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(Analog) Data Acquisition

This document outlines critical issues regarding analog data acquisition, including signal types (AC/DC), channel requirements, and signal characteristics such as voltage range and frequency. It discusses the role of A/D converters in recording data, the necessity of signal conditioning (amplification and filtering), and data storage formats for compatibility with software. Key insights include the use of AC-coupling for EMGs and DC-coupling for other signals, as well as recommendations for managing multiple channels and their sampling rates, ensuring accurate and efficient data acquisition in biomedical research.

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(Analog) Data Acquisition

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  1. (Analog) DataAcquisition D. Gordon E. Robertson, PhD, FCSB

  2. Issues • What type of signal? (AC or DC) • How many signals (channels) will be needed? • Characteristics of signal (range, frequency) • How does the A/D converter record the data? • Is signal conditioning necessary? (amplification, filtering) • How will the data be saved for use by other software?

  3. What Kind of Signal? • Use AC-coupling to remove biases from AC signals such as EMGs, EEGs or ECGs. • Use DC-coupling for all other signals • Piezoelectric signals, such as from accelerometers or force transducers, may require recording of bias levels for zeroing EMGs are AC Forces are DC

  4. How Many Channels? NI 32-channel cDAQ A/D • A/D converters sample multiple signals one channel at a time. • Typically 4, 8, 16, 32 or 64 channels are possible • Note, the more channels that are sampled the lower the maximum sampling rate • A/D converters have maximum rates (e.g., 100 kHz) that must be divided among the number of sampled channels • Most systems sample all channels at the same rate. Some allow different rates for each channel. • 1 Kistler force platform: 8 channels • 1 AMTI force platform: 6 channels • 1 3D head accelerometers: 9 channels • 1 joint elgon: 1–3 channels AMTI force platform Biometrics 2-channel electrogoniometer

  5. Characteristics of Signal • Voltage range: • 0 to +5 V, 0 to +10 V • +/–1.5 V, +/–5 V, +/–10 V • What is the signal’s frequency spectrum? • forces: DC to 10 Hz • audio: 20 Hz to 20 kHz • EMG: 20 to 500 Hz

  6. What Signal Conditioning is Done? • Amplification or attenuation to fit dynamic range of A/D converter • 1000x for EMGs • force transducers 1/100, 1/10, 1/1 • Analog filtering • band-pass filtering of EMGs Honeywell bridge amp Bortec 8-channel EMG, gains from 1x to 15000x

  7. How does the A/D converter Record the Data? • number of data bits: • 8 bits gives 0 to 255 • 12 bits gives 0 to 4095 • 16 bits gives 0 to 65 536 • 24 bits gives 0 to over 1 million • are the data stored in ones or twos complement or without sign bit • bit formats: Intel, DEC, Sun • software must be able to convert the integer data to floating point in volts or other measurement unit

  8. 8 bit 12 bit 16 bit +/–12 bit input V actual 255 128 0 4095 2048 0 216–1 215 0 2047 0 –2048 +10 V 0 V –10 V +5000 N 0 N –5000 N Relationship between Integer and Real Data • resolution = voltage range/bits • for 12 bit, +/–10 V; resolution = 20/4096 = 0.0048828 V/bit

  9. How Will the Data be Saved? • integer format is the most compact and fastest • integer format must be converted to floating point by other software (e.g., BioProc2/3, MATLAB, commercial software) • real format (machine language) requires less processing but more memory • ASCII requires greatest amount of memory but is easiest to read by other software (Excel, QuattroPro, SPSS) and may reduce accuracy due to rounding • public format: .C3D, .WAV, .EDF, .WK1 • spreadsheet: .WK1, .XLS, .QPW

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