BRIDGE2: DC-bridge measurement 261
imc CANSAS Users Manual - Doc. Version 1.9 - 05.12.2014© 2014 imc Meßsysteme GmbH
6.1.7 Sampling interval BRIDGE2
The analog/digital converters sample the measurement signal at a fixed rate of 5 kHz per channel. A built-
in low pass filter (AAF, anti-aliasing filer) of high order and a cutoff frequency of 1 kHz provide
suppression of frequencies in the measurement signal above the cutoff frequency which, as a result of
sampling, would otherwise disturb the interesting range of the measurement signal (i.e. would cause
aliasing). When slower sampling rates are selected, the signal processor (DSP) provides additional online
digital filtering in order to reduce aliasing.
The cutoff frequency is defined as the measurement signal's frequency component which is
dampened by 3 dB.
Note that at a sampling rate of 5 kHz, the anti-aliasing filter provides strong suppression of (undesirable)
signal components of half that frequency (2,5 kHz), since the cutoff frequency is already reached at 1 kHz.
But if a different sampling rate is set, a digital anti-aliasing filter is applied. This software filter only
provides 3
rd
order filtering, which is, however, perfectly adequate for most practical applications. Such
filters dampen the frequency of (sampling rate ¸ 2) by around 30dB, and by more at higher frequencies,
of course. 30dB is an acceptable level of damping for signal components whose strength is (in practice)
only a fraction of the input range. Such minor components can be disregarded after having been
dampened by 30dB. Note that the value 30dB applies to frequency components of around half the
sampling rate.
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