LM211M National Semiconductor, LM211M Datasheet - Page 8

• Manufacturer Part Number: LM211M
• Description: Voltage Comparator
• Manufacturer: National Semiconductor

www.national.com
7.0 LM311 Typical Performance Characteristics
Supply Current
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 to
100 k ), 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 in-
2. Certain sources will produce a cleaner comparator out-
3. When the signal source is applied through a resistive
4. When comparator circuits use input resistors (eg. sum-
puts. 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 .
put waveform if a 100 pF to 1000 pF capacitor C2 is con-
nected directly across the input pins.
network, R
of substantially the same value, both for DC and for dy-
namic (AC) considerations. Carbon, tin-oxide, and
metal-film resistors have all been used successfully in
comparator input circuitry. Inductive wirewound resistors
are not suitable.
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
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.
S
, it is usually advantageous to choose an R
S
= 10 k , as little as 5 inches of
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Supply Current
S
'
8
5. Since feedback to almost any pin of a comparator can
6. It is a standard procedure to use hysteresis (positive
7. When both inputs of the LM111 are connected to active
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 ca-
pacitive 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 in-
stalled. 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.)
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
not be reasonable to simply increase the value of the
positive feedback resistor above 510 k . The circuit of
Figure 3 could be used, but it is rather awkward. See the
notes in paragraph 7 below.
signals, or if a high-impedance signal is driving the posi-
tive input of the LM111 so that positive feedback would
be disruptive, the circuit of Figure 1 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 82
ply. This signal is centered around the nominal voltage at
pin 5, so this feedback does not add to the V
comparator. As much as 8 mV of V
out, using the 5 k
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resistor swings 240 mV below the positive sup-
S
(Continued)
is larger than 100 , such as 50 k , it would
Leakage Currents
pot and 3 k
resistor as shown.
OS
can be trimmed
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OS
of the