Datasheet

10
®
XTR112, XTR114
R
2
= RTD resistance at maximum measured temperature, T
MAX
where R
Z
= RTD resistance at the minimum measured temperature, T
MIN
R
LIN
= 1k (internal)
XTR112 RESISTOR EXAMPLE:
The measurement range is –100°C to +200°C for a 3-wire Pt100 RTD connection. Determine the values for R
S
, R
G
, R
LIN1
, and R
LIN2
. Look up the values
from the chart or calculate the values according to the equations provided.
METHOD 1: TABLE LOOK UP
T
MIN
= –100°C and T = 300°C (T
MAX
= +200°C),
Using Table II the 1% values are:
R
Z
= 604 R
LIN1
= 33.2k
R
G
= 750 R
LIN2
= 59k
METHOD 2: CALCULATION
Step 1: Determine R
Z
, R
1
, and R
2
.
R
Z
is the RTD resistance at the minimum measured temperature, T
MIN
= –100°C.
Using Equation (1) at right gives R
Z
= 602.5 (1% value is 604).
R
2
is the RTD resistance at the maximum measured temperature, T
MAX
= 200°C.
Using Equation (2) at right gives R
2
= 1758.4.
R
1
is the RTD resistance at the midpoint measured temperature,
T
MID
= (T
MIN
+ T
MAX
)/2 = (–100 + 200)/2 = 50°C. R
1
is NOT the average of R
Z
and R
2
.
Using Equation (2) at right gives R
1
= 1194.
Step 2: Calculate R
G
, R
LIN1
, and R
LIN2
using equations above.
R
G
= 757 (1% value is 750)
R
LIN1
= 33.322k (1% value is 33.2k)
R
LIN2
= 58.548k (1% value is 59k)
Calculation of Pt1000 Resistance Values
(according to DIN IEC 751)
Equation (1) Temperature range from –200°C to 0°C:
R
(T)
= 1000 [1 + 3.90802 • 10
–3
• T – 0.5802 • 10
–6
• T
2
– 4.27350 • 10
–12
• (T – 100) • T
3
]
Equation (2) Temperature range from 0°C to +850°C:
R
(T)
= 1000 (1 + 3.90802 • 10
–3
• T – 0.5802 • 10
–6
• T
2
)
where: R
(T)
is the resistance in at temperature T.
T is the temperature in °C.
TABLE I. Summary of Resistor Equations for Two-Wire and Three-Wire Pt1000 RTD Connections.
Table I summarizes the resistor equations for two-wire and
three-wire RTD connections. An example calculation is also
provided. To maintain good accuracy, at least 1% (or better)
resistors should be used for R
G
. Table II provides standard
1% R
G
values for a three-wire Pt1000 RTD connection with
linearization for the XTR112. Table III gives R
G
values for
the XTR114.
LINEARIZATION
RTD temperature sensors are inherently (but predictably)
nonlinear. With the addition of one or two external resistors,
R
LIN1
and R
LIN2
, it is possible to compensate for most of this
nonlinearity resulting in 40:1 improvement in linearity over
the uncompensated output.
General Equations
NOTE: Most RTD manufacturers provide reference tables for
resistance values at various temperatures.
Resistor values for other RTD types (such as Pt2000) can be
calculated using the XTR resistor selection program in the
Applications Section on Burr-Brown’s web site (www.burr-
brown.com)
I
REF
• 2.5 [R
1
(R
2
+ R
Z
) – 2 (R
2
R
Z
)]
(R
2
– R
1
)
=
0.4 • R
LIN
(R
2
– R
1
)
I
REF
• (2R
1
– R
2
– R
Z
)
=
I
REF
• 2.5 (R
2
– R
Z
) (R
1
– R
Z
)]
(R
2
– R
1
)
=
0.4 • R
LIN
(R
2
– R
1
)
I
REF
• (2R
1
– R
2
– R
Z
)
=
0.4 • (R
LIN
+ R
G
)(R
2
– R
1
)
I
REF
• (2R
1
– R
2
– R
Z
)
=
XTR112 (I
REF
= 0.25)
(see Table II)
R
G
R
LIN1
R
G
R
LIN1
R
LIN2
THREE-WIRE
TWO-WIRE
0.625 • [R
1
(R
2
+ R
Z
) – 2 (R
2
R
Z
)]
(R
2
– R
1
)
=
1.6 • R
LIN
(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
0.625 • (R
2
– R
Z
) (R
1
– R
Z
)]
(R
2
– R
1
)
=
1.6 • R
LIN
(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
1.6 • (R
LIN
+ R
G
)(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
0.25 • [R
1
(R
2
+ R
Z
) – 2 (R
2
R
Z
)]
(R
2
– R
1
)
=
4 • R
LIN
(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
0.25 • (R
2
– R
Z
) (R
1
– R
Z
)]
(R
2
– R
1
)
=
4 • R
LIN
(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
4 • (R
LIN
+ R
G
)(R
2
– R
1
)
(2R
1
– R
2
– R
Z
)
=
XTR114 (I
REF
= 0.1)
(see Table III)
R
1
= RTD resistance at the midpoint measured temperature, T
MID
= (T
MIN
+ T
MAX
)/2