ADCMP370/ADCMP371

Rev. A | Page 9 of 12

APPLICATIONS

BASIC COMPARATOR

In its most basic configuration, a comparator can be used to

convert an analog input signal to a digital output signal. The

analog signal on IN+ is compared to the voltage on IN−, and

the voltage at OUT is either high or low, depending on whether

IN+ is at a higher or lower potential than IN−, respectively.

The ADCMP370 and ADCMP371 have different digital output

structures. The ADCMP370 has an open-drain output stage

that requires an external resistor to pull OUT to the logic high

voltage level when the output transistor is switched off. This

voltage level can be as high as 22 V. The same 22 V tolerance

also applies to the inputs of the comparators. The pull-up

resistor should be large enough to avoid excessive power

dissipation but small enough to switch logic levels reasonably

quickly when the comparator output is connected to other

digital circuitry. A suitable value is between 1 kΩ and 10 kΩ.

The ADCMP371 has a push-pull output stage, which has an

internal PMOS pull-up and, therefore, does not require an

external resistor. Faster switching speeds between low and high

ADM331

OUT

V+

t

0V

ADCMP370

V+

Figure 18. Basic Comparator and Input and Output Signals

ADDING HYSTERESIS

To prevent oscillations at the output caused by noise or slowly

moving signals passing the switching threshold, positive feedback

can be used to add hysteresis to the differential input.

For the noninverting configuration, shown in Figure 19,

two resistors are used to create different switching thresholds,

depending on whether the input signal is increasing or

decreasing in magnitude. When the input voltage is increasing,

The upper input threshold level is given by

(

)

R2

R1

V

R2

R1

V

V

CC

REF

IN_HI

−

+

=

The lower input threshold level is given by

(

)

R2

R2

R1

V

V

REF

IN_LO

+

=

The hysteresis is the difference between these voltage levels

R2

R1

V

V

CC

Δ

In the example in Figure 19, Resistor R1 and Resistor R2 are

chosen to give 1 V hysteresis about the reference of 2.5 V, with

ADM331

ADCMP371

R2 = 1M

Ω

R1 = 200k

Ω

IN+

IN–

= 2V

= 3V

OUT

Figure 19. Noninverting Comparator Configuration with Hysteresis

With the inverting configuration, the upper and lower

switching thresholds are

R3

R1

R2

V

V

CC

IN_HI

+

=

||

()

)

(

)

1

(

)

(

1

R3

R2

R

R2

R3

R1

R3

R

R2

V

V

CC

IN_HI

×

+

×

+

×

+

×

=

(

)

()

R3

R2

R1

R3

R2

V

V

CC

IN_LO

||

||

+

=

)

3

(

)

(

)

3

(

3

2

R

R2

R1

R2

R

R1

R

R

V

V

CC

IN_LO

×

+

×

+

×

×

×

=