Application Information

BASIC COMPARATOR

A basic comparator circuit is used to convert analog input

signals to digital output signals. The comparator compares an

input voltage (V

voltage (V

output (V

V

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FIGURE 1. Basic Comparator

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 voltage). Hys-

teresis 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-in-

verting configuration with two resistors and positive feedback

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

up to V

V

The output will switch from high to low when V

where V

V

The Hysteresis is the difference between V

ΔV

= ((V

= V

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FIGURE 2. Non-Inverting Comparator Configuration

INPUT

The LMV761/LMV762 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 acti-

vated 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

drive SD while V

The logic threshold limits for SD are proportional to V

BOARD LAYOUT AND BYPASSING

The LMV761/LMV762 is designed to be stable and oscillation

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

pacitors and ground pickups. Ceramic 0.1

μF capacitors

should be placed at both supplies to provide clean switching.

Minimize the length of signal traces to reduce stray capaci-

tance.

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