XTR103 Burr-Brown Corporation, XTR103 Datasheet - Page 6

• Manufacturer Part Number: XTR103
• Description: 4-20mA Current Transmitter with RTD EXCITATION AND LINEARIZATION
• Manufacturer: Burr-Brown Corporation

APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for the XTR103.
The loop power supply, V
Output loop current is measured as a voltage across the
series load resistor, R
Two matched 0.8mA current sources drive the RTD and
zero-setting resistor, R
put of the XTR103 measures the voltage difference between
the RTD and R
the resistance of the RTD at the low-scale (minimum)
measurement temperature. R
4mA output at the minimum measurement temperature to
correct for input offset voltage and reference current mis-
match of the XTR103.
R
the XTR103 within their common-mode range. Resistor, R
sets the gain of the instrumentation amplifier according to
the desired temperature measurement range.
The transfer function through the complete instrumentation
amplifier and voltage-to-current converter is:
where V
connected (R
output current. With R
duces a 4-20mA output current. Other values for R
calculated according to the desired full-scale input voltage,
V
FIGURE 1. Basic RTD Temperature Measurement Circuit.
CM
FS
, with the formula in Figure 1.
provides an additional voltage drop to bias the inputs of
RTD
I
(V
IN
0.8mA
O
I
R
IN
= V
is the differential input voltage. With no R
=
G
®
in volts, R
=
Z
IN
XTR103
. The value of R
• (0.016 + 40/R
0.8mA
), a 0V to 1V input produces a 4-20mA
I
R
=
L
.
Z
G
V
. The instrumentation amplifier in-
G
IN
= 25 , a 0V to 10mV input pro-
PS
V
= V
= I
+
–
R
in ohms, R
IN
Z
provides power for all circuitry.
(1, 3)
R
+
Z
IN
(RTD – R
can be adjusted to achieve
– V
R
Z
(2, 3)
G
–
G
IN
is chosen to be equal to
) + 4mA,
4
5
6
3
Z
R
LIN
)
V
V
R
R
CM
–
R
+
G
G
13
IN
IN
8
0.01µF
I
LIN
= )
R
= 1.5k
XTR103
R
12
LIN
I
R
R
(3)
LIN
9
G
V+
I
O
can be
10
B
E
7
I = 4mA + V
15
11
O
G
G
,
NOTES: (1) R
Q
6
1
Negative input voltage, V
be less than 4mA. Increasingly negative V
output current to limit at approximately 3.6mA.
Increasingly positive input voltage (greater than V
produce increasing output current according to the transfer
function, up to the output current limit of approximately
34mA.
EXTERNAL TRANSISTOR
Transistor Q
4-20mA loop current. Using an external transistor isolates
the majority of the power dissipation from the precision
input and reference circuitry of the XTR103, maintaining
excellent accuracy.
Since the external transistor is inside a feedback loop its
characteristics are not critical. Requirements are: V
45V min,
requirements may be lower if the loop power supply voltage
is less than 40V. Some possible choices for Q
Figure 1.
The XTR103 can be operated without this external transistor
by connecting pin 11 to 14 (see Figure 2). Accuracy will be
somewhat degraded by the additional internal power dissipa-
tion. This effect is most pronounced when the input stage is
set for high gain (for low full-scale input voltage). The
typical performance curve “Input Offset Voltage vs Loop
Supply Voltage” describes this behavior.
IN
(2)
(3) See Table I for values.
(0.016 +
R
0.01µF
Z
G
Possible choices for Q
= RTD resistance at the minimum measured temperature.
=
40
R
2N4922
V
TIP29B
TIP31B
G
TYPE
1
FS
2500
)
= 40 min and P
1
– 1
conducts the majority of the signal-dependent
PACKAGE
TO-225
TO-220
TO-220
R
+
–
, where V
L
4-20 mA
1
(see text).
IN
, will cause the output current to
D
FS
= 800mW. Power dissipation
is Full Scale V
V
+
–
PS
IN
.
IN
will cause the
1
are listed in
FS
CEO
) will
=