REV. B

–6–

AD8200

CURRENT SENSING

High Line, High Current Sensing

Basic automotive applications making use of the large common-

mode range are shown in Figures 1 and 2. The capability of the

device to operate as an amplifier in primary battery supply circuits

is shown in Figure 1; Figure 2 illustrates the ability of the device

to withstand voltages below system ground.

Low Current Sensing

The AD8200 can also be used in low current sensing applica-

tions, such as the 4–20 mA current loop shown in Figure 4. In

such applications, the relatively large shunt resistor can degrade

the common-mode rejection. Adding a resistor of equal value in

the low impedance side of the input corrects for this error.

5V

OUTPUT

10

10

1%

NC = NO CONNECT

1%

+

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 4. 4–20 mA Current Loop Receiver

GAIN ADJUSTMENT

The default gain of the preamplifier and buffer are

10 and

2,

respectively, resulting in a composite gain of

20. With the

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

lowered, raised, or finely calibrated.

Gains Less than 20

Since the preamplifier has an output resistance of 100 k

Ω, an exter-

nal resistor connected from Pins 3 and 4 to GND will decrease the

Ω + R

10k

10k

100k

A2

A1

GND

–IN

OUT

NC

+IN

AD8200

OUT

2

GAIN =

GAIN

20 – GAIN

2

NC = NO CONNECT

Figure 5. Adjusting for Gains Less than 20

The overall bandwidth is unaffected by changes in gain 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, can be nulled by

inserting a resistor equal to 100 k

Ω minus the parallel sum of

Ω, in series with Pin 4. For example, with R

EXT

= 100 k

Ω (yielding a composite gain of 10), the optional offset

nulling resistor is 50 k

Ω (see Figure 11.)

Gains Greater than 20

Connecting a resistor from the output of the buffer amplifier to

its noninverting input, as shown in Figure 6, will increase the

100 k

Ω); for example, it is doubled for R

Ω. Overall

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

accuracy of the gain becomes critically dependent on resistor

value at high gains. Also, the effective input offset voltage at

Pins 1 and 8 (about six times the actual offset of A1) limits the

part’s use in very high gain, dc-coupled applications.

10k

10k

100k

A2

A1

–IN

NC

+IN

AD8200

GND

OUT

OUT

GAIN =

GAIN

GAIN – 20

2

2

NC = NO CONNECT

Figure 6. Adjusting for Gains Greater than 20

GAIN TRIM

Figure 7 shows a method for incremental gain trimming using a

The following approximation is useful for small gain ranges

∆Ω

GM

R

EXT

≈÷

()

10

%

Thus, the adjustment range would be

±2% for R

Ω;

5V

OUT

GAIN TRIM

20k

MIN

2

2

NC = NO CONNECT

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 7. Incremental Gain Trim