6

®

XTR104

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 (

≥2750Ω) strain gage sensor can be

The output terminals of the bridge are connected to the

The transfer function is:

a 4 to 20mA output current. With R

input produces a 4 to 20mA output current. Other values for

R

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.

increasing output current according to the transfer function,

up to the output current limit of approximately 34mA.

EXTERNAL TRANSISTOR

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

LOOP POWER SUPPLY

The voltage applied to the XTR104, V+, is measured with

voltage applied to the XTR104 according to the voltage drop

drop in the line).

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.

(2)

(1)

R

G

=

2500

1

V

FS

– 1

FIGURE 1. Bridge Sensor Application, Connected for Positive Nonlinearity.

XTR104

–

+

(1)

R

L

V

PS

–

+

11

0.01µF

–

+

13

7

(3)

4

6

5

1

(3)

12

10

2

(2)

3

(3)

8

9

V

+

LIN

R

G

V

–

LIN

V

+

IN

R

G

V

–

IN

I

O

E

B

V+

R

LIN

R

Ω

2500

– 1

(1)

(2) R

(3) See text — “Linearization”.

1

V

FS

where V

Bridge Sensor

V

R

I

4-20mA

Q

1

Possible choices for Q

Type

Package

2N4922

TIP29B

TIP31B

TO-225

TO-220

TO-220