Datasheet

B
V
=
RTD
MAX
- RTD
MIN
RTD
MID
-
2
RTD
MAX
+ RTD
MIN
G
LIN
=
(0.5 + B
V
)*RTD
MAX
- (0.5 - B
V
)*RTD
MIN
- 2*B
V
*R
Z
2*B
V
G
LIN_MAX
=
R
LIN
16
I
SET
=
50*A
PGA
*(RTD
MAX
- RTD
MIN
)
(I
OUT_MAX
- I
OUT_MIN
)*[1 - G
LIN
*(RTD
MAX
- R
Z
)]*R
VI
V
REF
= 1.193V
I
REF
=
R
SET
V
REF
I
REF_MIN
=
0.65*(5*I
SET
)
I
REF_MAX
=
1.35*(5*I
SET
)
Appendix www.ti.com
8.2 XTR108 Calibration Equations (Current-Output Mode)
The following section gives the equations used during the calibration procedure for current-output mode.
8.2.1 Initial Calculations
8.2.1.1 RTD Resistance
The equations in Table 30 are used to calculate the RTD resistance at the minimum temperature
(RTD
MIN
), medium temperature (RTD
MID
), and maximum temperature (RTD
MAX
). Pick an external zero
resistor, R
Z
, that is the closest standard value to RTD
MIN
.
Table 30. 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
]
(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 kΩ), and T = temperature in °C.
8.2.1.2 Relative Nonlinearity
Calculate the relative nonlinearity, B
V
, by using the RTD values calculated in the previous step.
8.2.1.3 Linearization Coefficient
Calculate the linearization coefficient, G
LIN
.
Calculate G
LIN_MAX
. If G
LIN
is larger than G
LIN_MAX
, the external resistor R
LIN
must be changed.
8.2.1.4 I
REF
DAC (Excitation Current Source)
Choose the minimum and maximum output current values (I
OUT_MIN
and I
OUT_MAX
). Select an initial PGA gain
setting (A
PGA
) and calculate the initial excitation current I
REF
. Ensure that the PGA gain is selected such
that I
REF_MIN
< I
REF
< I
REF_MAX
.
56
XTR108EVM-USB Evaluation Board and Software Tutorial SBOU123–March 2012
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