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Comparator

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Comparator

Comparator

AS111 RAD-TOL

AustinSemiconductor,Inc.reservestherighttochangeproductsorspecificationswithoutnotice.

10

AS111 RAD-TOL

Rev. 1.1 06/05

Austin Semiconductor, Inc.

PRELIMINARY SPECIFICATION

APPLICATION HINTS (CONTINUED)

4. When comparator circuits use input resistors (eg.

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

S

=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 damp. Twisting these input leads

tightly is the only(second best) alternative to placing re-

sistors 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

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 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

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

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.

However, if R

S

is larger than 100

Ω, such as 50 kΩ, it would

not be 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 active

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

OS

of the

comparator. As much as 8 mV of V

OS

can be trimmed out,

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.