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LMC660 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMC660 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Application Hints (Continued) Note that these capacitor values are usually significant smaller than those given by the older, more conservative formula: Using the smaller capacitors will give much higher band- width with little degradation of transient response. It may be necessary in any of the above cases to use a somewhat larger feedback capacitor to allow for unexpected stray ca- pacitance, or to tolerate additional phase shifts in the loop, or excessive capacitive load, or to decrease the noise or band- width, or simply because the particular circuit implementa- tion needs more feedback capacitance to be sufficiently stable. For example, a printed circuit board’s stray capaci- tance may be larger or smaller than the breadboard’s, so the actual optimum value for C F may be different from the one estimated using the breadboard. In most cases, the values of C F should be checked on the actual circuit, starting with the computed value. CAPACITIVE LOAD TOLERANCE Like many other op amps, the LMC660 may oscillate when its applied load appears capacitive. The threshold of oscilla- tion varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain follower. See Typical Performance Characteristics. The load capacitance interacts with the op amp’s output resistance to create an additional pole. If this pole frequency is sufficiently low, it will degrade the op amp’s phase margin so that the amplifier is no longer stable at low gains. As shown in Figure 3, the addition of a small resistor (50 Ω to 100 Ω) in series with the op amp’s output, and a capacitor (5 pF to 10 pF) from inverting input to output pins, returns the phase margin to a safe value without interfering with lower- frequency circuit operation. Thus larger values of capaci- tance can be tolerated without oscillation. Note that in all cases, the output will ring heavily when the load capacitance is near the threshold for oscillation. Capacitive load driving capability is enhanced by using a pull up resistor to V + (Figure 4). Typically a pull up resistor conducting 500 µA or more will significantly improve capaci- tive load responses. The value of the pull up resistor must be determined based on the current sinking capability of the amplifier with respect to the desired output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see Electrical Characteristics). PRINTED-CIRCUIT-BOARD LAYOUT FOR HIGH-IMPEDANCE WORK It is generally recognized that any circuit which must operate with less than 1000 pA of leakage current requires special layout of the PC board. When one wishes to take advantage of the ultra-low bias current of the LMC662, typically less than 0.04 pA, it is essential to have an excellent layout. Fortunately, the techniques for obtaining low leakages are quite simple. First, the user must not ignore the surface leakage of the PC board, even though it may sometimes appear acceptably low, because under conditions of high humidity or dust or contamination, the surface leakage will be appreciable. To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC660’s inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op amp’s inputs. SeeFigure 5. To have a significant effect, guard rings should be placed on both the top and bottom of the PC board. This PC foil must then be connected to a voltage which is at the same voltage as the amplifier inputs, since no leakage current can flow between two points at the same potential. For example, a PC board trace-to-pad resistance of 10 12 Ω, which is nor- mally considered a very large resistance, could leak 5 pA if the trace were a 5V bus adjacent to the pad of an input. This would cause a 100 times degradation from the LMC660’s actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a resistance of 10 11 Ω would 00876706 CS consists of the amplifier’s input capacitance plus any stray capacitance from the circuit board and socket. CF compensates for the pole caused by CS and the feedback resistors. FIGURE 2. General Operational Amplifier Circuit 00876705 FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance 00876723 FIGURE 4. Compensating for Large Capacitive Loads with a Pull Up Resistor www.national.com 8 |
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