16
Functional groups
Functional groups
Manual ADQ-250 series Rev. 0.9
3.2 Analog acquisition
Nyquist´s sampling theorem (Oversampling)
Nyquist´s sampling theorem tells us, that the sample rate for a periodic signal, whose maximum
frequency component should be f
Pmax
, must be at least twice as high, i. e. 2 • f
Pmax
or higher.
In practice we recommend to choose a sampling rate by the factor 5 or 10 higher than f
Pmax
to
replicate the signal form truely. This issue is also called "oversampling".
Example:
The max. frequency component f
Pmax.
(1/t
P
) of the signal frequency should be 10 kHz. The sam-
ple rate f
S
(1/t
S
) should be at least 5 • 10 kHz = 50 kHz.
Figure 5: Nyquist-Shannon sampling theorem
3.2.1 Voltage inputs
The ADQ-250 series provides 8 voltage inputs with electrical isolation of up to 700 V
RMS
bet-
ween each other and towards PC ground. Each of the 8 channels is assembled with a 16 bit or
18 bit ADC and can sample with up to 2.0 MS/s synchronously (depending on model).
The differential input channels are optimized for signal bandwidth of 280 kHz (-3 dB). Because
of the lower sample rate of the ADQ-255 you can acquire signals with a bandwidth of about
50 kHz without an appreciable decrease of the amplitude (0 dB) with this model. By the poten-
tial-free voltage inputs and the full differential sampling parasitic effects like ground loops and
common mode errors can be largely rejected. The input voltage range is standard from -10,24 V
to +10,24 V for signals with a full swing of ±10 V. Optional an input voltage range of ±102,4 V is
possible. The high A/D converter resolution of 16 bit resp. 18 bit (depending on model) enables
a very precise measurements without the need of switching between different measurement
ranges.
All channels are always sampled synchronously. The values can be acquired as single values
or timer-controlled. On demand the measurement can be started or stopped by two external
trigger inputs, which are isolated towards PC ground and to the analog channels.
Содержание ADQ-250 cPCI
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