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Comparator

Comparator

Comparator

Comparator

Comparator

AS111

AustinSemiconductor,Inc.reservestherighttochangeproductsorspecificationswithoutnotice.

7

AS111

Rev. 1.6 6/05

Austin Semiconductor, Inc.

TYPICAL PERFORMANCE CHARACTERISTICS (CONTINUED)

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

disccapacitors), 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

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)