AD8603/AD8607/AD8609

Rev. C | Page 13 of 16

PROXIMITY SENSORS

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.

COMPOSITE AMPLIFIERS

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

in parallel with R2 (see Figure 45) improves the phase margin.

values shown in Figure 45.

R1

1kΩ

99kΩ

R2

AD8603

AD8541

V+

V

–

V+

V

–

R3

R4

99kΩ

1kΩ

U5

Figure 45. High Gain Composite Amplifier

A composite amplifier can be used to optimize dc and ac

characteristics. Figure 46 shows an example using the AD8603

and the AD8541. This circuit offers many advantages. The band-

width is increased substantially, and the input offset voltage and

noise of the AD8541 become insignificant because they are divided

by the high gain of the AD8603.

The circuit in Figure 46 offers high bandwidth (nearly double

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

consumption of less than 100 μA.

R1

1kΩ

V+

V

–

100kΩ

R2

AD8541

100Ω

C3

1kΩ

R4

R3

C2

V–

V+

AD8603

Figure 46. Low Power Composite Amplifier

BATTERY-POWERED APPLICATIONS

The AD8603/AD8607/AD8609 are ideal for battery-powered

applications. The parts are tested at 5 V, 3.3 V, 2.7 V, and 1.8 V

and are suitable for various applications whether in single or

dual supply.

In addition to their low offset voltage and low input bias, the

AD8603/AD8607/AD8609 have a very low supply current of

40 μA, making the parts an excellent choice for portable electronics.

The TSOT package allows the AD8603 to be used on smaller

board spaces.

PHOTODIODES

Photodiodes have a wide range of applications from barcode

scanners to precision light meters and CAT scanners. The very

low noise and low input bias current of the AD8603/AD8607/

AD8609 make the parts very attractive amplifiers for I-V

conversion applications.

Figure 47 shows a simple photodiode circuit. The feedback

capacitor helps the circuit maintain stability. The signal band-

width can be increased at the expense of an increase in the total

noise; a low-pass filter can be implemented by a simple RC network

at the output to reduce the noise. The signal bandwidth can be

calculated by ½πR2C2, and the closed-loop bandwidth is the

intersection point of the open-loop gain and the noise gain.

The circuit shown in Figure 47 has a closed-loop bandwidth of

58 kHz and a signal bandwidth of 16 Hz. Increasing C2 to 50 pF

yields a closed-loop bandwidth of 65 kHz, but only 3.2 Hz of

signal bandwidth can be achieved.

C2

10pF

R2

1000MΩ

R1

1000MΩ

V–

V+

AD8603

C1

10pF

Figure 47. Photodiode Circuit