XTR104 Burr-Brown Corporation, XTR104 Datasheet - Page 6

• Manufacturer Part Number: XTR104
• Description: 4-20mA Current Transmitter with BRIDGE EXCITATION AND LINEARIZATION
• Manufacturer: Burr-Brown Corporation

APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for the XTR104.
The loop power supply, V
cuitry. Loop current is measured as a voltage across the
series load resistor, R
A high impedance ( 2750 ) strain gage sensor can be
excited directly by the 5V reference output terminal, V
The output terminals of the bridge are connected to the
instrumentation amplifier inputs, V
tor, R
required by the full-scale bridge voltage, V
The transfer function is:
With no R
a 4 to 20mA output current. With R
input produces a 4 to 20mA output current. Other values for
R
Where: V
Under-scale input voltage (negative) will cause the output
current to decrease below 4mA. Increasingly negative input
will cause the output current to limit at approximately
3.6mA.
Increasingly positive input voltage (above V
increasing output current according to the transfer function,
up to the output current limit of approximately 34mA.
FIGURE 1. Bridge Sensor Application, Connected for Positive Nonlinearity.
G
(2) R
(3) See text — “Linearization”.
(1)
I
Where: V
O
can be calculated as follows:
R
where V
= V
G
G
B
, sets the gain of the instrumentation amplifier as
=
V
R
2750
FS
IN
G
–
G
IN
V
V
FS
1
in
FS
2500
• (0.016 + 40/R
connected (R
is the full scale voltage applied to the V
–
IN
®
differential inputs (in Volts).
Bridge Sensor
IN
is Full Scale V
– 1
Otherwise add series resistance (see Figure 8).
is the voltage applied to the V
XTR104
R
differential inputs (in Volts.) R
.
G
–
L
.
IN
G
.
V
1
= ), a 0V to 1V input produces
R
PS
FS
2500
G
B
) + 4mA,
(2)
, provides power for all cir-
– 1
+
+
G
IN
= 25 , a 0V to 10mV
and V
R
1
FS
FS
–
) will produce
.
IN
+
IN
G
. The resis-
R
2
in
and
(3)
+
(3)
IN
.
R
and
G
(1)
(2)
(1)
R
1
3
5
6
4
2
.
V
V
R
R
V
V
+
+
G
G
–
–
12
6
IN
LIN
LIN
IN
V
R
XTR104
EXTERNAL TRANSISTOR
Transistor Q
4 to 20mA loop current. Using an external transistor isolates
the power dissipation from the precision input and reference
circuitry of the XTR104, maintaining excellent accuracy.
Since the external transistor is inside a feedback loop its
characteristics are not critical. Many common NPN types
can be used. Requirements for operation at the full loop
supply voltage are: V
800mW. Power dissipation requirements may be lower if the
maximum loop power supply voltage is less than 40V. Some
possible choices for Q
LOOP POWER SUPPLY
The voltage applied to the XTR104, V+, is measured with
respect to the I
40V. The loop supply voltage, V
voltage applied to the XTR104 according to the voltage drop
on the current sensing resistor, R
drop in the line).
If a low loop supply voltage is used, R
relatively low value to assure that V+ remains 9V or greater
for the maximum loop current of 20mA. It may, in fact, be
prudent to design for V+ equal or greater than 9V with loop
currents up to 34mA to allow for out-of-range input condi-
tions. The typical performance curve “Loop Resistance vs
Loop Power Supply” shows the allowable sense resistor
values for full-scale 20mA.
The low operating voltage (9V) of the XTR104 allows
operation directly from personal computer power supplies
(12V 5%). When used with the RCV420 Current Loop
Receiver (see Figure 9), load resistor voltage drop is only
1.5V at 20mA.
8
R
LIN
7
9
I
O
R
V+
LIN
10
B
E
(3)
I
O
13
11
1
= 4-20mA
conducts the majority of the signal-dependent
O
connection, pin 7. V+ can range from 9V to
Q
1
CEO
1
are listed in Figure 1.
Possible choices for Q
= 45V min,
0.01µF
Type
2N4922
TIP29B
TIP31B
L
PS
(plus any other voltage
, will differ from the
TO-225
TO-220
TO-220
Package
= 40 min and P
L
R
+
–
1
must be made a
L
(see text).
4-20mA
V
+
–
PS
D
=