AD8603/AD8607/AD8609
Optimum values for RS and CS are determined empirically;
Table 5 lists a few starting values.
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 loss in phase margin. Placing a small capacitor,
CF, in the feedback in parallel with R± (Figure 45) improves the
phase margin. Picking CF = 50 pF yields a phase margin of
about 45° for the values shown in Figure 45.
Table 5. Optimum Values for the Snubber Network
CL (pF)
RS (Ω)
CS (pF)
100~500
1500
500
100
680
330
1600~2000
400
100
PROXIMITY SENSORS
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
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.
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.
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.
R2
100kΩ
R1
R2
V
EE
AD8603
1kΩ
99kΩ
V
V
EE
CC
R1
R3
V
–
1kΩ
V+
R4
V
V+
1kΩ
CC
V+
V
IN
U5
V–
AD8603
V
–
100Ω
AD8541
C3
C2
AD8541
V+
V
CC
V
–
V
EE
V
V
CC
IN
V
EE
R3
R4
1kΩ
99kΩ
Figure 46. Low Power Composite Amplifier
Figure 45. High Gain Composite Amplifier
Rev. B | Page 13 of 20
ADI [ ADI ]
