BASIC COMPARATOR CIRCUITS l COMPARATOR WITH HYSTERESIS l

BASIC COMPARATOR CIRCUITS l COMPARATOR WITH HYSTERESIS l WINDOW COMPARATOR 1

BASIC COMPARATOR CIRCUITS v Effect of noise on comparator circuits 2

BASIC COMPARATOR CIRCUITS v Effect of noise on comparator circuits 3

BASIC COMPARATOR CIRCUITS v Effect of noise on comparator circuits v. In many practical situation , noise appears on the input line v. The noise cause a comparator to erratically switch output state v How to reduce? v. One way to reduce the effect of noise is by using a comparator with positive feedback called Hysterisis also called as Schmitt trigger v. The positive feedback produce two separates trip points that prevents a noisy input from producing false transitions for example UTP and LTP – Hysterisis 4

BASIC COMPARATOR CIRCUITS Schmitt trigger symbol q The symbol for Schmitt triggers in electronic diagrams is a triangle with a hysteresis symbol: 5

SCHMITT TRIGGER +V Vin Vout -V - Inverting Schmitt trigger R 1 R 2 6

SCHMITT TRIGGER Upper Threshold Voltage (VUTP) When Vout = +Vsat, the fed-back voltage is called the upper-threshold voltage, VUT. l For Vin < VUTP Vout is locked at +Vsat l If Vin > VUTP Vout begins to drop in value until –Vsat l 7

SCHMITT TRIGGER Lower Threshold Voltage (VLTP) When Vout = -Vsat, the fed-back voltage is called the lower-threshold voltage, VLT. l For Vin > VLTP Vout is locked at -Vsat l If Vin < VLTP Vout will switch back to +Vsat l 8

SCHMITT TRIGGER 9

BASIC COMPARATOR CIRCUITS What is Hysteresis? l Technique of showing comparator performance on one graph instead of two graphs Vout Hysteresis voltage +Vsat Vin VLTP -Vsat VUTP 10

BASIC COMPARATOR CIRCUITS The difference in voltage between VUTP and VLTP is called hysteresis voltage, VHYS l Whenever any circuit changes from one state to second state at some input signal and then reverts from the second state to the first state at a different input signal, the circuit is said to exhibit hysteresis. l If VHYS > Vnoise peak-peak there will be no false output crossings l VHYS tells us how much peak-peak noise the circuit can withstand l 11

Non-inverting Schmitt trigger Vout +Vsat Vout VHYS Vin VLTP R 1 VHTP R 2 -Vsat 12

Non-inverting Schmitt trigger Example 1 l Vin If Vsat = 13. 5 V, what are the trip points and hysteresis in Figure below? +15 V Answer : Trip points Vout -15 V 47 k 1 k Answer : Hysteresis voltage 13

BASIC COMPARATOR CIRCUITS Window Comparator It detects when the input voltage is between two limits called window. l Use two comparators with different thresholds. l 14

Window Comparator Inverting Window Comparator VUTP +V Vout Vin -V +V Vout +Vsat Vin VLTP -V VLTP V UTP 15

Window Comparator Example 2 l In previous figure, the VLTP and VUTP are changed to +3. 5 V and +4. 75 V. If Vsat = 12 V and the input is a sine wave with a peak of 10 V, what is the output voltage waveform? Vout Output is low when input is between 3. 5 and 4. 75 V 12 V 3. 5 V 4. 75 V Vin 16

Comparator Applications Smoke Detector Bar graph Display Over-Temperature sensing circuit 17

Comparator Applications A/D converter 18