Spectrum Controls 1769sc-IT6I - Manual (Page 42)

4-8 Compact™ IO Isolated Thermocouple Module
User’s Manual Pub. 0300244-01 Rev. A
Repeatability
Repeatability is the ability of the input module to register the same reading in successive
measurements for the same input signal. The repeatability for an input channel depends
upon the filter frequency selected for that channel. Table 4-6, above, describes the
repeatability for each of the range selections at the six available frequencies. This table
does not include the affects of unfiltered input noise. Choose the frequency that most
closely matches your requirements.
Module Update Time
The module update time is defined as the total time required for the module to sample
and convert the input signals of all enabled input channels and provide the resulting data
values to the processor. On an isolated module with one ADC (Analog to Digital
Converter) per channel, all channels convert the signal in parallel so the module update
time is equal to the slowest channel update time in normal conversion mode (normal
conversion mode is defined when CJC is not being sampled).
The update time can slow down for channels 0 and 5 during CJC sampling which occurs
once every 10 seconds. At the 10 second internal the ADC switches from measuring the
thermocouple voltage to measuring the CJC voltage. Once a sample of the CJC voltage
is taken, the ADC will switch back to measuring the thermocouple voltage. This
switching process increases the channel update time. Table 4-6 shows the increased
update time for channels 0 and 5 during the CJC sampling process. After the CJC
sampling process completes, channels 0 and 5 return to the, faster, normal conversion
mode as described above.
For channels 1, 2, 3 and 4 the update rate is fixed and unaffected by the CJC
measurement. Likewise, if all channels are configuring for measuring millivolts, then the
CJCs are never sampled and the module update time is equal to the slowest configured
channel update time.
Channel update time is also dependent upon the input filter selection. Table 4-6 shows
the channel update times based on filter selection.
Example 1 Channels enabled for different input types
Channel 0 Input: +/- 50 mV with 4.17 Hz Filter
Channel (1 – 5) Input: Type J TC with 10 Hz Filter
Module update time before CJC update
= slowest channel update time
= 246 ms
Module update time during CJC update
Channel 5 update time changes to 500 ms
= slowest channel update time
= 500 ms