Datasheet AD8682, AD8684 (Analog Devices) - 10

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AD8682/AD8684 APPLICATIONS INFORMATION
The AD8682 and AD8684 are dual and quad JFET op amps that current when the input exceeds the supply rail. The resistor are optimized for high speed at low power. This combination should be selected to limit the amount of input current below makes these amplifiers excellent choices for battery-powered or the absolute maximum rating. low power applications that require above average performance.
V+
Applications benefiting from this performance combination include telecommunications, geophysical exploration, portable
D2 R1 IN5711 AD8682/ 10kΩ
medical equipment, and navigational instrumentation.
AD8684 VIN D1 V+ IN5711 VOUT V– HIGH-SIDE SIGNAL CONDITIONING D3 IN5711
There are many applications requiring the sensing of signals near 3 the positive rail. The AD8682 and the AD8684 were tested and 03 8- 27 are guaranteed over a common-mode range (−11 V ≤ V
V–
06 CM ≤ +15 V) that includes the positive supply. Figure 33. Phase Reversal Solution Circuit The AD8682/AD8684 are commonly used in the sensing of power supply currents and in current sensing applications, such
VS = ±15V
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.
IV D 0.1Ω (5V/ 15V E 2 G 500kΩ R 100kΩ L LTA VO 100kΩ VOUT 1/2 AD8682
34 0 8-
V 500kΩ IN
32 27 0 8- 06 627
TIME (200µs/DIV)
0 Figure 32. High-Side Signal Conditioning Figure 34. No Phase Reversal
PHASE INVERSION ACTIVE FILTERS
Most JFET input amplifiers invert the phase of the input signal The wide bandwidth and high slew rates of the AD8682/AD8684 if either input exceeds the input common-mode range. For the make either one an excellent choice for many filter applications. AD8682/AD8684, a negative signal in excess of 11 V causes There are many active filter configurations, but the four most phase inversion. This is caused by saturation of the input stage popular configurations are: Butterworth, elliptic, Bessel, and leading to the forward-biasing of a gate-drain diode. Phase Chebyshev. Each type has a response that is optimized for a reversal in AD8682/AD8684 can be prevented by using Schottky given characteristic, as shown in Table 4. 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
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 Rev. B | Page 10 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION HIGH-SIDE SIGNAL CONDITIONING PHASE INVERSION ACTIVE FILTERS PROGRAMMABLE STATE VARIABLE FILTER OUTLINE DIMENSIONS ORDERING GUIDE