jl111sza National Semiconductor Corporation, jl111sza Datasheet - Page 11

• Manufacturer Part Number: jl111sza
• Description: Lm111jan Voltage Comparator
• Manufacturer: National Semiconductor Corporation

Application Hints
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 output
response will normally be fast and stable, assuming that the
power supplies have been bypassed (with 0.1 μF disc capac-
itors), 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 such as the LM111. To avoid oscillation
or instability in such a usage, several precautions are recom-
mended, as shown in Figure 1 below.
1.
2.
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.
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.
Leakage Currents
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3.
4.
5.
When the signal source is applied through a resistive
network, R
′
dynamic (AC) considerations. Carbon, tin-oxide, and
metal-film resistors have all been used successfully in
comparator input circuitry. Inductive wire wound resistors
are not suitable.
When comparator circuits use input resistors (e.g.
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 R
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.
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
ground plane under the LM111 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
of substantially the same value, both for DC and for
S
, it is usually advantageous to choose an R
S
=10 kΩ, as little as 5 inches of lead
Supply Current
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