IMC CANSAS - Page 128

128 Operation
imc CANSAS Users Manual - Doc. Version 1.9 - 05.12.2014© 2014 imc Meßsysteme GmbH
For these two options, the ID of the CAN-Bus message to be used must also be defined.
Time offset (Module type, Sampling frequency):
There is a slight time offset between a module's individual channels. This also applies to channels which
are equipped with simultaneous Sample And Hold or in which the AD converters work simultaneously.
This is because the individually conditioned channels possess analog components such as amplifiers and
filters. These have narrow tolerances, but also slight deviations in their frequency response and thus also
in the times for signal propagation through the components. However, in UNI8, for instance, these
discrepancies are typically around 10µs.
Synchronization really only ensures that the instant in which a sample is taken is the same. But the signal
propagation time through the module depends on many factors. These include the analog part with its
anti-aliasing filter and any digital filters it may have. The filters (and the digital filters in general) are
configured according to the sampling rate. This is because an anti-aliasing filter is selected which is
appropriate for the sampling rate set. Thus, the signal propagation time varies with the sampling rate.
Although this signal propagation time is constant for a given module type and a given sampling rate, if
either of these factors change, so does the propagation time, which becomes noticeable when working
with a variety of module types (e.g. UNI8 and P8). This offset is constant and can be corrected but it isn't
zero. To simplify matters, it's recommended to use only one module type for real phase or propagation
time measurements, and to set the same sampling rate for all measurement channels whose values are
to be compared with each other.
Data acquisition:
When imc CANSAS-modules work synchronously, this makes certain demands on the data acquisition
system. In general, it's necessary for the data acquisition system and the master (timer) of all the imc
CANSAS-units to be one and the same device. For an ideal match, use such devices as imc BUSDAQ and
imc CRONOS-PL. One such unit can then serve as the master, configured as per the CAN-1 protocol, and
the imc CANSAS-modules act as the slaves.