What is a hysteresis comparator?

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What is a hysteresis comparator? [Copy link]

The hysteresis comparator can also be understood as a single-limit comparator with positive feedback. For a single-limit comparator, if the input signal Uin has a slight interference near the threshold value, the output voltage will produce corresponding jitter (fluctuation). Introducing positive feedback in the circuit can overcome this shortcoming.
　　Figure 1a shows a hysteresis comparator. The familiar "Schmidt" circuit is a comparator with hysteresis. Figure 1b shows the transmission characteristics of the hysteresis comparator.
　

　　It is not difficult to see that once the output state is switched, as long as the interference near the jump voltage value does not exceed the value of ΔU, the output voltage value will be stable. But the resolution is reduced. Because for the hysteresis comparator, it cannot distinguish two input voltage values with a difference less than ΔU. The hysteresis comparator with positive feedback can speed up the response of the comparator, which is one of its advantages. In addition, since the positive feedback added to the hysteresis comparator is very strong, much stronger than the parasitic coupling in the circuit, the hysteresis comparator can also avoid the self-excited oscillation caused by the parasitic coupling of the circuit.
　　Hysteresis comparator
　　The output VO of the hysteresis comparator is not linearly related to the input VI, and the critical condition for the conversion of the output voltage is
　　Threshold voltage VP (voltage at the non-inverting input) ≈ VN (voltage at the inverting input) = VI (reference voltage)
　　VP = VN = [(R1 × VREF) / (R1 + R2)] + [(R2 × VO) / (R1 + R2)] (Formula-1)
　　According to the different values of the output voltage VO (VOH or VOL), the upper threshold voltage VT+ and the lower threshold voltage VT- can be calculated respectively:
　　VT+ = {[1 + (R1/R2)] × VREF} - [(R1/R2) × VOL] (Formula-2)
　　VT- = {[1 + (R1/R2)] × VREF} - [(R1/R2) × VOH] (Formula-3)
　　Then the threshold width is:
　　ΔVT = (R1/R2) × (VOH-VOL) (Formula - 4)
　

　　Given
　　Working voltage = 12VReference
　　voltage VREF = 1VInput
　　voltage VI = 1~5VR1
　　= 1000Ω = 1KΩR2
　　= 1000000Ω = 1MΩFeedback
　　coefficient = R1/(R1+R2) = 0.000999Comparator
　　output voltage VOH = 12V, VOL = 0VAnd comparator
　　threshold width/output voltage = feedback coefficient
　　That is, feedback coefficient × output voltage = threshold width0.000999
　　×12 = 0.011988≈0.012VAccording
　　to (Formula-2)VT+={[1+(R1/R2)]×VREF}-[(R1/R2)×VOL]
　　={[1+(1000/1000000)]×1}-[(1000/1000000)×0]
　　=1.001-0
　　=1.001(V)
　　According to (Formula 3) VT-={[1+(R1/R2)]×VREF}-[(R1/R2)×VOH]
　　={[1+(1000/1000000)]×1}-[(1000/1000000)×12]
　　=1.001-0.012
　　=0.989(V)
　　According to (Formula -4) ΔVT=(R1/R2)×(VOH-VOL)
　　=(1000/1000000)×12
　　=0.012(V)
　　Verification:
　　VT+-VT- =1.001-0.989=0.012(V)
　　The range of ΔVT can be adjusted by changing R2 to change the feedback coefficient.
　　For example, when R2 is changed to 10KΩ,
　　ΔVT = (R1/R2) × (VOH-VOL)
　　= (1000/10000) × 12
　　= 1.2 (V)
　　For example, when R2 is changed to 100KΩ, ΔVT = (R1/R2) × (VOH-VOL)
　　= (1000/100000) × 12
　　= 0.12 (V)