cmos schmitt trigger test presentation

7/27/2019 CMOS Schmitt Trigger Test Presentation

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CMOS Schmitt TriggerTest Circuit

Mitchell Belser, P.E.

Visiting Instructor

Department of Computer EngineeringJackson State University

[email protected]


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What is a Schmitt Trigger

A type of comparator.

A comparator is an electronic circuit

whose output state changes when its input

reaches a certain value.

A comparators output state will change at

the same input value whether the input

voltage is increasing or decreasing.

A noisy signal can cause the output to

change state randomly.


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Schmitt Trigger Operation

The Schmitt trigger is a comparator circuit thathas two threshold voltages.

When the input is higher than the upper

threshold, the output is high. When the input is lower than the lower threshold,

the output is low.


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Schmitt Trigger Operation

The Schmitt trigger employs positivefeedback.

When the input is between the two

thresholds the output state does notchange.

The difference between the two thresholdsis the hysteresis.


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Schmitt Trigger Transfer Curve


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Schmitt Trigger History

The Schmitt trigger was first invented by Otto H.Schmitt in the 1934.

Vacuum Tube based design.

A Thermionic Trigger, Journal of ScientificInstruments, 15 January 1938 pg 24-26


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Schmitt Trigger Implementations

Comparator with positive feedback

Bipolar transistor Schmitt trigger

CMOS transistor Schmitt trigger


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Comparator with Positive Feedback

U1

R11

R12

Vin

output

Comparator

with Positive Feedback


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Bipolar Schmitt Trigger

Upper switching point

voltage:

Vth = R4/(R4+R3)*Vdd

Lower switching pointvoltage:

Vtl = R4/(R4+R2)*Vdd

Q1

Q2R1

R2 R3

R4 R5

vdd

input

output

BJT Schmitt Trigger


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CMOS Schmitt Trigger

Upper switching point

voltage:

1/ 3 = W1L3 / W3L1

Lower switching pointvoltage:

5/ 6 = W5L6 / W6L5

M1

M2

M3

M4

M5

M6

GND

GND

VDD

VDD

U1A U3A U4Aoutputinput


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Applications

Input buffers for digital signals.

Crystal oscillator circuits.

Clean up noisy signals (switch debounce).

Speed up signals with slow edges.


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Problem Description and Motivation

for Improvement

The conventional method for characterizing a CMOS Schmitt trigger is to ramp up the

input voltage until the output state changes. The input voltage that results in the

output changing state is the high threshold voltage VTH. In a similar manner, the low

threshold voltage VTL is determined by reducing the input voltage until a state

change occurs at the output. This method is widely used. The major draw back is

the time that is required to search for the input voltage that results in a state change

at the output.

The proposed method would eliminate the need to perform a ramp search and thus

greatly reduce the time needed to determine the switching threshold voltage levels.


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Specific Improvements

Innovation : A CMOS Schmitt trigger circuit containing additional test circuitry that

allows trigger voltage levels to be detected at the input without needing to ramp the

input voltage.

Important components of innovation: The test circuitry consists of combinational

logic and a CMOS switch. Digital input signals determine whether the high or low

threshold voltage is measurable.


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Traditional Schmitt Trigger Circuit

M1

M2

M3

M4

M5

M6

GND

GND

VDD

VDD

U1A U3A U4Aoutputinput


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Schmitt Trigger Showing Effect of

Test Circuitry

Test circuitry is shown in blue.

Asserting test mode and vih input

results in sout being shorted to the

input. The gates of the feedback

transistors are driven to vdd. The

input is regulated to the highthreshold voltage VTH.

Asserting test mode and vil input

results in sout being shorted to the

input. The gates of the feedback

transistors are driven to gnd. The

input is regulated to the low

threshold voltage VTL.

In normal operating mode the

test circuitry has no effect on

Schmitt trigger operation.

M1

M2

M3

M4

M5

M6

GND

GND

VDD

VDD

U4Aoutputinput

U2A

7408J

U5A

7408J

U6A

7402N

U7A

7402N

U8A

7432N

VTL

VTH

Test_en

M7 M8 U1A

sout

soutsout

input


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Advantages of This Method

Typical method requires input to be driven and output voltage to be monitored in order todetermine the switching threshold voltage levels.

Test circuitry allow thresholds to be measured at the input without driving input ormonitoring the output.

Simply setting a digital input causes the threshold voltage to be present on the input pinof the Schmitt trigger. .

Simplifies test program and reduces test time resulting in increased profit margin.


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Measurement Data

Part VDD STATIC RAMP Frequency

C VCL-A4-5 VTH VTL VTH VTL

5 V 4.21 2.92 4.2 3.08 1

4.24 3.04 1k

3.3 V 2.63 1.69 2.64 1.84 1

2.64 1.76 1k

A VCL-A4-3 5 V 4.22 2.98 4.24 3.12 1

4.28 3.06 1k

3.3 V 2.64 1.73 2.66 1.80 1

2.64 1.72 1k


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Measurement Waveforms

VDD= 3.3V

Ramp frequency =

1kHz


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Summary

Schmitt trigger threshold test circuit.

Measure rising and falling threshold levels.

Ramping input and monitoring output state is no

longer necessary. Reduction in test time.

Increase in profit due to reduction in test time.

cmos schmitt trigger test presentation

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