Analog data acquisition: A/D converter
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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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[ADC resolution]
[ADC scan rate]
[ADC for motion
analysis]
[Typical analog data in motion analysis]
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