LM211, LM311
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7
TECHNIQUES FOR AVOIDING OSCILLATIONS IN COMPARATOR APPLICATIONS
When a high speed comparator such as the LM211 is used
with high speed input signals and low source impedances,
the output response will normally be fast and stable,
providing the power supplies have been bypassed (with
0.1 mF 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 (1.0 kW
to 100 kW), the comparator may burst into oscillation near
the crossing−point. This is due to the high gain and wide
bandwidth of comparators like the LM211 series. To avoid
oscillation or instability in such a usage, several precautions
are recommended, as shown in Figure 16.
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 mF 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 16. For the fastest response time, tie both balance pins
to V
CC
.
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. When the signal source is
applied through a resistive network, R1, it is usually
advantageous to choose R2 of the same value, both for DC
and for dynamic (AC) considerations. Carbon, tin−oxide,
and metal−film resistors have all been used with good results
in comparator input circuitry, but inductive wirewound
resistors should be avoided.
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 a very
short 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 R1 =
10 kW, as little as 5 inches of lead between the resistors and
the input pins can result in oscillations that are very hard to
dampen. Twisting these input leads tightly is the 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
groundplane under the LM211 circuitry (e.g., one side of a
double layer printed circuit board). Ground, 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 fast
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 no more than
a few inches away from the LM211, and a 0.01 mF capacitor
should be installed across Pins 5 and 6. 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
LM211.
A standard procedure is to add hysteresis to a comparator
to prevent oscillation, and to avoid excessive noise on the
output. In the circuit of Figure 17, the feedback resistor of
510 kW from the output to the positive input will cause about
3.0 mV of hysteresis. However, if R2 is larger than 100 W,
such as 50 kW, it would not be practical to simply increase
the value of the positive feedback resistor proportionally
above 510 kW to maintain the same amount of hysteresis.
When both inputs of the LM211 are connected to active
signals, or if a high−impedance signal is driving the positive
input of the LM211 so that positive feedback would be
disruptive, the circuit of Figure 16 is ideal. The positive
feedback is applied to Pin 5 (one of the offset adjustment
pins). This will be sufficient to cause 1.0 mV to 2.0 mV
hysteresis and sharp transitions with input triangle waves
from a few Hz to hundreds of kHz. The positive−feedback
signal across the 82 W 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 offset
voltage of the comparator. As much as 8.0 mV of offset
voltage can be trimmed out, using the 5.0 kW pot and 3.0 kW
resistor as shown.
