WebDAQ 504 User's Guide Functional Details
17
Figure 10. Measurement Error Introduced by Common-Mode Bias Current
Data rates
The frequency of the master timebase (f
m
) controls the data rate (f
s
) of the WebDAQ 504. The internal master
timebase frequency is 13.1072 MHz.
The data rates available on the WebDAQ 504 can be calculated with the following equation:
=
÷ 256
, where n is any integer from 1 to 31.
The data rate must remain within the appropriate data rate range. Using the internal master timebase of
13.1072 MHz results in data rates of 51.2 kS/s, 25.6 kS/s, 17.067 kS/s, and so on, down to 1.652 kS/s,
depending on the value of n.
Anti-aliasing filters
The WebDAQ 504 uses a combination of analog and digital filtering to provide an accurate representation of in-
band signals while rejecting out-of-band signals. The filters discriminate between signals based on the
frequency range, or bandwidth, of the signal. The three important bandwidths to consider are the passband, the
stopband, and the alias-free bandwidth.
The WebDAQ 504 represents signals within the passband, as quantified primarily by passband flatness and
phase nonlinearity. All signals that appear in the alias-free bandwidth are either unaliased signals or signals that
have been filtered by at least the amount of the stopband rejection.
Passband
The signals within the passband have frequency-dependent gain or attenuation. The small amount of variation in
gain with respect to frequency is called the passband flatness. Digital filters adjust the frequency range of the
passband to match the data rate, so the amount of gain or attenuation at a given frequency depends on the data
rate. Figure 11 shows typical passband response for the WebDAQ 504.
Figure 11. Typical passband response
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