(Analog) DataAcquisition

D. Gordon E. Robertson, PhD, FCSB

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?

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

How Many Channels?

• 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

NI 32-channelcDAQ A/D

Biometrics2-channelelectrogoniometer

AMTI force platform

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

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

Bortec 8-channel EMG,gains from 1x to 15000x

Honeywell bridge amp

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

Relationship between Integer and Real Data

• resolution = voltage range/bits• for 12 bit, +/–10 V; resolution = 20/4096 = 0.0048828 V/bit

8 bit 12 bit 16 bit +/–12 bit input V actual

+5000 N

0 N

–5000 N

+10 V

0 V

–10 V

216–1

215

0

4095

2048

0

2047

0

–2048

255

128

0

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