AD8682/AD8684
Rev. B | Page 10 of 16
APPLICATIONS INFORMATION
The AD8682 and AD8684 are dual and quad JFET op amps that
are optimized for high speed at low power. This combination
makes these amplifiers excellent choices for battery-powered or
low power applications that require above average performance.
Applications benefiting from this performance combination
include telecommunications, geophysical exploration, portable
medical equipment, and navigational instrumentation.
HIGH-SIDE SIGNAL CONDITIONING
There are many applications requiring the sensing of signals near
the positive rail. The AD8682 and the AD8684 were tested and
are guaranteed over a common-mode range (−11 V ≤ V
CM
≤
+15 V) that includes the positive supply.
The AD8682/AD8684 are commonly used in the sensing of
power supply currents and in current sensing applications, such
as the partial circuit shown in Figure 32. In this circuit, the voltage
drop across a low value resistor, such as the 0.1 Ω shown here, is
amplified and compared to 7.5 V. The output can then be used
for current limiting.
1/2
AD8682
100kΩ
500kΩ
100kΩ
500kΩ
0.1
15V
R
L
06278-032
Figure 32. High-Side Signal Conditioning
PHASE INVERSION
Most JFET input amplifiers invert the phase of the input signal
if either input exceeds the input common-mode range. For the
AD8682/AD8684, a negative signal in excess of 11 V causes
phase inversion. This is caused by saturation of the input stage
leading to the forward-biasing of a gate-drain diode. Phase
reversal in AD8682/AD8684 can be prevented by using Schottky
diodes to clamp the input terminals to each other and to the
supplies. In the simple buffer circuit below, D1 protects the op
amp against phase reversal. R1, D2, and D3 limit the input
current when the input exceeds the supply rail. The resistor
should be selected to limit the amount of input current below
the absolute maximum rating.
06278-033
D1
IN5711
V+
AD8682/
AD8684
+
V
OUT
IN
V–
V–
D2
IN5711
R1
10kΩ
D3
IN5711
Figure 33. Phase Reversal Solution Circuit
06278-034
TIME (200µs/DIV)
VOLTAGE (5V/DIV)
2
V
S
= ±15V
V
IN
V
OUT
Figure 34. No Phase Reversal
ACTIVE FILTERS
The wide bandwidth and high slew rates of the AD8682/AD8684
make either one an excellent choice for many filter applications.
There are many active filter configurations, but the four most
popular configurations are: Butterworth, elliptic, Bessel, and
Chebyshev. Each type has a response that is optimized for a
given characteristic, as shown in Table 4.
Table 4.
Type Selectivity Overshoot Phase Amplitude (Pass Band) Amplitude (Stop Band)
Butterworth Moderate Good Maximum flat
Chebyshev Good Moderate Nonlinear Equal ripple
Elliptic Best Poor Equal ripple Equal ripple
Bessel (Thompson) Poor Best Linear
