Datasheet
14
®
XTR112, XTR114
FIGURE 5. Input Bypassing Technique with Linearization.
14
11
12
13
4
3
2
R
G
XTR112
XTR114
R
CM
7
1
0.01µF
0.01µF 0.01µF
R
Z
(1)
RTD
6
NOTE: (1) Bypass capacitors can be connected
to either the I
RET
pin or the I
O
pin.
R
G
R
G
V
IN
–
V
IN
+
V
LIN
I
R1
I
R2
V
REG
V+
I
RET
I
O
E
B
8
0.01µF
9
10
1kΩ
R
LIN1
R
LIN2
1kΩ
XTR112
XTR114
7
V+
I
O
E
B
V
PS
10
0.01µF
R
L
D
1
(1)
9
8
NOTE: (1) Zener Diode 36V: 1N4753A or General
Semiconductor Transorb
TM
1N6286A. Use lower
voltage zener diodes with loop power supply
voltages less than 30V for increased protection.
See “Over-Voltage Surge Protection.”
Maximum V
PS
must be
less than minimum
voltage rating of zener
diode.
The diode bridge causes
a 1.4V loss in loop supply
voltage.
1N4148
Diodes
6
I
RET
FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.
Most surge protection zener diodes have a diode character-
istic in the forward direction that will conduct excessive
current, possibly damaging receiving-side circuitry if the
loop connections are reversed. If a surge protection diode is
used, a series diode or diode bridge should be used for
protection against reversed connections.
RADIO FREQUENCY INTERFERENCE
The long wire lengths of current loops invite radio frequency
interference. RF can be rectified by the sensitive input
circuitry of the XTR112 and XTR114 causing errors. This
generally appears as an unstable output current that varies
with the position of loop supply or input wiring.
If the RTD sensor is remotely located, the interference may
enter at the input terminals. For integrated transmitter as-
semblies with short connection to the sensor, the interfer-
ence more likely comes from the current loop connections.
Bypass capacitors on the input reduce or eliminate this input
interference. Connect these bypass capacitors to the I
RET
terminal as shown in Figure 5. Although the dc voltage at the
I
RET
terminal is not equal to 0V (at the loop supply, V
PS
) this
circuit point can be considered the transmitter’s “ground.”
The 0.01µF capacitor connected between V+ and I
O
may
help minimize output interference.