7.3.1 Basic Comparator

A basic comparator circuit is used to convert analog input signals to digital output signals. The comparator compares an input voltage (V_IN) at the noninverting input to the reference voltage (V_REF) at the inverting pin. If V_IN is less than V_REF the output (V_O) is low (V_OL). However, if V_IN is greater than V_REF, the output voltage (V_O) is high (V_OH).

Figure 19. Basic Comparator Without Hysteresis

Figure 20. Basic Comparator

7.3.2 Hysteresis

The basic comparator configuration may oscillate or produce a noisy output if the applied differential input is near the input offset voltage of the comparator, which 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). 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 noninverting configuration with two resistors and positive feedback Figure 22. The output will switch from low to high when V_IN rises up to V_IN1, where V_IN1 is calculated by Equation 1:

The output will switch from high to low when V_IN falls to V_IN2, where V_IN2 is calculated by Equation 2:

The Hysteresis is the difference between V_IN1 and V_IN2, as calculated by Equation 3:

Figure 21. Basic Comparator With Hysteresis

Figure 22. Noninverting Comparator Configuration

7.3.3 Input

The LMV76x devices have near-zero input bias current, which allows very high resistance circuits to be used without any concern for matching input resistances. This near-zero input bias 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.

7.4.1 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 20 nA and the comparator is disabled. Driving SD high will turn the comparator on. The SD pin must 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. Do not three-state the SD pin.

The maximum input voltage for SD is 5.5 V referred to ground and is not limited by V_CC. This allows the use of
5-V logic to drive SD while V_CC operates at a lower voltage, such as 3 V. The logic threshold limits for SD are proportional to V_CC.