VTI Instruments EX1401, User Manual

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EX1401 Instrument Drivers 85
There is a Sampling interface in the Measurement interface as well as the individual Channel
interface. The properties in the global Sampling interface that apply to all channels as well as DIO
can be configured from the Measurement interface, while the Channel interface allows the user to
configure each channel independently.
ClockFrequency
The ClockFrequency specifies the A/D conversion rate. This property determines the highest
SampleRate of the data acquisition session. The highest rate can be achieved by bypassing all
decimation filters. The EX1401 supports all integers from 10000 to 20000 as valid values of
ClockFrequency . ClockFrequency can also be set to -1 to allow the firmware to choose the value
automatically based on the requested SampleRate . ClockFrequency is a global parameter. It cannot
have different values for each channel.
Prescaler
This is the sampling rate divider before the ADC data is decimated by the ½ decimation filter stages.
The EX1401 only supports a value of 1 for
Prescaler , which has no effect on measurement data.
DownsamplingFactor
This is the amount of unfiltered downsampling that will be performed after the decimation filtering
by the selected
FilterType . The EX1401 supports all integers from 1 to 65536.
FilterType
This specifies the type of digital filter that will be used to decimate and filter the data. Four
options are available;
High Performance , Medium Latency , Low Latency , and None . None disables
all digital filtering, such that effective SampleRate is equal to ClockFrequency divided by
DownsamplingFactor . HighPerformance is an FIR filter with 0-16 divide-by-2 stages. This
provides optimal performance but has high group delay. The decimation ratio can only be powers
of two up to 2
16
. This filter’s response can be customized by providing normalized filter
coefficients. LowLatency is a CIC filter capable of any integer decimation ratio from 4 to 8192.
This provides the lowest group delay, but is only suitable for use with DC inputs. MedLatency
pairs the CIC filter with a Compensating FIR (with decimation ratio of 4) at its output. This
increases the group delay, slightly, while providing a better response across a variety of input
frequencies. Valid decimation ratios are all multiples of 4 from 16 to 65536. The
MedLatency
filter’s CFIR can also be customized by supplying normalized filter coefficients.
SampleRate
This is the effective data rate. The inverse of SampleRate specifies the interval between data samples
that the user can obtain. The ClockFrequency , DownsamplingFactor , and SampleRate determines
the amount of decimation and filtering performed by the filter indicated by FilterType . Some
instruments support setting different SampleRate values on channels in a single instrument. For
EX1401, the SampleRate is a global property and will always be the same for all channels in the
same instrument. The decimation ratio of the selected filter will be set to ClockFrequency /
DownsamplingFactor / SampleRate , rounded down to the next valid value for the chosen FilterType .
Span
The Span is the nominal frequency range of the acquired signal. The value of Span is determined
by the decimation filter’s cutoff frequency. The ratio between
Span and SampleRate varies based
on the selected FilterType . See the instrument specifications table for the precise value for each
filter.