Rockwell Automation 1771-IXHR , D17716.5.80 HIGH RESOL.THERMOCOUPLE - Manual (Page 50)

Appendix A
Specifications
A-3
Allowable Distances
The open thermocouple detection circuit injects a current of approximately 7.3
nanoamps into the thermocouple cable. A total lead resistance of 1370 ohms
(685 ohms one–way) cable resistance will produce 10uV of error.
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 offset 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.
Hardware
The analog input module has hardware–based high frequency filters on all
channels to reduce the effect of electrical noise on the input signal. In addition,
a 6–pole digital filter, which begins rolling off at 8.0Hz, is also incorporated.
Software
A first order programmable filter is also available. Filter time constants range
from 0 (disabled) to 255 (6.4 seconds).
Lead Resistance
Compensation
Filtering