C careful selection and placement of external compo

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c) Careful selection and placement of external compo- nents will preserve the high frequency performance of the OPA687. Resistors should be a very low reactance type. Surface-mount resistors work best and allow a tighter over- all layout. Metal-film and carbon composition, axially-leaded resistors can also provide good high frequency performance. Again, keep their leads and PC board trace length as short as possible. Never use wirewound type resistors in a high frequency application. Since the output pin and inverting input pin are the most sensitive to parasitic capacitance, always position the feedback and series output resistor, if any, as close as possible to the output pin. Other network components, such as non-inverting input termination resis- tors, should also be placed close to the package. Where double-side component mounting is allowed, place the feed- 25k 110k 15k I S Control –V S +V S V DIS Q1 FIGURE 9. Simplified Disabled Control Circuit. In normal operation, base current to Q1 is provided through the 110k resistor while the emitter current through the 15k resistor sets up a voltage drop that is inadequate to turn on the two diodes in Q1’s emitter. As V DIS is pulled low, additional current is pulled through the 15k resistor, eventually turning on these two diodes ( 100 µ A). At this point, any further current pulled out of V DIS goes through those diodes holding the emitter-based voltage of Q1 at approximately zero volts. This shuts off the collector current out of Q1, turning the amplifier off. The supply current in the disable mode are only those required to operate the circuit of Figure 9. THERMAL ANALYSIS The OPA687 will not require heatsinking or airflow in most applications. Maximum desired junction temperature will set the maximum allowed internal power dissipation as described below. In no case should the maximum junction temperature be allowed to exceed 175 ° C. Operating junction temperature (T J ) is given by T A + P D θ JA . The total internal power dissipation (P D ) is the sum of quiescent power (P DQ ) and additional power dissipated in the output stage (P DL ) to deliver load power. Quiescent power is simply the specified no-load supply current times the total supply voltage across the part. P DL will depend on the required output signal and load but would, for a grounded resistive load, be at a maximum when the output is fixed at a voltage equal to 1/2 either supply voltage (for equal bipolar supplies). Under this condition P DL = V S 2 /(4 • R L ) where R L includes feedback network loading. This is the absolute highest power that can be dissipated for a given R L . All actual applications will dissipate less power in the output stage.
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16 OPA687 SBOS065A back resistor directly under the package on the other side of the board between the output and inverting input pins. Even with a low parasitic capacitance shunting the external resis- tors, excessively high resistor values can create significant time constants that can degrade performance. Good axial metal-film or surface-mount resistors have approximately 0.2pF in shunt with the resistor. For resistor values > 2.0k
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