AMP04

REV. A

–11–

not interact with one another. Calibration is simple and easy

with the NULL adjusted first, followed by SPAN adjust. The

entire circuit can be remotely placed, and powered from the

4-20 mA 2-wire loop.

4-20 mA Loop Receiver

At the receiving end of a 4-20 mA loop, the AMP04 makes a

convenient differential receiver to convert the current back to a

usable voltage (Figure 13). The 4-20 mA signal current passes

through a 100

Ω sense resistor. The voltage drop is differentially

amplified by the AMP04. The 4 mA offset is removed by the

offset correction circuit.

7

1

6

5

4

2

3

8

AMP-04

6

100k

0.15

µF

4–20mA

TRANS-

MITTER

POWER

SUPPLY

+–

2

3

+–

100

Ω 1%

1k

1k

WIRE RE-

SISTANCE

IN4002

–0.400V

OP-177

AD589

10k

27k

–15V

+15V

V

OUT

0–1.6V FS

–15V

4–20mA

4–20mA

+

–

Figure 13. 4-to-20 mA Line Receiver

Low Power, Pulsed Load-Cell Amplifier

Figure 14 shows a 350

Ω load cell that is pulsed with a low duty

cycle to conserve power. The OP295’s rail-to-rail output capa-

bility allows a maximum voltage of 10 volts to be applied to the

bridge. The bridge voltage is selectively pulsed on when a mea-

surement is made. A negative-going pulse lasting 200 ms should

be applied to the MEASURE input. The long pulse width is

necessary to allow ample settling time for the long time constant

of the low-pass filter around the AMP04. A much faster settling

time can be achieved by omitting the filter capacitor.

7

1

6

5

4

2

3

8

AMP-04

0.22

µF

+12V

1/2

OP-295

REF-01

GND

OUT

IN

+12V

330

Ω

1k

10k

10V

50k

2N3904

V

OUT

1N4148

350

Ω

MEASURE

Figure 14. Pulsed Load Cell Bridge Amplifier

Single Supply Programmable Gain Instrumentation Amplifier

Combining with the single supply ADG221 quad analog switch,

the AMP04 makes a useful programmable gain amplifier that

can handle input and output signals at zero volts. Figure 15

shows the implementation. A logic low input to any of the gain

control ports will cause the gain to change by shorting a gain-

set resistor across AMP04’s Pins 1 and 8. Trimming is required

at higher gains to improve accuracy because the switch ON-

resistance becomes a more significant part of the gain-set

resistance. The gain of 500 setting has two switches connected

in parallel to reduce the switch resistance.

1

2

3

4

8

7

6

5

AMP-04

V–

REF

V+

–

+

INPUT

V

OUT

+5V

TO +30V

R

G

R

G

0.1

µF

0.22

µF

100k

10.9k

3

14

715

Ω

200

Ω

200

Ω

6

11

4

5

ADG221

+5V TO +30V

13

10

9

7

8

15

16

2

1

12

+

10

µF

0.1

µF

GAIN OF 500

WR

GAIN OF 100

GAIN OF 10

Figure 15. Single Supply Programmable Gain Instrumen-

tation Amplifier

The switch ON resistance is lower if the supply voltage is

12 volts or higher. Additionally the overall amplifier’s tempera-

ture coefficient also improves with higher supply voltage.