R
CM
523
:
R
Z2
R
Z3
R
Z4
R
Z5
R
Z1
18.7
:
RTD
PT100
+
PGA
XTR108
T=-200°C
18.52
:
500
P
A
500
P
A
I
1
I
2
V
CM =
0.52V
0.532V
0.532V
1
0
0
0
P
A
5
0
0
P
A
5
0
0
P
A
Input differential voltage is
approximately 0V at T
MIN
MUX
Appendix
52
SBOU123A – March 2012 – Revised September 2016
Copyright © 2012–2016, Texas Instruments Incorporated
XTR108EVM-USB Evaluation Board and Software Tutorial
8
Appendix
This section contains information related to the operation of the XTR108, the equations used during the
calibration procedure, and the mnemonic command syntax displayed by the
Debug
window.
8.1
XTR108 Operation
This section describes the operation of key XTR108 functional blocks.
8.1.1
Input MUX
The primary function of the XTR108 input multiplexer (MUX) is to allow a single hardware module to
function for multiple RTD ranges and types. In the typical XTR108 configuration, one MUX channel is used
for the RTD and five other channels remain for different ranges. R
Z1
through R
Z5
are used to the set the
minimum temperature (T
MIN
) of a particular range. In the example shown below, where T
MIN
= –200°C, R
Z1
is selected to match the resistance of the RTD at this temperature (18.7
Ω
is the closest standard value).
This setting forces the PGA differential input voltage to be approximately 0 V at T
MIN
. R
CM
generates a
common-mode voltage so that the XTR108 common-mode input voltage range is not violated.
Figure 36. XTR108 Input MUX Example (T
MIN
= –200°C, PT100 RTD)
