Datasheet

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SBOS146A − OCTOBER 1986 − REVISED AUGUST 2004
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11
EXAMPLE 1
An RTD transducer is shown in Figure 9.
Given a process with temperature limits of +25°C and
+150°C, configure the XTR101 to measure the
temperature with a platinum RTD which produces 100Ω at
0°C and 200Ω at +266°C (obtained from standard RTD
tables). Transmit 4mA for +25°C and 20mA for +150°C.
COMPUTING R
S
:
The sensitivity of the RTD is ∆R/∆T = 100Ω/266°C. When
excited with a 1mA current source for a 25°C to 150°C
range (a 125°C span), the span of e
IN
is
1mA × (100Ω/266°C) × 125°C = 47mV = ∆e
IN
.
From Equation 1, R
S
+
40
DI
O
ńDe
IN
* 0.016
amps
volt
R
S
+
40
16mAń47mV * 0.016AńV
+
40
0.3244
+ 123.3W
Span adjustment (calibration) is accomplished by
trimming R
S
.
COMPUTING R
4
:
At ) 25
o
C, eȀ
2
+ 1mA
(
R
T
) DR
T
)
+ 1mA
ƪ
100W )
100W
266
o
C
25
o
C
ƫ
+ 1mA
(
109.4W
)
+ 109.4mV
In order to make the lower range limit of 25°C correspond
to the output lower range limit of 4mA, the input circuitry
shown in Figure 9 is used.
e
IN
, the XTR101 differential input, is made 0 at 25°C or:
eȀ
225
o
C
* V
4
thus, V
4
+ eȀ
225
o
C
+ 109.4mV
R
4
+
V
4
1mA
+
109.4mV
1mA
+ 109.4W
COMPUTING R
2
AND CHECKING CMV:
At ) 25
o
C, eȀ
2
+ 109.4mV
At ) 150
o
C, eȀ
2
+ 1mA
(
R
T
) DR
T
)
+ 1mA
ƪ
100W )
100W
266
o
C
150
o
C
ƫ
+ 156.4mV
Since both eȀ
2
and V
4
are small relative to the desired 5V
common-mode voltage, they may be ignored in computing
R
2
as long as the CMV is met.
R
2
+
5V
2mA
+ 2.5kW
e
2
min + 5V ) 0.1094V
e
2
max + 5V ) 0.1564V
e
1
+ 5V ) 0.1094V
ǁ
The 4V to 6V CMV
requirement is met.
+
−
8
10
3
5
6
4
R
S
e
IN
+
−
XTR101
R
2
7
0.01
µ
F
D
1
e
1
e
2
e’
2
+
0.01
µ
F
−
R
T
11
V
4
R
4
+
−
+
24V
−−
+
R
L
e
L
Figure 9. Circuit for Example 1
EXAMPLE 2
A thermocouple transducer is shown in Figure 10.
Given a process with temperature (T
1
) limits of 0°C and
+1000°C, configure the XTR101 to measure the
temperature with a type J thermocouple that produces a
58mV change for 1000°C change. Use a semiconductor
diode for cold junction compensation to make the
measurement relative to 0°C. This is accomplished by
supplying a compensating voltage (V
R6
) equal to that
normally produced by the thermocouple with its cold
junction (T
2
) at ambient. At a typical ambient of +25°C, this
is 1.28mV (obtained from standard thermocouple tables
with reference junction of 0°C). Transmit 4mA for T
1
= 0°C
and 20mA for T
1
= +1000°C. Note: e
IN
= e
2
− e
1
indicates
that T
1
is relative to T
2
.
+
−
XTR101
1mA
1mA
R
6
51
Ω
−
+
2.5k
Ω
e
1
e
2
3
11
10
8
7
4
R
4
V
4
0.01
µ
F
V
TC
Thermocouple
T
TC
Temperature T
1
Temperature T
2
=T
D
−
+
+
−
D
R
5
2k
Ω
e
IN
Figure 10. Thermocouple Input Circuit with Two
Temperature Regions and Diode (D) Cold
Junction Compensation