Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 203

Figure 6.1. Frequency response of some measuring devices
Several types of dynamic acquisition devices are available. We recommend use
of devices that can carry out several acquisitions simultaneously ( track and hold ),
thereby eliminating some signal aligning corrections. Acquisition devices must be
fitted with anti-aliasing “hardware” filters before the analog-to-digital conversion,
to prevent frequencies near the acquisition devices getting into the range of interest,
because of aliasing during the conversion into digital format. As an example, Figure
6.2 illustrates a 0.4 second long 95 Hz signal. Suppose that the sampling frequency
of the data acquisition device were 100 Hz, the black spots representing the digitized
values. The system would then record a 0.2 s-period signal instead of the real one,
and the latter would have aliased to 5 Hz. In this case, use of a 50 Hz “hardware”
filter would ensure the original 95 Hz signal almost completely died down before
being digitized.
Controling the cut-off frequencies of these filters is essential, as they may change
from test to test. The frequency response curve of the hardware filter should also be