Data Sheet

AD8208

Rev. C | Page 13 of 20

4 mA to 20 mA Current Loop Receiver

The AD8208 can also be used in low current-sensing applica-

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

in Figure 29. 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

AD8208

BATTERY

10Ω

1%

10Ω

1%

NC = NO CONNECT

OUTPUT

5V

+

–

Figure 29. 4 mA to 20 mA Current Loop Receiver

GAIN ADJUSTMENT

The default gain of the preamplifier and buffer are 10 V/V and

2 V/V, respectively, resulting in a composite gain of 20 V/V. With

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

be lowered, raised, or finely calibrated.

Gains Less than 20

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 30):

GND

NC

–IN

+IN

A1

A2

OUT

AD8208

NC = NO CONNECT

OUTPUT

GAIN =

5V

GAIN

20 – GAIN

+

–

+

–

Figure 30. Adjusting for Gains Less than 20

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 10 V/V), the optional offset nulling resistor is 50 kΩ.

Gains Greater than 20

Connecting a resistor from the output of the buffer amplifier to

its noninverting input, as shown in Figure 31, 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

AD8208

NC = NO CONNECT

OUTPUT

GAIN =

5V

GAIN

GAIN – 20

+

–

+

–

Figure 31. Adjusting for Gains Greater than 20

GAIN TRIM

Figure 32 shows a method for incremental gain trimming by

The following approximation is useful for small gain ranges:

For example, using this equation, the adjustment range is ±2%

GND

NC

–IN

+IN

A1

A2

OUT

AD8208

5V

NC = NO CONNECT

OUTPUT

GAIN TRIM

20kΩ MIN

+

–

+

–

Figure 32. Incremental Gain Trimming