Rockwell Automation 1771-IXE Thermo/MV Input Installation Instructions - Manual (Page 14)

Thermocouple/Millivolt Input Module 14
Publication 1771Ć5.49 - October 1996
Source Impedance Compensation for Millivolt Inputs
Source resistance causes similar errors to occur with millivolt inputs.
If source resistance is less than 100 ohms, no compensation is
necessary to maintain stated accuracy. If source resistance is greater
than 100 ohms, the error can be calculated as follows:
Error (in calibration counts) =
309329 R
s
(0.22-Vin)
R
s
+ 15M ohms
-
Where R
s
= source resistance (one–way cable resistance)
V
in
= applied input voltage
When using thermocouples, Vin is the approximate thermocouple
voltage of the temperature of interest.
+
+
-
-
.22V
V
in
2V
RS
30megohms
Vo
2R
S
V
O
= V
IN
+ 2V
RS
Error = 2V
RS
Internal
Module
Circuitry
To maintain a display error of < 5uV at V
in
= OV, R
S
should be <
341 ohms. Refer to NBS NM–125 Thermocouple Reference Tables
for determining actual thermocouple voltage versus temperature
readings.
The analog input module has hardware–based high frequency filters
on all channels to reduce the effect of electrical noise on the input
signal. These are 6–pole filters, which begin rolling off at 8.0Hz.
This filtering is in addition to the software-based digital filtering
selected in the module’s BTW configuration.
Filtering