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AD647SH データシート(PDF) 5 Page - Analog Devices |
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AD647SH データシート(HTML) 5 Page - Analog Devices |
5 / 6 page AD647 REV. A –5– APPLICATION NOTES The AD647 is fully specified under actual operating conditions to insure high performance in any application, but there are some steps that will improve on even this high level of performance. The bias current of a JFET amplifier doubles with every 10 °C increase in junction temperature. Any heat source that can be eliminated or minimized will significantly improve bias current performance. To account for normal power dissipation, the larg- est contributor to chip self-heating, the bias currents of the AD647 are guaranteed fully warmed up with ±15 V supplies. A decrease in supply voltage will decrease power consumption, resulting in a corresponding drop in bias currents. Open loop gain and bias currents, to some extent, are affected by output loading. In applications where high linearity is essen- tial, load impedance should be kept as high as possible to mini- mize degradation of open loop gain. The outstanding ac and dc performance of the AD647 make it an ideal choice for critical instrumentation applications. In such applications, leakage paths, line losses and external noise sources should be considered in the layout of printed circuit boards. A guard ring surrounding the inputs and connected to a low impedance potential (at the same level as the inputs) should be placed on both sides of the circuit board. This will eliminate leakage paths that could degrade bias current performance. All signal paths should be shielded to minimize noise pickup. Figure 23. AD647 Used as DAC Output Amplifier A CMOS DAC AMPLIFIER The output impedance of a CMOS DAC, such as the AD7541, varies with digital input code. This causes a corresponding variation in the noise gain of the DAC-amplifier combination. This noise gain modulation introduces a nonlinearity whose magnitude is dependent on the amount of offset voltage present. Laser wafer drift trimming lowers the initial offset voltage and the offset voltage drift of the AD647, therefore minimizing the effect of this nonlinearity and its drift with temperature. This, in conjunction with the low bias current and high open loop gain, makes the AD647 ideal for DAC output amplifier applications. THE AD647 USED WITH THE AD7546 Figure 24 shows the AD647 used with the AD7546 16-bit segment DAC. In this application, amplifier performance is critical to the overall performance of the AD7546. A1 is used as a dual precision buffer. Here the offset voltage match, low offset voltage and high open loop gain of the AD647 ensure monoto- nicity and high linearity over the entire operating temperature range. A2 serves a dual function amplifier A is a Track and Hold circuit that deglitches the DAC output and amplifier B acts as an output amplifier. The performance of the amplifiers of A2 is crucial to the accuracy of the system. The errors of these amplifiers are added to the errors due strictly to DAC im- perfections. For this reason great care should be used in the se- lection of these amplifiers. The matching characteristics, low bias current and low temperature coefficients of the AD647 make it ideal for this application. Figure 24. AD647 Used with AD7546 16-Bit DAC USING THE AD647 IN LOG AMPLIFIER APPLICATIONS Log amplifiers or log ratio amplifiers are useful in a wide range of analog computational applications, ranging from the simple linearization of exponential transducer outputs to the use of logarithms in computations involving multi-term products or ar- bitrary exponents. Log amps also facilitate the compression of wide ranging analog input signals into a range that can be easily handled using standard circuit techniques. Figure 25. Log-Ratio Amplifier |
同様の部品番号 - AD647SH |
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同様の説明 - AD647SH |
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