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LMC660CMX Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LMC660CMX Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 12 page Application Hints (Continued) Every amplifier has some capacitance between each input and AC ground, and also some differential capacitance be- tween the inputs. When the feedback network around an amplifier is resistive, this input capacitance (along with any additional capacitance due to circuit board traces, the socket, etc.) and the feedback resistors create a pole in the feedback path. In the following General Operational Amplifier circuit, Figure 2 the frequency of this pole is where C S is the total capacitance at the inverting input, in- cluding amplifier input capcitance and any stray capacitance from the IC socket (if one is used), circuit board traces, etc., and R P is the parallel combination of RF and RIN. This for- mula, as well as all formulae derived below, apply to invert- ing and non-inverting op-amp configurations. When the feedback resistors are smaller than a few k Ω, the frequency of the feedback pole will be quite high, since C S is generally less than 10 pF. If the frequency of the feedback pole is much higher than the “ideal” closed-loop bandwidth (the nominal closed-loop bandwidth in the absence of C S), the pole will have a negligible effect on stability, as it will add only a small amount of phase shift. However, if the feedback pole is less than approximately 6 to 10 times the “ideal” −3 dB frequency, a feedback capacitor, C F, should be connected between the output and the invert- ing input of the op amp. This condition can also be stated in terms of the amplifier’s low-frequency noise gain: To main- tain stability a feedback capacitor will probably be needed if 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 inverting 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 significant 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 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 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 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. DS008767-6 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 www.national.com 6 |
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