and upward. Full-scale inputs greater than 0.5V are recom-
mend to minimize the effect of offset voltage and drift of A1.
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.”
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.
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 VREG or VREF. Current drawn from these terminals
must be returned to IRET, pin 3. This IRET pin is a “local
ground” for input circuitry driving the XTR115/6.
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.
The XTR115/6 is a current-input device with a gain of 100.
A current flowing into pin 2 produces IO = 100 • IIN. The
input voltage at the IIN pin is zero (referred to the IRET pin).
A voltage input is created with an external input resistor, as
shown. Common full-scale input voltages range from 1V
XTR115
XTR116
IREG
IO
5V
VREG
8
+5V
Regulator
V+
7
XTR115: 2.5V
XTR116: 4.096V
IREF
(1)
VREF
Voltage
Reference
1
VLOOP
RIN
20kΩ
B
6
IIN
10nF
Q1
VIN
IIN
2
Input
Circuitry
RL
A1
E
5
RLIM
3
IRET
R1
2.475kΩ
R2
25Ω
All return current
from IREG and IREF
IO
4
I = 100 • IIN
For IO = 4mA to 20mA
IN = 40µA to 200µA
I
NOTE: (1) See also Figure 5.
With RIN = 20kΩ
IN = 0.8V to 4V
V
Possible choices for Q1 (see text).
TYPE
PACKAGE
2N4922
TIP29C
TIP31B
TO-225
TO-220
TO-220
FIGURE 1. Basic Circuit Connections.
XTR115, XTR116
5
SBOS124A
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