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 ≤ VCM ≤

+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

0.1Ω

15V

RL

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.

D1

IN5711

V+

AD8682/

AD8684

V+

VOUT

VIN

V–

V–

D2

IN5711

R1

10kΩ

D3

IN5711

Figure 33. Phase Reversal Solution Circuit

TIME (200µs/DIV)

2

VS = ±15V

VIN

VOUT

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