7

®

XTR115, XTR116

V

PS

0.01

µF

R

L

D

1

(1)

NOTE: (1) Zener Diode 36V: 1N4753A or Motorola

P6KE39A. Use lower voltage zener diodes with loop

power supply voltages less than 30V for increased

protection. See “Over-Voltage Surge Protection.”

Maximum V

less

than

minimum

voltage rating of zener

diode.

The diode bridge causes

a 1.4V loss in loop supply

voltage.

1N4148

Diodes

8

1

2

3

XTR115

XTR116

V

IN

V

REF

I

IN

I

RET

V

REG

V+

I

O

E

B

6

7

5

4

Q

1

R

IN

FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.

REVERSE-VOLTAGE PROTECTION

The XTR115/6 low compliance voltage rating (7.5V) per-

mits the use of various voltage protection methods without

compromising operating range. Figure 4 shows a diode

bridge circuit which 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 approxi-

mately 9V—satisfactory for most applications. A diode can

be inserted in series with the loop supply voltage and the V+

pin to protect against reverse output connection lines with

only a 0.7V loss in loop supply voltage.

OVER-VOLTAGE SURGE PROTECTION

Remote connections to current transmitters can sometimes be

subjected to voltage surges. It is prudent to limit the maximum

surge voltage applied to the XTR115/6 to as low as practical.

Various zener diode 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

several production lots showed no damage with loop supply

voltages up to 65V.

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 input circuitry of the

XTR115/6 or preceding circuitry. This generally appears as

an unstable output current that varies with the position of

loop supply or input wiring.

Interference may also enter at the input terminals. For

integrated transmitter assemblies with short connection to

the sensor, the interference more likely comes from the

current loop connections.