VTI Instruments EX1629, Руководство пользователя

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EX1629 Basic Operation 53
Unique in its design, the EX1629 features a secondary “confidence” measurement system that runs
in parallel with the main bridge measurement system. This confidence system, if enabled,
produces data at a fixed rate of approximately 500 Sa/s. If the sampling rate is set at or below
500 Sa/s, the confidence system can produce fresh data for every main bridge output reading. If,
however, the sampling rate is set above 500 Sa/s, the returned data will periodically be returned
without confidence data.
The default setting is 1000.
U
NITS
Each EX1629 input channel can be individually configured with respect to the units of its strain
conversions, either strain (ε) or microstrain (µε). The default units setting is strain. The results of
voltage and linear conversions are always in volts, and ratiometric conversions are unitless.
If the tare feature is employed, it should be noted that an entered tare value is unaffected by a
change in the units setting. For example, if a tare value of 100 is entered when the units are
microstrain, it will be considered as 100 µε. If the unit’s setting is then changed to strain, this tare
value will be considered as 100 ε. To avoid this confusion, it is best to set the desired units first
and then enter any tare value.
T
ARE
Each EX1629 input channel can be assigned a tare, or relative, value. The tare value is subtracted
from the nominal conversion results to produce the final readings, yielding measurements that are
referenced to a nonzero point. Tare can be used on strain as well as non-strain EU conversions.
A subtle characteristic of the tare value is that it is not linked to the units setting of the instrument.
For example, if a tare value of 100 is entered when the units are microstrain, it will be considered
as 100 µε. However, if the units setting is then changed to strain, this tare value will be considered
as 100 ε. Similarly, if instead the conversion is changed to voltage, this tare value will be
considered as 100 V. To avoid this confusion, it is best to set the desired EU conversion and units
first and then enter any tare value.
Tare values apply only to main bridge conversions, not to confidence system measurements.
The default tare value is 0.
D
IGITAL
F
ILTER
The EX1629 finite impulse response (FIR) filters provide near Nyquist bandwidths as indicated in
Table B-1. The passband ripple for sample frequencies (
f
s
) greater than or equal to 3125 Hz
is -0.01 dB, while it is ±0.001 dB for f
s
< 3125 Hz. The filters provide an alias rejection of 100 dB
and have linear phase. Some sampling frequencies result in very high attenuation of 60 Hz signals.
The FIR filters are solely a function of sampling and are not user configurable.
Each EX1629 channel can be individually configured with respect to infinite impulse response
(IIR) filtering. The user can select either Butterworth or Bessel type filters, or may choose no IIR
filter. If IIR filtering is turned on, the -3 dB frequencies can be chosen continuously from
f
s
/1000
to f
c
max as indicated in Table B-1. The order of the filters can be set by the user as well.
Butterworth filters can be set from 0 - 10 and Bessel filters can be set from 1 to 10. When the user
order is specified as 0, the EX1629 calculates the order based on an analog prototype Butterworth
filter with -200 dB attenuation at
f
s
/2, given the sampling frequency and the -3 dB frequency. The
on-board DSP will design IIR filters on-the-fly given a sampling frequency, the -3 dB frequency, a
user order between 1 and 10, the filter type, and the transform.