20
Span Adjust
at the transducer. Thus the XTR101 is, in general, very
suitable for individualized and special purpose applications.
EXAMPLE 1
RTD Transducer shown in Figure 9.
15
i
0
(mA)
10
Elevated
Zero
Range
5
Suppressed
Zero
Range
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
(i.e., 125°C span), the span of e
IN
is 1mA
X
(100Ω/266°C)
X
125°C = 47mV =
∆e
IN
.
From equation 1, R
S
=
40
∆I
O
/∆e
IN
– 0.016Ω
=
40
= 123.3Ω
0.3244
0
–0+
e
IN
(V)
FIGURE 7. Elevation and Suppression Graph.
1mA
1mA
e
IN
–
+
1mA
1mA
–
e
IN
+
+
V
4
–
R
4
e'
2
–
2mA
+
R
T
e'
2
+
R
T
–
+
R
4
2mA
–
Span adjustment (calibration) is accomplished by trimming
R
S
.
COMPUTING R
4
:
At +25°C, e'
2
= 1mA (R
T
+
∆R
T
)
= 1mA [100Ω +
X
25°C]
266°C
= 1mA (109.4Ω) = 109.4mV
100Ω
V
4
e
IN
= (e'
2
–V
4
)
V
4
= 1mA
X
R
4
e'
2
= 1mA
X
R
T
(a) Elevated Zero Range
e
IN
= (e'
2
+V
4
)
V
4
= 1mA
X
R
4
e'
2
= 1mA
X
R
T
(b) Suppressed Zero Range
2mA
–
e
IN
+
2mA
–
e'
2
+
–
e
IN
+
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
e'
2 25°C
– V
4
= 0
+
V
4
–
R
4
+
e'
2
–
+
V
4
–
2mA
2mA
R
4
thus, V
4
= e'
2 25°C
= 109.4mV
R
4
=
V
4
1mA
=
109.4mV
1mA
= 109.4Ω
e
IN
= (e'
2
–V
4
)
V
4
= 2mA
X
R
4
(c) Elevated Zero Range
e
IN
= (e'
2
+V
4
)
V
4
= 2mA
X
R
4
(d) Suppressed Zero Range
COMPUTING R
2
AND CHECKING CMV:
At +25°C, e'
2
= 109.4mV
At +150°C, e'
2
= 1mA (R
T
+
∆R
T
)
= 1mA [100Ω +(
= 156.4mV
Since both e'
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 = 2.5kΩ
e
2
min = 5V + 0.1094V
e
2
max = 5V + 0.1564V
e
1
= 5V + 0.1094V
®
FIGURE 8. Elevation and Suppression Circuits.
APPLICATION INFORMATION
The small size, low offset voltage and drift, excellent linear-
ity, and internal precision current sources, make the XTR101
ideal for a variety of two-wire transmitter applications. It can
be used by OEMs producing different types of transducer
transmitter modules and by data acquisition systems manu-
facturers who gather transducer data. Current mode trans-
mission greatly reduces noise interference. The two-wire
nature of the device allows economical signal conditioning
100Ω
X
150°C)]
266°C
9
Ω
R
S
=
40
16mA/47mV – 0.016
The +4V to +6V CMV
requirement is met.
XTR101
BURR-BROWN [ BURR-BROWN CORPORATION ]
