LM397MFX datasheet(4/8 Pages) NSC

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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

IN) at the non-inverting pin to the reference voltage

REF) at the inverting pin. If VIN is less than VREF the output

O) is low (VOL). However, if VIN is greater than VREF, the

output voltage (V

O) is high (VOH). Refer to Figure 2.

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

IN) at the inverting node is less than

non-inverting node (V

T), the output is high. The equivalent

circuit for the three resistor network is R

1 in parallel with R3

and in series with R

2. The lower threshold voltage VT1 is

calculated by:

T1 = ((VS R2) / (((R1 R3)/(R1 +R3))+R2))

When V

IN is greater than VT, the output voltage is low. The

equivalent circuit for the three resistor network is R

2 in

parallel with R

3 and in series with R1. The upper threshold

voltage V

T2 is calculated by:

T2 =VS ((R2 R3)/(R2 +R3))/(R1 + ((R2 R3)/(R2 +

3)))

The hysteresis is defined as

∆V

IN =VT1 –VT2

20022110

20022111

FIGURE 2. Basic Comparator

20022112

20022113

FIGURE 3. Inverting Configured Comparator – LM397

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