Datasheet ADCMP370, ADCMP371 (Analog Devices) - 9

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Data Sheet ADCMP370/ADCMP371 APPLICATIONS BASIC COMPARATOR
The upper input threshold level is given by In its most basic configuration, a comparator can be used to V (R1+ R2) = convert an analog input signal to a digital output signal. The V REF IN_HI R2 analog signal on IN+ is compared to the voltage on IN−, and the voltage at OUT is either high or low, depending on whether The lower input threshold level is given by IN+ is at a higher or lower potential than IN−, respectively. V ( + − REF R1 R2) V R1 V CC = IN_LO The ADCMP370 and ADCMP371 have different digital output R2 structures. The ADCMP370 has an open-drain output stage The hysteresis is the difference between these voltage levels that requires an external resistor to pull OUT to the logic high V R1 voltage level when the output transistor is switched off. This V CC ∆ = IN voltage level can be as high as 22 V. The same 22 V tolerance R2 also applies to the inputs of the comparators. The pull-up In the example in Figure 19, Resistor R1 and Resistor R2 are resistor should be large enough to avoid excessive power chosen to give 1 V hysteresis about the reference of 2.5 V, with dissipation but small enough to switch logic levels reasonably VCC = 5 V. quickly when the comparator output is connected to other digital circuitry. A suitable value is between 1 kΩ and 10 kΩ.
VCC = 5V
The ADCMP371 has a push-pull output stage, which has an internal PMOS pull-up and, therefore, does not require an
ADM331 ADCMP371
external resistor. Faster switching speeds between low and high rails are possible, but the logic high level is limited to V
IN–
CC.
VREF = 2.5V OUT IN+ VCC VIN R1 = 200k
Ω
V+ ADM331 ADCMP370 R2 = 1M
Ω
VIN OUT V V REF OUT V+ VOUT VIN VIN_LO VIN_HI = 2V = 3V
04745-018
VREF
Figure 19. Noninverting ADCMP371 Comparator Configuration with
0V
Hysteresis
t VIN
With the inverting configuration, the upper and lower 04745-017 switching thresholds are Figure 18. Basic Comparator and Input and Output Signals V R2 V CC = IN_HI
ADDING HYSTERESIS
(R1||R3)+ R2 To prevent oscillations at the output caused by noise or slowly V × R2( 1 R + R3) V CC = IN_HI moving signals passing the switching threshold, positive feedback (R1× ) R3 + (R2 × R ) 1 + (R2 × ) R3 can be used to add hysteresis to the differential input. V (R2 || R3) V CC = For the noninverting configuration, shown in Figure 19, two IN_LO R1+ (R2 || R3) resistors are used to create different switching thresholds, depending on whether the input signal is increasing or V × R2 × R3 V CC = IN_LO decreasing in magnitude. When the input voltage is increasing, (R1× R ) 3 + (R2 × ) R1 + (R2 × R ) 3 the threshold is above VREF, and when it is decreasing, the threshold is below VREF. Rev. D | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram Table of Contents Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configurations and Function Description Typical Performance Characteristics Applications Basic Comparator Adding Hysteresis Outline Dimensions Ordering Guide