Application Notes

Basic Comparators

A comparator is quite often used to convert an analog signal

to a digital signal. The comparator compares an input volt-

age (V

(V

(V

output voltage (V

Hysteresis

The basic comparator configuration may oscillate or produce

a noisy output if the applied differential input is near the

comparator’s input offset voltage. This tends to occur when

the voltage on the input is equal or very close to the other

input voltage. Adding hysteresis can prevent this problem.

Hysteresis creates two switching thresholds (one for the

rising input voltage and the other for the falling input volt-

age). Hysteresis is the voltage difference between the two

switching thresholds. When both inputs are nearly equal,

hysteresis causes one input to effectively move quickly pass

the other. Thus, effectively moving the input out of region that

oscillation may occur.

For an inverting configured comparator, hysteresis can be

added with a three resistor network and positive feedback.

When input voltage (V

non-inverting node (V

circuit for the three resistor network is R

and in series with R

calculated by:

V

When V

equivalent circuit for the three resistor network is R

parallel with R

voltage V

V

R

The hysteresis is defined as

∆V

20022110

20022111

FIGURE 2. Basic Comparator

20022112

20022113

FIGURE 3. Inverting Configured Comparator – LM397

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