VTI Instruments EX1401, User Manual

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EX1401 Instrument Drivers 87
characteristics. If the ClockFrequency is set to -1, the firmware will automatically select the largest
possible value that best achieves the user-specified SampleRate . FilterType selects which type of
digital decimation filter will be used, and will affect both the granularity of available sample rates
and the spectral performance of the decimated output.
DownsamplingFactor is applied after all
digital filtering to reduce sample rate with no group delay by simply throwing away N-1 in N
samples. Finally, setting
SampleRate will specify the decimation ratio of the chosen filter so that
SampleRate = ClockFrequency / DecimationRatio / DownsamplingFactor . DecimationRatio will be
rounded down to the next valid value (and thus SampleRate rounded up) if the supplied value is not
exactly possible.
DIO Sample Rate
The EX1401’s DIO port can record its input state at the same rate as the analog input channels.
Because this data is purely digital and binary, it cannot be decimated. All DIO data is downsampled
with no filtering such that the DIO sample rate is always equal to the analog channels’ final sample
rate. For that reason, only
ClockFrequency and SampleRate are available in the DIO.Sampling
interface – FilterType is not supported. Attempting to set either the channel or DIO sample rate will
always cause the other to be set to the same value.
O
VERLOADS AND
O
PEN
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RANSDUCER
D
ETECTION
When the analog signal amplitude exceeds the input range, the digitized samples are truncated
resulting in a distorted waveform. To avoid this, users can increase the input range of the instrument,
attenuate the signal level, or discard overloaded data from processing. Apart from over-range, there
are other fault conditions that can invalidate measurement data, such as an open transducer. The
EX1401 is capable of detecting some of these common fault conditions and reporting them, allowing
the user to act.
The fault status can be queried from the Digitizer driver’s API. The status can be indicated at the
front panel LED, external trigger, and DIO outputs. The information is also associated to the
acquired data record and returned in the
AdditionalData string.
The Digitizer driver defines each fault status as an Overload Status bit field. The user can configure
which fault conditions to reported, if an LED indicator is shown, or it can latch so that a momentarily
fault won’t be overlooked.
Bit Mask Name Description
0x00000002 Transducer Upper Limit
The signal is above the defined range of the
transducer’s EU conversion. This is either the limit of
the NIST polynomials, for standard thermocouple
types, or the user-supplied value of
Channel.Tranducer.UpperLimit.
0x00000004 Transducer Lower Limit
The signal is below the defined range of the
transducer’s EU conversion. This is either the limit of
the NIST polynomials, for standard thermocouple
types, or the user-supplied value of
Channel.Tranducer.LowerLimit.
0x00000008 User Upper Limit
The signal is above the user-defined value of
Channel.Overload.UserUpperLimit, and
Channel.Overload.UserLimitEnabled is True.
0x00000010 User Lower Limit
The signal is below the user-defined value of
Channel.Overload.UserLowerLimit, and
Channel.Overload.UserLimitEnabled is True.