Application Hints

Basic Comparator

A basic comparator circuit is used to convert analog input

signals to digital output signals. The comparator compares

an input voltage (V

ence voltage (V

V

than 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 one 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 past

the other. Thus, moving the input out of the region in which

oscillation may occur.

Hysteresis can easily be added to a comparator in a non-

inverting configuration with two resistors and positive feed-

back Figure 2. The output will switch from low to high when

V

V

The output will switch from high to low when V

where V

V

The Hysteresis is the difference between V

∆V

= ((V

=V

Input

The LMV761/762 have near zero input bias current. This

allows very high resistance circuits to be used without any

concern for matching input resistances. This also allows the

use of very small capacitors in R-C type timing circuits. This

reduces the cost of the capacitors and amount of board

space used.

Shutdown Mode

The LMV761 features a low-power shutdown pin that is

activated by driving SD low. In shutdown mode, the output is

in a high impedance state, supply current is reduced to 20nA

and the comparator is disabled. Driving SD high will turn the

comparator on. The SD pin should not be left unconnected

due to the fact that it is a high impedance input. When left

unconnected, the output will be at an unknown voltage. Also

do not three-state the SD pin.

The maximum input voltage for SD is 5.5V, referred to

ground and is not limited by V

logic to drive SD while V

as 3V. The logic threshold limits for SD are proportional to

V

Board Layout and Bypassing

The LMV761/762 is designed to be stable and oscillation

free, but it is still important to include the proper bypass

capacitors and ground pickups. Ceramic 0.1µF capacitors

should be placed at both supplies to provide clean switching.

Minimize the length of signal traces to reduce stray capaci-

tance.

20037028

20037031

FIGURE 1. Basic Comparator

20037026

20037027

FIGURE 2. Non-Inverting Comparator Configuration

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