Transcript comparator - UniMAP Portal

BASIC COMPARATOR
CIRCUITS
 COMPARATOR WITH HYSTERESIS
 WINDOW COMPARATOR
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BASIC COMPARATOR
CIRCUITS
 Effect of noise on comparator circuits
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BASIC COMPARATOR
CIRCUITS
 Effect of noise on comparator circuits
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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
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BASIC COMPARATOR
CIRCUITS
Schmitt trigger symbol
 The symbol for Schmitt triggers in
electronic diagrams is a triangle with a
hysteresis symbol:
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SCHMITT TRIGGER
+V
Vin
-
Vout
+
-V
- Inverting Schmitt
trigger
R1
R2
VUTP
R2
 Vsat 

R1  R2
VLTP
R2
 Vsat 

R1  R2
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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
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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
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SCHMITT TRIGGER
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BASIC COMPARATOR
CIRCUITS
What is Hysteresis?
 Technique
of
showing
comparator
performance on one graph instead of two
graphs
Vout
Hysteresis
voltage
+Vsat
Vin
VLTP
-Vsat
VUTP
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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.
VHYS  VUTP  VLTP
 If VHYS > Vnoise peak-peak  there will be no false
output crossings
 VHYS tells us how much peak-peak noise the
circuit can withstand
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Non-inverting Schmitt
trigger
Vout
+Vsat
Vout
VHYS
+
Vin
VLTP
Vin
R1
VHTP
R2
-Vsat
VUTP
R1
 Vsat 

R2
VLTP
R1
 Vsat 

R2
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Non-inverting Schmitt
trigger
Example 1
 If Vsat = 13.5 V, what are the trip points
and hysteresis in Figure below?
Vin
+15V
-
VUTP
1 k
 13.5V   0.281V

48 k
VLTP
1 k
 13.5V   0.281V

48 k
Vout
+
-15V
47 k
1 k
Answer :Trip points
Answer :Hysteresis voltage
VHYS  VUTP  VLTP  0.281V   0.281V   0.562V
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BASIC COMPARATOR
CIRCUITS
Window Comparator
 It detects when the input voltage is
between two limits called window.
 Use two comparators with different
thresholds.
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Window Comparator
Inverting Window Comparator
+V
VUTP
+
Vout
-
-V
+V
Vin
Vout
+Vsat
+
VLTP
Vin
-
-V
VLTP V
UTP
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Window Comparator
Example 2
 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 4.75V
12V
3.5V 4.75V
Vin
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Comparator Applications
Smoke Detector
Bar graph Display
Over-Temperature sensing circuit
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Comparator Applications
A/D converter
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