Austin Semiconductor, Inc.
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 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
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.)
oltag
Volta g e
Comparator
AS111
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 100Ω, such as 50 kΩ, it would
not be reasonable to simply increase the value of the
positive feedback resistor above 510 kΩ. 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
82Ω 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 kΩ 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.
AS111
Rev. 1.6 6/05
Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.
8
AUSTIN [ AUSTIN SEMICONDUCTOR ]
