C. Selects the MUX channel, issues sample/hold and start-of-conversion commands, and signals end of conversion D. Converts the binary word to analog Answer: C. It sequences the multiplexer, S/H and ADC and tells the computer when data are ready. (Key point: Control logic/clock) ## Related in Digital Control System - [Data acquisition system and tracking/hold modes](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-tracking-hold) — Digital Control System, chapter 1 - [Data acquisition, conversion and distribution](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-conversion-distribution) — Digital Control System, chapter 1, asked 9× - [Data acquisition block diagram and DCS advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-advantages) — Digital Control System, chapter 1 ## Related in other subjects - [Digital data acquisition system block diagram](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Adigital-data-acquisition-system) — Instrumentation I, chapter 6 - [Real-time versus ordinary software and OS; data acquisition](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-vs-non-real-time) — Software Engineering, chapter 4 - [Knowledge acquisition in expert system](https://www.bctengineer.com/question.md?key=semester_6%2Fartificial_intelligence%3Aknowledge-acquisition-es-architecture) — Artificial Intelligence, chapter 7, asked 2× - [Distributed system in big data example](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Adistributed-system-example) — Big Data Technologies, chapter 1