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LMC6042AIM Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LMC6042AIM Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Typical Performance Characteristics V S = ±7.5V, TA = 25˚C unless otherwise specified (Continued) Applications Hints AMPLIFIER TOPOLOGY The LMC6042 incorporates a novel op-amp design topology that enables it to maintain rail-to-rail output swing even when driving a large load. Instead of relying on a push-pull unity gain output buffer stage, the output stage is taken directly from the internal integrator, which provides both low output impedance and large gain. Special feed-forward compensa- tion design techniques are incorporated to maintain stability over a wider range of operating conditions than traditional micropower op-amps. These features make the LMC6042 both easier to design with, and provide higher speed than products typically found in this ultra-low power class. COMPENSATING FOR INPUT CAPACITANCE It is quite common to use large values of feedback resis- tance with amplifiers with ultra-low input curent, like the LMC6042. Although the LMC6042 is highly stable over a wide range of operating conditions, certain precautions must be met to achieve the desired pulse response when a large feedback resistor is used. Large feedback resistors and even small values of input capacitance, due to transducers, photo- diodes, and circuit board parasitics, reduce phase margins. When high input impedances are demanded, guarding of the LMC6042 is suggested. Guarding input lines will not only re- duce leakage, but lowers stray input capacitance as well. (See Printed-Circuit-Board Layout for High Impedance Work). The effect of input capacitance can be compensated for by adding a capacitor. Place a capacitor, C f, around the feed- back resistor (as in Figure 1 ) such that: or R1 C IN ≤ R2 Cf Since it is often difficult to know the exact value of C IN,Cf can be experimentally adjusted so that the desired pulse re- sponse is achieved. Refer to the LMC660 and the LMC662 for a more detailed discussion on compensating for input ca- pacitance. CAPACITIVE LOAD TOLERANCE Direct capacitive loading will reduce the phase margin of many op-amps. A pole in the feedback loop is created by the combination of the op-amp’s output impedance and the ca- pacitive load. This pole induces phase lag at the unity-gain crossover frequency of the amplifier resulting in either an os- cillatory or underdamped pulse response. With a few exter- nal components, op amps can easily indirectly drive capaci- tive loads, as shown in Figure 2. In the circuit of Figure 2, R1 and C1 serve to counteract the loss of phase margin by feeding the high frequency compo- Stability vs Capacitive Load DS011137-43 Stability vs Capacitive Load DS011137-44 DS011137-5 FIGURE 1. Cancelling the Effect of Input Capacitance DS011137-6 FIGURE 2. LMC6042 Noninverting Gain of 10 Amplifier, Compensated to Handle Capacitive Loads www.national.com 7 |
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