Software Overview
33
SBOU123A – March 2012 – Revised September 2016
Copyright © 2012–2016, Texas Instruments Incorporated
XTR108EVM-USB Evaluation Board and Software Tutorial
5.2.1
Step 1: Enter Operating Conditions
This area of the
Find Resistors
tab allows the user to input system operating conditions, including the
global system conditions as well as individual conditions for each channel. Global conditions consist of
current- or voltage-output mode, output span, and overscale and underscale limiting. Channel-specific
conditions include channel enable or disable, temperature span, RTD type, and Callendar-van Dusen
(CVD) coefficients.
5.2.1.1
Mode
This control allows the user to select current-output mode or voltage-output mode. Ensure that the proper
power-supply voltage is applied for each mode (see the
Connecting Loop Power Supply to the
section) and that the correct Sensor Board jumper settings are used
(see
).
5.2.1.2
Output Span
This control allows the user to set the desired XTR108 output span. The most common output span
settings are 4 mA to 20 mA for current-output mode and 0.5 V to 4.5 V for voltage-output mode. These
controls automatically update the units (mA or V) to reflect the status of the
Mode
control.
5.2.1.3
Overscale and Underscale
This control allows the user to enable or disable overscale and underscale limiting as well as set the
overscale and underscale output levels. The current and voltage levels available in the overscale and
underscale output controls correspond to the values listed in
and
. These controls
automatically update the units (mA or V) to reflect the status of the
Mode
control.
5.2.1.4
Channel Enable
The XTR108 supports up to five input channel configurations, allowing the same board design to be used
for multiple input conditions. Only one channel can be calibrated at a time. The
Channel Enable
control
allows the user to enable or disable the five input channels, as required by their application.
5.2.1.5
Temperature
This control allows the user to input the minimum and maximum RTD temperature of each input channel
in °C. The lowest possible minimum temperature is –200°C and the highest possible maximum
temperature is +850°C and matches the operating temperature ranges of most commercially-available
RTDs.
5.2.1.6
RTD Type
The XTR108 is primarily designed to interface to PT100- and PT1000-type RTDs. PT100 RTDs have a
resistance of 100
Ω
at 0°C; PT1000 RTDs have a resistance of 1000
Ω
(1 k
Ω
) at 0°C. The
RTD Type
control allows the user to specify what type of RTD is used at each input channel.
5.2.1.7
CVD Coefficients
The resistance of an RTD across its temperature range is defined by a set of polynomial equations, as
shown in
. A, B, and C are known as Callendar-van Dusen (CVD) coefficients. The industry-
standard values for A, B, and C are loaded into the XTR108EVM-USB software by default, but custom
values for each channel can be entered in the
CVD Coefficients
control. The
Reset CVD Coefficients
button restores the default values when clicked.
(1)
A = 3.9083E-3, B = –5.775E-7, C = –4.183E-12, R
0
= RTD value at 0°C (typically 100
Ω
or 1 k
Ω
), and T = temperature in °C.
Table 26. Standard RTD Equations
(1)
If –200°C < T < 0°C
If T > 0°C
R
t
= R
0
[1 + A × T + B × T
2
+ C × T
3
(T – 100°C)]
R
t
= R
0
[1 + A × T + B × T
2
]
