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LMC6032IM Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LMC6032IM Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Application Hints (Continued) where is the amplifier’s low-frequency noise gain and GBW is the amplifier’s gain bandwidth product. An amplifier’s low-frequency noise gain is represented by the formula regardless of whether the amplifier is being used in an invert- ing or non-inverting mode. Note that a feedback capacitor is more likely to be needed when the noise gain is low and/or the feedback resistor is large. If the above condition is met (indicating a feedback capacitor will probably be needed), and the noise gain is large enough that: the following value of feedback capacitor is recommended: If the feedback capacitor should be: Note that these capacitor values are usually significantly smaller than those given by the older, more conservative for- mula: 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 value of C F should be checked on the actual circuit, starting with the computed value. CAPACITIVE LOAD TOLERANCE Like many other op amps, the LMC6032 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 the Typical Performance Characteristics. The load capacitance interacts with the op amp’s output re- sistance 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 100Ω)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 ca- pacitance 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 con- ducting 500 µA or more will significantly improve capacitive load responses. The value of the pull up resistor must be de- termined based on the current sinking capability of the ampli- fier 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 LMC6032, typically less DS011135-4 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 resistor. FIGURE 2. General Operational Amplifier Circuit DS011135-5 FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance DS011135-22 FIGURE 4. Compensating for Large Capacitive Loads with a Pull Up Resistor www.national.com 7 |
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