LM111/LM211/LM311
7.0 LM311 Typical Performance Characteristics
Leakage Currents
(Continued)
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 kΩ to
100 kΩ), 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
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
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 R
S
'
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 kΩ, as little as 5 inches of
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10
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.)
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
the feedback from the output to the
positive input will cause about 3 mV of hysteresis. How-
ever, if R
S
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
could be used, but it is rather awkward. See the
notes in paragraph 7 below.
6.
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