Datasheet

XTR108
19
SBOS187C
www.ti.com
FIGURE 8. Reverse Voltage and Over-Voltage Protection.
XTR108
11
V+
I
O
V
PS
10
0.01µF
R
L
D
1
(1)
14
13
NOTE: (1) Zener Diode 36V: 1N4753A or General
Semiconductor Transorb
TM
1N6286A. Use lower
voltage zener diodes with loop power supply
voltages less than 30V for increased protection.
See “Over-Voltage Surge Protection.”
Maximum V
PS
must be
less than minimum
voltage rating of zener
diode.
The diode bridge causes
a 1.4V loss in loop supply
voltage.
1N4148
Diodes
12
I
RET
RTD APPLICATION
The values to be entered into the DAC control registers are
given by the formulas in Table XV.
1) For a chosen temperature range, using an industry-stan-
dard polynomial set as shown in Table XVI, calculate
RTD values at min, max, and the middle temperatures:
RR andR
MIN MAX MID
,,
()
2) Calculate a relative nonlinearity B
V
using the RTD val-
ues from above:
B
R
RR
RR
V
MID
MAX MIN
MAX MIN
=
+
–
–
2
3) Pick an external zero resistor, R
Z
closest to R
MIN
. Select-
ing R
Z
greater than R
MIN
will cause a voltage offset that
must be corrected by the PGA zero adjustment.
4) Calculate the linearization coefficient::
G
B
BR BR BR
LIN
V
V MAX V MIN V Z
=
+
()()
2
05 05 2.–.––
If the value of G
LIN
is larger than G
LIN MAX
= (16/
R
LIN
) the external resistor R
LIN
has to be changed. If
G
LIN
is significantly smaller (> 10 times) than G
LIN
MAX
, the R
LIN
value should be increased to minimize
the DAC quantization errors. For 100Ω RTD sensors
the required linearization coefficients are in the range
from 0.3 to 0.6 mA/V (1/kΩ) for all measurement
ranges. Therefore an external R
LIN
value of 15.8kΩ is
good setting the full-scale G
LIN MAX
~ = 1mA/V. For
1kΩ RTD’s the R
LIN
should be increased proportion-
ally.
5) Choose the output zero and full-scale level values, for
instance: I
OUT
MIN
= 4mA, I
OUT
MAX
= 20mA.
EXCITATION CURRENT I
REF
Coarse DAC code
N round
IR
V
REF
SET
REF
11
64
320=
•
−






Fine DAC Code
N round
IR
V
N
REF
SET
REF
10 11
1024
5120 16=
•
−−•






ZERO OUTPUT I
ZERO
Coarse DAC Code
N round
IR
V
ZERO
VI
REF
13
32
5
140=
•
•
−






Fine DAC Code
N round
IR
V
N
ZERO
VI
REF
12 13
512
5
2240 16=
•
•
−−•






LINEARIZATION COEFFICIENT G
LIN
Lin DAC Code
N round G R
LIN LIN14
16=•
()
TABLE XV. Equations for DAC Code Calculation.
TABLE XVI. Standard RTD Descriptive Equations.
This procedure allows calculation of the parameters needed
to calculate the DAC codes for an RTD sensors application.
S dard RTD Polynomials
R R At B C t C t for C t C
R R At B for C t C
Ae
Be
Ce
R base RTD value at C or k
t
O
t
O
O
tan :
.
.
.
=+++−°
()
[]
−°<<°
=++
[]
°<< °
=−
=− −
=− −
−°
()
1 1 100 200 0
1 1 0 850
3 9083 3
5 775 7
4 183 12
0 100 1
23
2
ΩΩ