5

®

XTR115, XTR116

APPLICATIONS INFORMATION

The XTR115 and XTR116 are identical devices except for

the reference voltage output, pin 1. This voltage is available

for external circuitry and is not used internally. Further

discussions that apply to both devices will refer to the

“XTR115/6.”

Figure 1 shows basic circuit connections with representative

simplified input circuitry. The XTR115/6 is a two-wire

current transmitter. Its input signal (pin 2) controls the output

current. A portion of this current flows into the V+ power

supply, pin 7. The remaining current flows in Q1. External

input circuitry connected to the XTR115/6 can be powered

ground” for input circuitry driving the XTR115/6.

The XTR115/6 is a current-input device with a gain of 100.

A voltage input is created with an external input resistor, as

shown. Common full-scale input voltages range from 1V

and upward. Full-scale inputs greater than 0.5V are recom-

mend to minimize the effect of offset voltage and drift of A1.

EXTERNAL TRANSISTOR

The external transistor, Q1, conducts the majority of the full-

scale output current. Power dissipation in this transistor can

approach 0.8W with high loop voltage (40V) and 20mA

output current. The XTR115/6 is designed to use an external

transistor to avoid on-chip thermal-induced errors. Heat

produced by Q1 will still cause ambient temperature changes

that can affect the XTR115/6. To minimize these effects,

locate Q1 away from sensitive analog circuitry, including

XTR115/6. Mount Q1 so that heat is conducted to the

outside of the transducer housing.

The XTR115/6 is designed to use virtually any NPN transis-

tor with sufficient voltage, current and power rating. Case

style and thermal mounting considerations often influence

the choice for any given application. Several possible choices

are listed in Figure 1. A MOSFET transistor will not improve

the accuracy of the XTR115/6 and is not recommended.

R

IN

20k

Ω

I

IN

2

V

IN

I

RET

3

XTR115

XTR116

+5V

Regulator

R

2

25

Ω

R

LIM

E

5

I

O

4

B

6

V+

7

R

1

2.475k

Ω

R

L

V

LOOP

A1

Voltage

Reference

V

REG

5V

V

REF

8

1

XTR115: 2.5V

XTR116: 4.096V

Possible choices for Q

2N4922

TIP29C

TIP31B

TYPE

TO-225

TO-220

TO-220

PACKAGE

I = 100 • I

IN

I

O

10nF

I

REF

I

IN

All return current

from I

For I

I

With R

V

I

REG

Q

1

Input

Circuitry

FIGURE 1. Basic Circuit Connections.