Rtd Accuracy - Spectrum Controls 1734 User Manual

6
The module update time is purely the sum of the conversion times for each enabled channel except
when a channel is configured for a 3-wire RTD. For each channel configured for a 3-wire RTD, double
the conversion time.
Chapter 3: Configuring the 1734sc-IF4U for RSLogix 5000
Channel Disable (Bit 0)
Enable (Default) = 0, Disable = 1. If the channel is disabled, all other fields are
ignored.
Filter Frequency (Bits 1 and 2)
The filter selection affects how the module attenuates the input signal at the
specified frequency. It also affects the update time of the input data which is
reflected in the minimum conversion time for each channel. Lower filter
frequencies are recommended for RTD and Thermocouple modes. The total
update time for the module can be calculated using the table below. Default
setting is 17 Hz.
Table 3-2. Channel Conversion Time
ADC Filter Name
(Update Frequency)
470 Hz
62 Hz
17 Hz
4 Hz
Example:
Channels 0/1 are configured for 3-wire RTD and 4 Hz filter:
Channel 2 is configured for 17 Hz voltage.
Channel 3 is configured for 62 Hz current.
The RTD channel pair has a conversion time of 1024 ms (512*2).
The module scan time will be 1024+153+62 = 1239 ms

3.5.2 RTD Accuracy

The RTD Algorithms used have a curve-fit accuracy that is generally within two
to three decimal places of a degree C across the span of measured resistance
ranges. This conversion accuracy has little effect on the final result. RTD
conversion is therefore heavily dependent on its ability to accurately measure
resistance.
CJC Disable (Bit 3)
When this bit is set, no CJC compensation is applied to the channel if it is
configured as a thermocouple. Default setting is enabled.
Open Circuit Response (Bits 4 and 5)
The open circuit response defines the state of the channel data when an open-
Step Response/Module update
Conversion
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Time
Rate
37 ms
65 ms
= Sum of conversion times for each
enabled channel
153 ms
512 ms
User's Manual Pub. 0300266-02 Rev. B
3-13