Voltage
Comparator
AS111
Micross Components reserves the right to change products or specifi cations 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 RS=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 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 amplifi er 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 RS 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 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 suffi cient 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 VOS of the
comparator. As much as 8 mV of VOS can be trimmed out,
using the 5 kΩ pot and 3 kΩ resistor as shown.
8. These application notes apply specifi cally 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.