Analog data acquisition: A/D converter

A hardware converter can be used to measure analog voltages and transform them to digital information suitable for signal processing and interpretation by computers.
In motion analysis A/D converters are used to measure kinematic parameters as well as dynamic parameters.

A/D converter resolution

Most converters have an input range from -10 V to +10 V. Some of them have a special setting called gain which changes this interval to +/- 5 V, +/- 2.5 V or similar values.

Typical converters have a resolution of 12 or 16 bits, i.e. they divide the given input range in 4096 or 65536 small units. Changes of voltage smaller than these units cannot be measured.

+/- 10 Volt +/- 5 Volt
12 bit 4096 4.88 mV/bit 2.44 mV/bit
16 bit 65536 0.31 mV/bit 0.15 mV/bit
16 bit converters can measure with an accuracy of 0.31 mV (+/- 10 V input)

Real world examples:

For a measurement of countermovement jumps the following settings are given:
Amplifier range: +/- 10 V, maximum force that can be measured: 5000 N
A/D converter with 12 bit resolution:
5000 N/2048 bit = 2.4 N per bit
This means that forces in the range of +/- 5000 N can be measured with a resolution of 2.4 N.
For gait analysis, where the amplifier can be configured for maximum forces of 2000 N the resolution is 0.98 N.
In comparison to that a 16 bit A/D converter has a resolution of 0.15 and 0.06 N per bit, respectively.

A/D converter scan rate

Most A/D converters do not measure all channels synchronously. They have a scan rate of typically 200,000 samples per second (200 kS/s). This means that the channels are measured one after another with a very small delay determined by the scan rate.

Real world examples:

A force platform (8 channels) is measured with 1000 Hz (=one sample per millisecond).
Using a 200 kS/s A/D converter the first sample of the first channel is measured at 0.000 ms, the second channel at 0.005 ms, the third at 0.010 ms and the eighth at 0.035 ms.
The second sample of the eight channels will be measured at 1.000 ms, 1.005 ms, 1.035 ms, respectively.
The small time delay between channels is acceptable for most applications.
When using more channels and higher sampling frequencies the problem gets bigger (32 channels, 5000 Hz, with 200 kS/s):
First sample: first channel at 0.000 ms, 32nd channel at 0.155 ms
Second sample: first channel at 0.200 ms, 32nd channel at 0.355 ms
As you can easily see the last channel of the first sample is in terms of time quite close to the first channel of the second sample.

Typical A/D converters for motion analysis purposes

Data-Translation DT-3001 PCI 330 kS/s 16 channels 12 bit
Data-Translation DT-3002 PCI 330 kS/s 32 channels 12 bit
Data-Translation DT-3003 PCI 330 kS/s 64 channels 12 bit
Data-Translation DT-3005 PCI 200 kS/s 16 channels 16 bit
Data-Translation DT-3010 PCI 1250 kS/s 32 channels 12 bit
Data-Translation DT-9801 USB 100 kS/s 16 channels 12 bit
Data-Translation DT-9803 USB 100 kS/s 16 channels 16 bit
National-Instruments DAQPad-6015 USB 200 kS/s 16 channels 16 bit
National-Instruments DAQPad-6020E USB 100 kS/s 16 channels 12 bit
National-Instruments PCI-6024E PCI 200 kS/s 16 channels 12 bit
National-Instruments DAQCard-6024E PCMCIA 200 kS/s 16 channels 12 bit
National-Instruments PCI-6036E PCI 200 kS/s 16 channels 16 bit
National-Instruments PCI-6052E PCI 333 kS/s 16 channels 16 bit
National-Instruments DAQPad-6052E FireWire 333 kS/s 16 channels 16 bit
National-Instruments PCI-6070E FireWire 1250 kS/s 16 channels 12 bit
National-Instruments PCI-6071E PCI 1250 kS/s 64 channels 12 bit
National-Instruments PCI-6220 PCI 250 kS/s 16 channels 16 bit
National-Instruments PCI-6224 PCI 250 kS/s 32 channels 16 bit
National-Instruments PCI-6250 PCI 1000 kS/s 16 channels 16 bit
National-Instruments PCI-6254 PCI 1000 kS/s 32 channels 16 bit
A/D converters typically used in motion analysis systems

Typical analog data in motion analysis

Force platforms have 6 to 8 channels of analog raw data and are usually measured at 300 to 1000 Hz for gait analysis and jump series.
EMG systems require one channel for each pair of electrodes and are measured at 1000 to 3000 Hz.
Other sensors (accelerometers, goniometers) require one channel per sensor. The measurement frequency varies from 300 to 5000 Hz.




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