1. COMPARATOR WITH HYSTERESIS WINDOW COMPARATOR
BASIC COMPARATOR CIRCUITS
COMPARATOR WITH HYSTERESIS
WINDOW COMPARATOR

2. BASIC COMPARATOR CIRCUITS
Effect of noise on comparator circuits

3. BASIC COMPARATOR CIRCUITS
Effect of noise on comparator circuits

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

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

6. SCHMITT TRIGGER R1 R2
Vin
Vout +V -V
- Inverting Schmitt trigger

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

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

9. SCHMITT TRIGGER

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

11. BASIC COMPARATOR CIRCUITS
The difference in voltage between VUTP and VLTP is called hysteresis voltage, VHYS
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.
If VHYS > Vnoise peak-peak  there will be no false output crossings
VHYS tells us how much peak-peak noise the circuit can withstand

12. Non-inverting Schmitt trigger
Vin
Vout
+Vsat
-Vsat
VHTP
VLTP
VHYS R1 R2
Vout
Vin

13. Non-inverting Schmitt trigger
Example 1
If Vsat = 13.5 V, what are the trip points and hysteresis in Figure below?
Vin
Vout
-15V
+15V
47 k
1 k
Answer :Trip points
Answer :Hysteresis voltage

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

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

16. Window Comparator Example 2
In previous figure, the VLTP and VUTP are changed to +3.5 V and V. If Vsat = 12 V and the input is a sine wave with a peak of 10 V, what is the output voltage waveform?
Vin
Vout
12V
3.5V
4.75V
Output is low when input is between 3.5 and 4.75V

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

18. Comparator Applications
A/D converter