Datasheet

XTR117
SBOS344C − SEPTEMBER 2005 − REVISED MAY 2012
www.ti.com
5
APPLICATIONS INFORMATION
BASIC OPERATION
The XTR117 is a precision current output converter
designed to transmit analog 4-20mA signals over an
industry-standard current loop. Figure 1 shows basic
circuit connections with representative simplified input
circuitry. The XTR117 is a two-wire current transmitter.
Its input current (pin 2) controls the output current. A
portion of the output current flows into the V+ power
supply, pin 7. The remaining current flows in Q
1
.
External input circuitry connected to the XTR117 can be
powered from V
REG
. Current drawn from these
terminals must be returned to I
RET
, pin 3. The I
RET
pin is
a local ground for input circuitry driving the XTR117.
The XTR117 is a current-input device with a gain of 100.
A current flowing into pin 2 produces I
O
= 100 x I
IN
. The
input voltage at the I
IN
pin is zero (referred to the I
RET
pin). A voltage input is converted to an input current with
an external input resistor, R
IN
, as shown in Figure 1.
Typical full-scale input voltages range from 1V and
upward. Full-scale inputs greater than 0.5V are
recommend to minimize the effects of offset voltage and
drift of A1.
EXTERNAL TRANSISTOR
The external transistor, Q
1
, 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 XTR117 is
designed to use an external transistor to avoid on-chip,
thermal-induced errors. Heat produced by Q
1
will still
cause ambient temperature changes that can influence
the XTR117 performance. To minimize these effects,
locate Q
1
away from sensitive analog circuitry, including
XTR117. Mount Q
1
so that heat is conducted to the
outside of the transducer housing.
The XTR117 is designed to use virtually any NPN
transistor 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
XTR117 and is not recommended.
A1
(V
REF
)
XTR117
+5V
Regulator
Input
Circuitry
Q1
I
O
R
LIM
R
2
25
Ω
R
1
2.475k
Ω
R
IN
20k
Ω
I
RET
I
IN
I
IN
V
IN
V
REG
I
REG
from I
REG
and I
REF
All return current
I
O
V
LOOP
R
L
E
B
V+
C
OUT
10nF
I=100(I
IN
)
5V
6
5
4
8
2
3
7
MJE3440
TIP41C
MJD3340
SOT−32
TO−220
D−PAK
TYPE PACKAGE
Possible choices for Q1 (see text):
REF3140
REF3130
REF3125
4.096V
3.0V
2.5V
DEVICE VOLTAGE
Use REF32xx for lower drift.
For improved precision use an external
voltage reference.
Figure 1. Basic Circuit Connections