LM111-N, LM211-N, LM311-N
SNOSBJ1E –MAY 1999–REVISED MARCH 2013
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APPLICATION HINTS
CIRCUIT TECHNIQUES FOR AVOIDING
OSCILLATIONS IN COMPARATOR APPLICATIONS
When a high-speed comparator such as the LM111-N is used with fast input signals and low source impedances,
the output 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 kΩ to 100 kΩ), the comparator may burst into oscillation near the crossing-point. This is due to the high gain
and wide bandwidth of comparators like the LM111-N. To avoid oscillation or instability in such a usage, several
precautions are recommended, as shown in Figure 40 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 40.
2. Certain sources will produce a cleaner comparator output 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, RS, 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. summing resistors), their value and placement are
particularly 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 RS=10 kΩ, 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-N circuitry, for example,
one side of a double-layer circuit card. Ground foil (or, positive supply or negative supply foil) should extend
between 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 surrounded 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-N, 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 located within a couple inches of the LM111-N. (Some other comparators require the power-supply
bypass to be located 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 41, the feedback from the output to the positive input will cause about 3 mV of
hysteresis. However, if RS is larger than 100Ω, such as 50 kΩ, it would not be reasonable to simply increase
the value of the positive feedback resistor above 510 kΩ. The circuit of Figure 42 could be used, but it is
rather awkward. See the notes in paragraph 7 below.
7. When both inputs of the LM111-N are connected to active signals, or if a high-impedance signal is driving the
positive input of the LM111-N so that positive feedback would be disruptive, the circuit of Figure 40 is ideal.
The positive feedback is to pin 5 (one of the offset adjustment pins). It is sufficient to cause 1 to 2 mV
hysteresis and sharp transitions with input triangle waves from a few Hz to hundreds of kHz. The positive-
feedback signal across the 82Ω resistor swings 240 mV below the positive supply. This signal is centered
around the nominal voltage at pin 5, so this feedback does not add to the VOS of the comparator. As much as
8 mV of VOS can be trimmed out, using the 5 kΩ pot and 3 kΩ resistor as shown.
8. These application notes apply specifically to the LM111-N, LM211-N, LM311-N, and LF111 families of
comparators, and are applicable to all high-speed comparators in general, (with the exception that not all
comparators have trim pins).
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