Schmitt trigger basics

1. Comparator and Schmitt Trigger

d frequent application in
Irumentation systems.

— Understand the Op-Amp Comparator with and without
an offset and its uses

— Understand the Schmitt Trigger and its benefits in
noise rejection and improved switching speed

Comparator and Schmidt Trigger K Craig 1

2. The prototype of op- u
amp comparator.

The circuit does not onl

3. nin that characterizes open-

E op-amp (A > 105), any small
[input voltages will cause large
| go into saturation at either
[voltage supply values and the
ifference.

e of this property to generate

shes E ST Vs

Consider the following.

A
Vin) I
int) @)

4. la

aan a D
analog-to-digital conven il <i ali
a continuous waveform ff Isela values | ui
comparator output con MIT

levels. |

A al:

15

You (t)

Lar

O)

02 03 04 05 06 07 08 0.9

Time, s

5. t an inverting comparator by
rting terminal to ground and
the inverting terminal.

Von)

Yin (1)
A

E

02 03 04 05 06 07 08 09 1
Time, s

6. — Comparator with Offset
« Asimple modificatioy
consists of connecti

one of the input ter |
reference voltage is I
level at which the col
extreme to the other

Comparator and Schmidt Trigger K Craig 6

7. f a comparator with a reference
input voltage of sin(wt).

0.1 02 03 04 05 06 0.7 08 09 1
Time, s

Note that the comparator output is no longer a

symmetric square wave.

Comparator and Schmidt Trigger

8. Another useful interpret “al Mi

bil Il i (ll
comparator can be tal li il
output transfer cod

on

9. sfer characteristic for a
ing type with a nonzero

vs

Vout

4
F
1

Vout À

Von

>
Vin

Veet

—Vsat

10. — Very often, in convertin
representation, one wol
other than +V.. Com
this type of switching ci

— Special-purpose integr
available that are speciälf
comparators. These tyhil
large inputs and have pro

desired reference voltage Ss

— An example is the LM3 ich provides an open-
collector output. The open- collector output allows t
user to connect the output transistor to any supply
voltage of choice by means of an external pull-up
resistor, thus completing the output circuit.

Comparator and Schmidt Trigger K.Craig 10

11. resistor is not critical, since
ed in the saturation mode;
undred and a few thousand

12. « Schmitt Trigger
— One of the typical applidé
comparator is in detecti
exceeds a present thre:
threshold is then repre
connected to the non-irl|
voltage source is conn

The presence of noise and
practical op-amps requires specia

Two improvements are discussed:

» How to improve the switching speed o
comparator.

« How to design a circuit that can operate correctly
even in the presence of noisy signals.

Comparator and Schmidt Trigger K Craig 12

13. 0.4 0.5
Time, s
Noisy input waveform

0.2 0.3 0.4 0.5 0.6 0.7
Time, s
Comparator output waveform

14. One very effective way ft

of the comparator is by a

Positive feedback can it
of the comparator and

same time.

Ra

Ta Ry Yt

Ry
Ri+R

15. about a voltage other than
je can also be connected to

positive-going transition

negative-going transition

16. |

— In effect, the Schmitt tri

| MIA
li
range equa | to + Veo [RUE il li I
comparator cannot swi “| ji | Il)
sh rene 20 pi il
range, the Schmitt rico] | AE

triggering.

i

ii

|
Ka a a =

N ia (il
0 0.1 0.2 03 0.4 0.5 0.6 0.7 0.8 0.9 1
Time, s
Noisy input waveform

aa

0.1 0.2 0.3 04 0.5 0.7 0.8 09 1

Schmitt trigger output waveform

Comparator and Schmidt Trigger K.Craig 16