REVERSE-VOLTAGE 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.
The XTR105’s low compliance rating (7.5V) permits the use
of various voltage protection methods without compromising
operating range. Figure 4 shows a diode bridge circuit that
allows normal operation even when the voltage connection
lines are reversed. The bridge causes a two diode drop
(approximately 1.4V) loss in loop-supply voltage. This results
in a compliance voltage of approximately 9V—satisfactory
for most applications. If a 1.4V drop in loop supply is too
much, a diode can be inserted in series with the loop-supply
voltage and the V+ pin. This protects against reverse output
connection lines with only a 0.7V loss in loop-supply voltage.
RADIO FREQUENCY INTERFERENCE
The long wire lengths of current loops invite radio frequency
(RF) interference. RF can be rectified by the sensitive input
circuitry of the XTR105 causing errors. This generally ap-
pears as an unstable output current that varies with the
position of loop supply or input wiring.
SURGE PROTECTION
If the RTD sensor is remotely located, the interference may
enter at the input terminals. For integrated transmitter as-
semblies with short connections to the sensor, the interfer-
ence more likely comes from the current loop connections.
Remote connections to current transmitters can sometimes be
subjected to voltage surges. It is prudent to limit the maximum
surge voltage applied to the XTR105 to as low as practical.
Various zener diodes and surge clamping diodes are specially
designed for this purpose. Select a clamp diode with as low a
voltage rating as possible for best protection. For example, a
36V protection diode will assure proper transmitter operation
at normal loop voltages, yet will provide an appropriate level
of protection against voltage surges. Characterization tests on
three production lots showed no damage to the XTR105 within
loop-supply voltages up to 65V.
Bypass capacitors on the input reduce or eliminate this input
interference. Connect these bypass capacitors to the IRET
terminal (see Figure 5). Although the dc voltage at the IRET
terminal is not equal to 0V (at the loop supply, VPS), this
circuit point can be considered the transmitter’s “ground.”
The 0.01µF capacitor connected between V+ and IO may
help minimize output interference.
NOTE: (1) Zener Diode 36V: 1N4753A or General
Semiconductor TransorbTM 1N6286A. Use lower
voltage zener diodes with loop-power supply
voltages less than 30V for increased protection.
See the Surge Protection section.
10
V+
0.01µF
1N4148
Diodes
(1)
B
E
D1
XTR105
9
8
Maximum VPS must be
less than minimum
voltage rating of zener
diode.
RL
VPS
IO
The diode bridge causes
a 1.4V loss in loop-supply
voltage.
7
IRET
6
FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.
XTR105
12
SBOS061B
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