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ADCMP370AKSZ-REEL bảng dữ liệu(PDF) 9 Page - Analog Devices |
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9 / 12 page 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 rails are possible, but the logic high level is limited to VCC. ADM331 OUT V+ VREF t VOUT 0V ADCMP370 VCC V+ VIN VREF VIN 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 threshold is above VREF, and when it is decreasing, the threshold is below VREF. 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 IN = 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 VCC = 5 V. ADM331 ADCMP371 VCC = 5V R2 = 1M Ω R1 = 200k Ω VREF = 2.5V IN+ IN– VIN VOUT VIN VIN_LO = 2V VIN_HI = 3V OUT Figure 19. Noninverting Comparator Configuration with Hysteresis With the inverting configuration, the upper and lower switching thresholds are () R2 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 × + × + × × × = |
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