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LM111 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM111 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 23 page 7.0 LM311 Typical Performance Characteristics (Continued) Leakage Currents 00570472 8.0 Application Hints 8.1 CIRCUIT TECHNIQUES FOR AVOIDING OSCILLATIONS IN COMPARATOR APPLICATIONS When a high-speed comparator such as the LM111 is used with fast input signals and low source impedances, the out- put response will normally be fast and stable, assuming that the power supplies have been bypassed (with 0.1 µF disc capacitors), and that the output signal is routed well away from the inputs (pins 2 and 3) and also away from pins 5 and 6. However, when the input signal is a voltage ramp or a slow sine wave, or if the signal source impedance is high (1 k Ω to 100 k Ω), the comparator may burst into oscillation near the crossing-point. This is due to the high gain and wide band- width of comparators like the LM111. To avoid oscillation or instability in such a usage, several precautions are recom- mended, as shown in Figure 1 below. 1. The trim pins (pins 5 and 6) act as unwanted auxiliary inputs. If these pins are not connected to a trim-pot, they should be shorted together. If they are connected to a trim-pot, a 0.01 µF capacitor C1 between pins 5 and 6 will minimize the susceptibility to AC coupling. A smaller capacitor is used if pin 5 is used for positive feedback as in Figure 1. 2. Certain sources will produce a cleaner comparator out- put waveform if a 100 pF to 1000 pF capacitor C2 is connected directly across the input pins. 3. When the signal source is applied through a resistive network, R S, it is usually advantageous to choose an RS' of substantially the same value, both for DC and for dynamic (AC) considerations. Carbon, tin-oxide, and metal-film resistors have all been used successfully in comparator input circuitry. Inductive wirewound resistors are not suitable. 4. When comparator circuits use input resistors (eg. sum- ming resistors), their value and placement are particu- larly important. In all cases the body of the resistor should be close to the device or socket. In other words there should be very little lead length or printed-circuit foil run between comparator and resistor to radiate or pick up signals. The same applies to capacitors, pots, etc. For example, if R S=10 k Ω, as little as 5 inches of lead between the resistors and the input pins can result in oscillations that are very hard to damp. Twisting these input leads tightly is the only (second best) alternative to placing resistors close to the comparator. 5. Since feedback to almost any pin of a comparator can result in oscillation, the printed-circuit layout should be engineered thoughtfully. Preferably there should be a groundplane under the LM111 circuitry, for example, one side of a double-layer circuit card. Ground foil (or, posi- tive supply or negative supply foil) should extend be- tween the output and the inputs, to act as a guard. The foil connections for the inputs should be as small and compact as possible, and should be essentially sur- rounded by ground foil on all sides, to guard against capacitive coupling from any high-level signals (such as the output). If pins 5 and 6 are not used, they should be shorted together. If they are connected to a trim-pot, the trim-pot should be located, at most, a few inches away from the LM111, and the 0.01 µF capacitor should be installed. If this capacitor cannot be used, a shielding printed-circuit foil may be advisable between pins 6 and 7. The power supply bypass capacitors should be lo- cated within a couple inches of the LM111. (Some other comparators require the power-supply bypass to be lo- cated immediately adjacent to the comparator.) 6. It is a standard procedure to use hysteresis (positive feedback) around a comparator, to prevent oscillation, and to avoid excessive noise on the output because the comparator is a good amplifier for its own noise. In the circuit of Figure 2, the feedback from the output to the positive input will cause about 3 mV of hysteresis. How- ever, if R S is larger than 100 Ω, such as 50 kΩ, it would not be reasonable to simply increase the value of the positive feedback resistor above 510 k Ω. The circuit of Figure 3 could be used, but it is rather awkward. See the notes in paragraph 7 below. www.national.com 10 |
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