Op Amp Applications Ch 5, Part B by W. Jung(ed)

By W. Jung(ed)

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We now replace the AD847 with an OP-90. The OP-90 is a DC precision amplifier and so has a limited bandwidth. In fact, its open loop gain is less than 10dB at 10kHz (see Figure 5-91(B)). This is not to imply that the AD847 is in all cases better than the OP-90. It is a case of misapplying the OP-90. 8Ω 1000pF OUT + –VS AD847 (OP-90) Q = 10 H = 1 F0 = 10kHz Figure 5-89: 1kHz multiple feedback bandpass filter From the output for the OP-90, also shown in Figure 5-90, we see that the magnitude of the output has been reduced, and the center frequency has shifted downward.

This increases the cost and size of the filter. Not only is the cost of components increased, so are the manufacturing costs, both for loading and tuning the filter. Furthermore, success will be still limited by the number of parts that are used, their tolerance, and their tracking, both over temperature and time. A more practical way is to use a circuit analysis program to determine the response using standard values. The program can also evaluate the effects of component drift over temperature.

Very large values tend to be more prone to parasitics since smaller capacitances will couple more easily into larger impedance levels. Noise also increases with the square root of the resistor value. Larger value resistors also will cause larger offsets due to the effects of the amplifier bias currents. 102 ANALOG FILTERS PRACTICAL PROBLEMS IN FILTER IMPLEMENTATION Parasitic capacitances due to circuit layout and other sources affect the performance of the circuit. They can form between two traces on a PC board (on the same side or opposite side of the board), between leads of adjacent components, and just about everything else you can (and in most cases can't) think of.

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