APPLICATIONS INFORMATION
The XTR115 and XTR116 are identical devices except for
the reference voltage output, pin 1. This voltage is available
for external circuitry and is not used internally. Further
discussions that apply to both devices will refer to the
“XTR115/6.”
Figure 1 shows basic circuit connections with representative
simplified input circuitry. The XTR115/6 is a two-wire
current transmitter. Its input signal (pin 2) controls the output
current. A portion of this current flows into the V+ power
supply, pin 7. The remaining current flows in Q1. External
input circuitry connected to the XTR115/6 can be powered
from V
REG
or V
REF
. Current drawn from these terminals
must be returned to I
RET
, pin 3. This I
RET
pin is a “local
ground” for input circuitry driving the XTR115/6.
The XTR115/6 is a current-input device with a gain of 100.
A current flowing into pin 2 produces I
O
= 100 • I
IN
. The
input voltage at the I
IN
pin is zero (referred to the I
RET
pin).
A voltage input is created with an external input resistor, as
shown. Common full-scale input voltages range from 1V
and upward. Full-scale inputs greater than 0.5V are recom-
mend to minimize the effect of offset voltage and drift of A1.
EXTERNAL TRANSISTOR
The external transistor, Q1, conducts the majority of the full-
scale output current. Power dissipation in this transistor can
approach 0.8W with high loop voltage (40V) and 20mA
output current. The XTR115/6 is designed to use an external
transistor to avoid on-chip thermal-induced errors. Heat
produced by Q1 will still cause ambient temperature changes
that can affect the XTR115/6. To minimize these effects,
locate Q1 away from sensitive analog circuitry, including
XTR115/6. Mount Q1 so that heat is conducted to the
outside of the transducer housing.
The XTR115/6 is designed to use virtually any NPN transis-
tor with sufficient voltage, current and power rating. Case
style and thermal mounting considerations often influence
the choice for any given application. Several possible choices
are listed in Figure 1. A MOSFET transistor will not improve
the accuracy of the XTR115/6 and is not recommended.
I
REG
XTR115: 2.5V
XTR116: 4.096V
I
REF
XTR115
XTR116
5V
V
REG
8
V
REF(1)
1
Voltage
Reference
+5V
Regulator
V+
7
I
O
V
LOOP
Input
Circuitry
V
IN
R
IN
20kΩ
I
IN
B
I
IN
2
A1
E
5
3
R
LIM
R
1
2.475kΩ
R
2
25Ω
I
RET
6
Q
1
10nF
R
L
All return current
from I
REG
and I
REF
For I
O
= 4mA to 20mA
I
IN
= 40µA to 200µA
With R
IN
= 20kΩ
V
IN
= 0.8V to 4V
I
O
4
I = 100 • I
IN
NOTE: (1) See also Figure 5.
Possible choices for Q
1
(see text).
TYPE
2N4922
TIP29C
TIP31B
PACKAGE
TO-225
TO-220
TO-220
FIGURE 1. Basic Circuit Connections.
XTR115, XTR116
SBOS124A
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