APPLICATIONS INFORMATION
Figure 1 shows the basic connection diagram for the XTR106.
The loop power supply, V
PS
, provides power for all cir-
cuitry. Output loop current is measured as a voltage across
the series load resistor, R
L
. A 0.01µF to 0.03µF supply
bypass capacitor connected between V+ and I
O
is recom-
mended. For applications where fault and/or overload con-
ditions might saturate the inputs, a 0.03µF capacitor is
recommended.
A 2.5V or 5V reference is available to excite a bridge sensor.
For 5V excitation, pin 14 (V
REF
5) should be connected to the
bridge as shown in Figure 1. For 2.5V excitation, connect
pin 13 (V
REF
2.5) to pin 14 as shown in Figure 3b. The output
terminals of the bridge are connected to the instrumentation
+
–
amplifier inputs, V
IN
and V
IN
. A 0.01µF capacitor is shown
connected between the inputs and is recommended for high
impedance bridges (> 10kΩ). The resistor R
G
sets the gain
of the instrumentation amplifier as required by the full-scale
bridge voltage, V
FS
.
Lin Polarity and R
LIN
provide second-order linearization
correction to the bridge, achieving up to a 20:1 improvement
in linearity. Connections to Lin Polarity (pin 12) determine
the polarity of nonlinearity correction and should be con-
nected either to I
RET
or V
REG
. Lin Polarity should be con-
nected to V
REG
even if linearity correction is not desired.
R
LIN
is chosen according to the equation in Figure 1 and is
dependent on K
LIN
(linearization constant) and the bridge’s
nonlinearity relative to V
FS
(see “Linearization” section).
The transfer function for the complete current transmitter is:
I
O
= 4mA + V
IN
• (40/R
G
)
V
IN
in Volts, R
G
in Ohms
where V
IN
is the differential input voltage. As evident from
the transfer function, if no R
G
is used (R
G
=
∞),
the gain is
zero and the output is simply the XTR106’s zero current.
A negative input voltage, V
IN
, will cause the output current
to be less than 4mA. Increasingly negative V
IN
will cause the
output current to limit at approximately 1.6mA. If current is
being sourced from the reference and/or V
REG
, the current
limit value may increase. Refer to the Typical Performance
Curves, “Under-Scale Current vs I
REF
+ I
REG
” and “Under-
Scale Current vs Temperature.”
Increasingly positive input voltage (greater than the full-
scale input, V
FS
) will produce increasing output current
according to the transfer function, up to the output current
limit of approximately 28mA. Refer to the Typical Perfor-
mance Curve, “Over-Scale Current vs Temperature.”
The I
RET
pin is the return path for all current from the
references and V
REG
. I
RET
also serves as a local ground and
is the reference point for V
REG
and the on-board voltage
references. The I
RET
pin allows any current used in external
circuitry to be sensed by the XTR106 and to be included in
the output current without causing error. The input voltage
range of the XTR106 is referred to this pin.
(1)
For 2.5V excitation, connect
pin 13 to pin 14
V
REF
5
V
REF
2.5
14
13
5
+
V
IN
V
REG
Possible choices for Q
1
(see text).
TYPE
R
LIN(3)
2N4922
TIP29C
TIP31C
7.5V to 36V
PACKAGE
TO-225
TO-220
TO-220
11
R
LIN
1
10
V+
I
O
4-20 mA
V
REG
5V
(5)
R
1
(5)
C
IN
0.01µF
(2)
R
B
–
R
G
4
(4)
R
G
XTR106
R
G
V
IN
I
RET
6
V
REG
or
(1)
–
R
2
+
Bridge
Sensor
B
9
Q
1
C
OUT
0.01µF
V
O
+
V
PS
–
3
E
Lin
(1)
Polarity
12
I
O
7
8
R
L
2
I
O
= 4mA + V
IN
• ( 40 )
R
G
NOTES:
(1) Connect Lin Polarity (pin 12) to I
RET
(pin 6) to correct for positive
bridge nonlinearity or connect to V
REG
(pin 1) for negative bridge
nonlinearity. The R
LIN
pin and Lin Polarity pin must be connected to
V
REG
if linearity correction is not desired. Refer to “Linearization”
section and Figure 3.
(2) Recommended for bridge impedances > 10kΩ
( 3) R
LIN
= K
LIN
•
4B
1 – 2B
(K
LIN
in
Ω)
(4) R
G
= (V
FS
/400µA) •
1 + 2B
1 – 2B
(V
FS
in V)
where K
LIN
= 9.905kΩ for 2.5V reference
K
LIN
= 6.645kΩ for 5V reference
B is the bridge nonlinearity relative to V
FS
V
FS
is the full-scale input voltage
(5) R
1
and R
2
form bridge trim circuit to compensate for the initial
accuracy of the bridge. See “Bridge Balance” text.
FIGURE 1. Basic Bridge Measurement Circuit with Linearization.
®
9
XTR106
BURR-BROWN [ BURR-BROWN CORPORATION ]
