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AD8684ARZ-REEL データシート(PDF) 10 Page - Analog Devices |
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AD8684ARZ-REEL データシート(HTML) 10 Page - Analog Devices |
10 / 16 page AD8682/AD8684 Rev. A | 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, 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 VIN 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 |
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