AMP04

REV.

–11–

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.

AMP04

1k

3

2

4

6

7

+15V

–15V

100

1%

1N4002

WIRE

RESISTANCE

1k

4–20mA

4–20mA

TRANSMITTER

100k

POWER

SUPPLY

0.15 F

4–20mA

0–1.6V FS

5

8

1

OP177

2

3

AD589

–15V

6

27k

10k

–0.400V

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 pulsewidth 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.

AMP04

3

2

7

6

IN

GND

OUT

REF01

1/2

OP295

MEASURE

1N4148

10k

1k

50k

12V

5

4

330

0.22 F

8

1

2N3904

12V

350

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.

8

7

6

5

1

2

3

4

V–

REF

V+

AMP04

ADG221

13

10

9

7

8

15

16

2

1

12

5V TO

30V

0.22 F

5

4

11

6

14

3

715

10.9k

200

200

0.1 F

10 F

5V TO 30V

GAIN OF 500

GAIN OF 100

GAIN OF 10

WR

GAIN

CONTROL

0.1 F

INPUT

100k

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 temperature coeffi-

cient also improves with higher supply voltage.

C