PRELIMINARY SPECIFICATION
Austin Semiconductor, Inc.
Schematic Diagram
20
oltag
Volta g e
Comparator
AS111 RAD-TOL
NOTE 20:
Pin connections shown on schematic diagram are for the TO08 can package.
CIRCUIT TECHNIQUES FOR AVOIDING
OSCILLATIONS IN COMPARATOR
APPLICATIONS
When a high-speed comparator such as the AS111 is used
with fast input signals and low source impedances, the output
response will normally be fast and stable, assuming that the
power supplies have been bypassed (with 0.1 µF disc capaci-
tors), 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 kW to
100 kW), the comparator may burst into oscillation near the
crossing-point. This is due to the high gain and wide
bandwidth of comparators like the AS111. To avoid oscillation
or instability in such a usage, several precautions are recom-
mended, as shown in Figure 1 below.
1. The trim pins (pins 5 and 6) act as unwanted auxiliary
inputs. If these pins are not connected to a trim-pot, they
AS111 RAD-TOL
Rev. 1.1 06/05
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 1.
2. Certain sources will produce a cleaner comparator
out put waveform if a 100 pF to 1000 pF capacitor C2 is
connected directly across the input pins.
3. When the signal source is applied through a resistive
network, R
S
, it is usually advantageous to choose an R
S
’ of
substantially the same value, both for DC and for dynamic
(AC) considerations. Carbon, tin-oxide, and metal-film
resistors have all been used successfully in comparator
input circuitry. Inductive wirewound resistors are not
suitable.
(continued)
Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.
9
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