Data Sheet

AD8209

Rev. C | Page 13 of 16

4 mA to 20 mA Current Loop Receiver

The AD8209 can also be used in low current-sensing applica-

tions, such as the 4 mA to 20 mA current loop receiver shown

in Figure 28. In such applications, the relatively large shunt

resistor may degrade the common-mode rejection. Adding a

resistor of equal value on the low impedance side of the input

corrects this error.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

BATTERY

10Ω

1%

10Ω

1%

NC = NO CONNECT

OUTPUT

+

–

Figure 28. 4 mA to 20 mA Current Loop Receiver

GAIN ADJUSTMENT

The default gain of the preamplifier and buffer are 7 V/V and

2 V/V, respectively, resulting in a composite gain of 14 V/V. With

the addition of external resistor(s) or trimmer(s), the gain can

be lowered, raised, or finely calibrated.

Gains Less than 14

Because the preamplifier has an output resistance of 100 kΩ, an

external resistor connected from Pin 3 and Pin 4 to GND decreases

the gain by the following factor (see Figure 29):

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

NC = NO CONNECT

OUTPUT

GAIN =

GAIN

14 – GAIN

+

–

+

–

Figure 29. Adjusting for Gains Less than 14

The overall bandwidth is unaffected by changes in gain by using

this method, although there may be a small offset voltage due to

the imbalance in source resistances at the input to the buffer. In

many cases, this can be ignored, but if desired, the offset voltage can

be nulled by inserting a resistor in series with Pin 4. The resistor

gain of 7 V/V), the optional offset nulling resistor is 50 kΩ.

Gains Greater than 14

Connecting a resistor from the output of the buffer amplifier to

its noninverting input, as shown in Figure 30, increases the gain.

The gain is now multiplied by the factor

high as 50 are achievable in this way. Note that the accuracy of

the gain becomes critically dependent on the resistor value at

high gains. In addition, the effective input offset voltage at Pin 1

and Pin 8 (which is about six times the actual offset of A1) limits

the use of the part in high gain, dc-coupled applications.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

NC = NO CONNECT

POINT X

(SEE TEXT)

OUTPUT

GAIN =

GAIN

GAIN – 14

+

–

+

–

Figure 30. Adjusting for Gains Greater than 14

A small offset voltage arises from an imbalance in source

resistances and the finite bias currents inherently present at the

input of A2. In most applications, this additional offset error is

comparable to the specified offset range and therefore introduces

negligible skew. However, it can be essentially eliminated by the

addition of a resistor in series with the parallel combination of

77 kΩ, the series resistor placed at point X is 23 kΩ.