REF

1

2

CC

1

IN2

2

V

(R

R )

V

R

V

R

CC

IN1

1

A

IN

1

2

(V

V

)R

V

V

R

R

REF

1

2

IN1

2

V

(R

R )

V

R

'

VCC

VO

R2

R1

VA

+

-

VREF

RL

VIN

LMV7271, LMV7272, LMV7275

SNOSA56I – FEBRUARY 2003 – REVISED SEPTEMBER 2015

www.ti.com

Device Functional Modes (continued)

7.4.2 Noise

Most comparators have rather low gain. This allows the output to alternate between high and low when the input

signal changes slowly. The result is the output may oscillate between high and low when the differential input is

near zero and triggers on noise. The high gain of this comparator eliminates this problem. Less than 1

μV of

change on the input will drive the output from one rail to the other rail. If the input signal is noisy, the output

cannot ignore the noise unless some hysteresis is provided by positive feedback. (See Hysteresis.)

7.4.3 Hysteresis

It is a standard procedure to use hysteresis (positive feedback) around a comparator to prevent oscillation due to

the comparator triggering its own noise on slowly ramping signals. The following sections will describe various

ways to apply hysteresis.

7.4.3.1 Noninverting Comparator With Hysteresis

Figure 27. Noninverting Comparator With Hysteresis

A noninverting comparator with hysteresis requires a two resistor network, and a voltage reference (VREF) at the

inverting input. When VIN is low, the output is also low. For the output to switch from low to high, VIN must rise up

to VIN1 where VIN1 is calculated by:

(1)

As soon as VO switches to VCC, VA steps to a value greater than VREF which is given by:

(2)

To make the comparator switch back to its low state, VIN must equal VREF before VA will again equal VREF. VIN2

can be calculated by:

(3)

The hysteresis of this circuit is the difference between VIN1 and VIN2.

ΔVIN = VCCR1 / R2

(4)

Figure 28. Noninverting Comparator Thresholds

14

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