Data Sheet

AD8603/AD8607/AD8609

APPLICATIONS

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Rev. D | 14 of 17

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 pharma-

ceutical 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 ach-

ieved 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 (see Figure 45) improves the phase margin. Picking

CF = 50 pF yields a phase margin of about 45° for the values shown

in Figure 45.

Figure 45. High Gain Composite Amplifier

A composite amplifier can be used to optimize dc and ac charac-

teristics. 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.

Figure 46. Low Power Composite Amplifier

BATTERY-POWERED APPLICATIONS

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

cations. 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 scan-

ners 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 applica-

tions.

Figure 47 shows a simple photodiode circuit. The feedback capaci-

tor 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.

Figure 47. Photodiode Circuit