LM311 LM211

6

MOTOROLA ANALOG IC DEVICE DATA

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

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 (1.0 k

Ω

to 100 k

Ω), 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 15.

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 15. For the fastest response time, tie both balance

pins to VCC.

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 k

Ω, 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

µF 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 16, the feedback resistor of

510 k

Ω from the output to the positive input will cause about

3.0 mV of hysteresis. However, if R2 is larger than 100

Ω,

such as 50 k

Ω, it would not be practical to simply increase the

value of the positive feedback resistor proportionally above

510 k

Ω 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 15 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

Ω 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 k

Ω pot and 3.0 kΩ

resistor as shown.

Figure 17. Zero–Crossing Detector

Driving CMOS Logic

Figure 18. Relay Driver with Strobe Capability

VCC = +15 V

3.0 k

10 k

VCC

5.0 k

LM311

Inputs

VEE

VEE = –15 V

Output

to CMOS Logic

Balance

Adjust

Balance

Input

Gnd

*D1

VCC2

VCC1

VEE

VEE

VCC

Output

Inputs

LM311

Gnd

1.0 k

Q1

Balance/Strobe

2N2222

or Equiv

*Zener Diode D1

protects the comparator

from inductive kickback

and voltage transients

on the VCC2 supply line.

TTL

Strobe

+

+