Voltage

Comparator

AS111

Micross Components reserves the right to change products or specifications without notice.

8

AS111

Rev. 1.7 01/10

(continued)

APPLICATION HINTS (CONTINUED)

4. When comparator circuits use input resistors (eg.

sum-

ming resistors), their value and placement are

par-

ticularly 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

the resistors and the input pins can result 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 AS111 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

con-

nections 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

cou-

pling from any high-level signals (such as the output). If

pins 5 and 6 are not used, they should be shorted

to-

gether. If they are connected to a trim-pot, the trim-pot

should be located, at most, a few inches away from the

AS111, 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 located within a couple

inches of the AS111. (Some other comparators require the

power-supply bypass to be located immediately adjacent

to the comparator.)

6. 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

com-

parator 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.

However,

if R

reasonable to simply increase the value of the

positive

feedback resistor above 510 k

Ω. The circuit of Figure 3

could be used, but it is rather awkward. See the notes in

paragraph 7 below.

7. When both inputs of the AS111 are connected to ac-

tive signals, or if a high-impedance signal is driving the

positive input of the AS111 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

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 V

comparator. As much as 8 mV of V

using the 5 k

Ω pot and 3 kΩ resistor as shown.

8. These application notes apply specifically to the AS111

family of comparators, and are applicable to all high-speed

comparators in general, (with the exception that not all

comparators have trim pins).

NOTE: Pin connections shown are for the TO08 can package.