ADLINK USB-2405 - Manual (Page 41)

Operation 31
USB-2405
3.2.4 Data Transfer
Software Polling Data Transfer (Non-Buffering Pro-
grammed I/O)
Polling mode supports flexible timing and is suitable for retrieving
recent data without FIFO buffering latency. The USB-2405 contin-
uously updates the most recently acquired data onto a data port
for specific channels. Data not retrieved in time is overwritten with
new data without notification. The UD-DASK Function Library Ref-
erence provides corresponding software API details.
Continuous Acquisition Mode
Differs from software polling mode only in the generation of block
data in continuous acquisition mode without the need to consider
data overwriting or acquiring repeat data in software polling mode.
This mode is suitable for when continuous data is to be acquired in
a fixed and precise time interval. Please note the data buffer size
must be a multiple of 256 in continuous acquisition mode. The
UD-DASK Function Library Reference provides corresponding
software API details.
3.2.5 Anti-Aliasing Filter and Frequency Response
The filter limits the bandwidth of the signal path and rejects
out-of-band noise. The digital filter of the Sigma-Delta ADC sets
the overall frequency response, using multi-stage FIR topology to
provide linear phase with minimal passband ripple and high stop-
band attenuation. These image frequencies, if present in the sig-
nal and not externally filtered, will fold back (or alias) into the
passband, causing errors. The stopband of the ADC provides
100dB attenuation of frequencies that begin just beyond the pass-
band and continue out to ADC over-sampling rate. The USB-2405
provides an anti-aliasing, low-pass, simple RC filter in front of the
ADC inputs to limit possible high amplitude out-of-band signals
and noise.