VTI Instruments EX1000A, User Manual

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VTI Instruments Corp.
60 EX10xxA Triggering
The user has configuration control over the reporting of the measured CJC data. This control only
affects the display of their data, not the actual measurement of them. They are updated with every
scan, regardless of their reporting status. If they are reported, their values are provided with units of
ºC, unaffected by the °C/°F configuration setting of the input channels.
The EX10xxA also accommodates the use of an external cold junction that is maintained and
measured by the user. In this application, the cold junction temperature in ºC is entered into the
EX10xxA and enabled on a per channel basis. That is, the use of internal and user-defined CJC
inputs can be mixed throughout the unit. This control is unaffected by the °C/°F configuration setting
of the input channels.
The default selections are:

CJC reporting is disabled

User-defined CJC temperatures are 0.0

User-defined CJC temperatures are disabled for all channels
T
EMPERATURE
U
NITS
The EX10xxA can output its temperature data with units of ºC or ºF. This is controlled on a global
basis, such that all input channels are configured with one setting. This selection applies to input
channel data only. CJC measurement data and the input values for external CJC temperatures are
fixed at ºC. Similarly, input channels configured for an EU conversion of voltage are unaffected by
this setting.
The default selection is °C.
S
AMPLING
R
ATE
/ N
OISE
P
ERFORMANCE
The EX10xxA can be configured for a sampling rate up to a maximum of 1 kSa/s, regardless of the
number of channels included in the scan list. The selected sampling rate refers to the frequency at
which the entire scan list is measured. As such, all channels are measured at the same sampling rate.
When the requested sampling rate is significantly less than 1 kSa/s, however, the EX10xxA
automatically takes and averages multiple samples. This offers improved noise performance, while
maintaining the requested data output rate. For example, 4096 readings of a K type thermocouple at
25 ºC are displayed with the following typical noise profiles at different selected sampling rates:
Sampling Rate* Noise (ºCp-p)
≤200 Hz
0.01
500 Hz 0.19
1 kHz 0.37
*4 Hz filter applied
T
ABLE
3-4: T
YPE
T T
HERMOCOUPLE
N
OISE
P
ROFILE
V
S
. S
AMPLING
R
ATE
As implied by Table 3-4, the maximum averaging occurs once the sampling rate reaches 200 Hz
and represents the ultimate noise floor of the instrument. Decreasing the sampling rate further does
not result in a lower noise profile. In practice, the noise performance delivered by the EX10xxA is
a function of the slope of the thermocouple characteristic for the sensor being used. That is, type E
thermocouples (59 µV/ºC) will deliver inherently quieter measurements than type K thermocouples
(39 µV/ºC). However, the relative relationship between noise and sampling rate shown above will
be true for any thermocouple type. Consequently, for best instrument noise performance, the
sampling rate should be set as low as the data collection requirements allow.
NOTE The described averaging mechanism occurs only if the trigger model is configured with a TRIG
source of Timer or LXI alarm. For more details, see
Maximizing Measurement Performance .