AD8203YRMZ-R7 datasheet(14/20 Pages) AD

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AD8203

Rev. B | Page 14 of 20

APPLICATIONS

10k

100k

GND

–IN

OUT

+VS

+IN

AD8203

OUT

+VS

REXT

VCM

VDIFF

GAIN =

14REXT

REXT + 100kΩ

REXT = 100kΩ

GAIN

14 – GAIN

VDIFF

NC = NO CONNECT

The AD8203 difference amplifier is intended for applications

that require extracting a small differential signal in the presence

of large common-mode voltages. The input resistance is nominally

320 kΩ, and the device can tolerate common-mode voltages

higher than the supply voltage and lower than ground.

The open collector output stage sources current to within

20 mV of ground and to within 200 mV of V .

CURRENT SENSING

High Line, High Current Sensing

Basic automotive applications making use of the large common-

mode range are shown in Figure 2 and Figure 3. The capability

of the device to operate as an amplifier in primary battery sup-

ply circuits is shown in

Figure 43. Adjusting for Gains < 14

Figure 2. Figure 3 illustrates the ability

of the device to withstand voltages below system ground.

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.

This can often be ignored, but if desired, it can be nulled by

inserting a resistor equal to 100 kΩ minus the parallel sum of

Low Current Sensing

The AD8203 is also used in low current sensing applications,

such as the 4 to 20 mA current loop shown in Figure 42. In such

applications, the relatively large shunt resistor can degrade the

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

low impedance side of the input corrects this error.

EXT

and 100 kΩ, in series with Pin 4. For example, with

REXT = 100 kΩ (yielding a composite gain of ×7), the optional

offset nulling resistor is 50 kΩ.

Gains Greater Than 14

OUTPUT

NC = NO CONNECT

GND

–IN

+IN

+VS

OUT

AD8203

Connecting a resistor from the output of the buffer amplifier to

its noninverting input, as shown in Figure 44, increases the

gain. The gain is now multiplied by the factor R

/(R

EXT

−

100 kΩ); for example, the gain is doubled for REXT = 200 kΩ.

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

accuracy of the gain becomes critically dependent on the

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

voltage at Pin 1 and Pin 8 (about six times the actual offset of

A1) limits the part’s use in high gain, dc-coupled applications.

10k

100k

GND

–IN

OUT

+VS

+IN

AD8203

OUT

+VS

REXT

VCM

VDIFF

GAIN =

14REXT

REXT – 100kΩ

REXT = 100kΩ

GAIN

GAIN – 14

VDIFF

NC = NO CONNECT

Figure 42. 4 to 20 mA Current Loop Receiver

GAIN ADJUSTMENT

The default gain of the preamplifier and buffer are ×7 and ×2,

respectively, resulting in a composite gain of ×14. With the

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

lowered, raised, or finely calibrated.

Gains Less Than 14

Figure 44. Adjusting for Gains > 14

Since 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 a factor REXT/(100 kΩ + REXT), as shown

in Figure 43.

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