Data Acquisition Flashcards
What are three things on which the fidelity of signal representation depends?
1) frequency content of analog signal
2) size of time increment between each discrete member
3) total sample period of measurement
What is meant by alias frequency?
Higher frequencies in analog signal appear as false lower frequencies in discrete representation.
What is the Nyquist frequency?
Frequency above which all frequencies appear as alias frequencies of less than Nyquist frequency in sampled signal.
How can aliasing be avoided?
1) Meet sampling theorem.
2) Use anti-aliasing filter (low pass filter that removes everything above Nyquist frequency)
3) For unknown signals, set sampling frequency as large as possible.
Define conversion time
Total time required to record and process a sampled value.
When can Discrete Fourier Transform be used to reconstruct dynamic signal from discrete data?
1) sampling frequency meets sampling theorem, AND
2) total sample period is an integer of fundamental period of continuous waveform.
What is leakage and when does it happen?
Signal is truncated.
Total sample period is NOT integer multiple of fundamental period of continuous waveform.
What is resolution?
smallest change in voltage that will cause a bit change (LSB).
What is the saturation error of AD / DA converters?
Converter is limited to minimum and maximum range. Output saturates an no longer changes if input exceeds either limit.
What is the conversion error of AD / DA converters?
Elemental errors that arise due to conversion process.
Hysteresis, linearity, sensitivity, zero and repeatability errors.
What is the purpose of Data Acquisition Systems?
Quantify, process, and store data continuously.
Describe three types of DAS.
Embedded systems: use dedicated microprocessors for real-time processing and feedback control via I/O ports.
PLC-based: cost-effective; store process variables on memory or flash.
Computer-based: use external modules (via bus, I/O board, or wireless) to interface with instruments and connect to a PC.
What do analog filters and digital filters do in analog signal conditioning respectively?
Analog filters control frequency content of signal being sampled (e.g., to remove aliasing).
Digital filters are used for signal analysis after sampling (NOT FOR ALIASING).
What do analog amplifiers do in analog signal conditioning?
Amplification or attenuation before A/D conversion to prevent saturation or loss of resolution.
What do shunt resistors do in analog signal conditioning?
Convert current signal into voltage signal.
Describe analog IO communication that uses single-ended connections.
- one signal line that is measured relative to common ground.
- short wires reduce EMI noise
- only applicable when all analog signals can be measured relative to common ground.
- avoid common mode voltage
Describe analog IO communication that uses differential-ended connections.
- allows measuring voltage difference between two distinct input signals
- high and low signals are isolated from ground (floating inputs)
- used for signals from multiple instruments (less prone to ground loops; can still be affected by high enough common mode voltages)
- preferred for low voltage (twisted pairs are less susceptible to EMI; include common mode resistor to reduce drift or sudden spikes in signal)
