AD8682/AD8684

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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

+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

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, negative signals in excess of approximately

14 V cause phase inversion. This is caused by saturation of the

input stage leading to the forward-biasing of a drain-gate diode.

A simple fix for this in noninverting applications is to place

a resistor in series with the noninverting input. This limits the

amount of current through the forward-biased diode and prevents

shutting down of the output stage. For the AD8682/AD8684,

a value of 200 kΩ has been found to work; however, it adds

a significant amount of noise.

15

10

5

0

–5

–10

–15

15

10

5

0

–5

–10

–15

Figure 33. AD8682 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, elliptical, 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

Elliptical

Best

Poor

Equal ripple

Equal ripple

Bessel (Thompson)

Poor

Best

Linear