AD8607ARMZ-R2 datasheet(13/20 Pages) AD

13 / 20 page

AD8603/AD8607/AD8609

Rev. B | Page 13 of 20

COMPOSITE AMPLIFIERS

Optimum values for RS and CS are determined empirically;

Table 5 lists a few starting values.

A composite amplifier can provide a very high gain in

applications where high closed-loop dc gains are needed. The

high gain achieved by the composite amplifier comes at the

expense of a loss in phase margin. Placing a small capacitor,

CF, in the feedback in parallel with R2 (

Table 5. Optimum Values for the Snubber Network

CL (pF)

RS (Ω)

CS (pF)

100~500

500

680

Figure 45) improves the

phase margin. Picking CF = 50 pF yields a phase margin of

about 45° for the values shown in

1500

100

330

1600~2000

400

100

Figure 45.

PROXIMITY SENSORS

A composite amplifier can be used to optimize dc and ac

characteristics.

Proximity sensors can be capacitive or inductive and are used in

a variety of applications. One of the most common applications

is liquid level sensing in tanks. This is particularly popular in

pharmaceutical environments where a tank must know when to

stop filling or mixing a given liquid. In aerospace applications,

these sensors detect the level of oxygen used to propel engines.

Whether in a combustible environment or not, capacitive

sensors generally use low voltage. The precision and low voltage

of the AD8603/AD8607/AD8609 make the parts an excellent

choice for such applications.

Figure 46 shows an example using the AD8603

and the AD8541. This circuit offers many advantages. The

bandwidth is increased substantially, and the input offset

voltage and noise of the AD8541 become insignificant since

they are divided by the high gain of the AD8603.

The circuit of Figure 46 offers a high bandwidth (nearly double

that of the AD8603), a high output current, and a very low

power consumption of less than 100 μA.

V+

–

V+

–

VIN

100k

AD8603

AD8541

100

VCC

VEE

VCC

VEE

VCC

VEE

VIN

99k

AD8603

AD8541

V+

–

V+

–

99k

Figure 46. Low Power Composite Amplifier

Figure 45. High Gain Composite Amplifier

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