Analog Electronics WorkshopInput/Output Limitations

Rev 0.2

March 13, 2013

What’s Wrong?

Common Mode Voltage Definition

+

+

–

+

–

GNDGND

pV nV

GND

+

-

oV

2

VVV

npcm

+

+

– 2

VidGND

+

-dmido AVV

+

–

GND

+

–

2

Vid

cmV

+

-idV

DifferentialAmplifier

dmAGain alDifferenti

+

-V+

V-

10V

Vcm

Input and Output Swing

Vcm Range

Vout

Output Swing

Vcm – Two Examples

VEE 18

VCC 18

RF 10k

Vout

-

+ +

R1 2k

V3 5

VEE 18

VCC 18

RF 10k

Vout

-

+ +

R1 2k

V3 5

Vcm = 0V Vcm = 5V

Input Stage

Q 1

Q 2

V O UTVBE

VSAT

Output Stage

Q4Q3

IS1

Q1Q2

-Vsupply

+Vsupply

VIN+

VIN-

to second stage

VCE =0.6V

Vsat=0.1V

Vgs=0.9V

Vgs=0.9V

Translating the Data Sheet

-2.6V< Vcm < 1.0V

-2.95< Vout < 2.95V

-2.6V< Vcm < 1.0V

Parameter Conditions Min Typ Max Unit

Input Voltage Range

Common-Mode Voltage Range VCM (V-) – 0.1V (V+)-1.5 V

Output

Voltage Output Vout RL = 10kΩ 20 50 mV

Vcm or Output Swing Problem?V+

V-

V1 2.5

V2 2.5

V+

V-

+

-

+

U1 OPA735

Vout

+

Vin

• Remember• -2.95V<Vout<2.95V• -2.6V<Vcm<1.0V

• What is the common-mode voltage?• This is a Vcm violation!

Vcm=Vin!

T

Time (s)0.00 500.00u 1.00m 1.50m 2.00m

Vin

-2.00

2.00

Vout

-1.99

999.94m

opa735.TSC

Vcm Lab •Measurement

9

NI myDAQ Exercise-Vcm

• Populate U1 with OPA735• Set J2 to 1-2 position

V+

V-

R5 0

-

+ +3

2

6

74

U1-OPA735

+

AO(1)

R6 DNP

J2

AI(0+)U1OutAO(0)

12

3

NI myDAQ Exercise-Vcm

• Launch Scope• Scope Settings

– Scale V/Div = 500mV– Time/Div = 200us– Trigger Type = Edge

• Run

NI myDAQ Exercise-Vcm

• Launch FGEN• FGEN Settings

– Triangle Wave– Frequency=1kHz– Amplitude=4Vpp– Signal Route=AO(1)

• Run

Further Reading

1

Understanding Operational Amplifier Limitations

and Long-Term Stability

By Marek LisSr Application Engineer

Texas Instruments -Tucson

a1

NI myDAQ Exercise-Vcm

T

Time (s)0.00 500.00u 1.00m 1.50m 2.00m

Vin

-1.50

1.50

Vout

-1.50

999.90m

TINA Results Lab Results

Output Swing Lab

• Simulation • Measurement

15

TINA Exercise

V+

V-

V1 2.5

V2 2.5

V+

V-

Vout+

Vin

R1 34.8kR2 1k

-

++

U1 OPA277_TG

opa277.TSC

TINA Exercise

• Vin Settings • Triangular Wave Settings

TINA Exercise

• Analysis->Transient

• View->Separate Curves

T

Time (s)

0.00 500.00u 1.00m 1.50m 2.00m

Vin

-80.00m

80.00m

Vout

-2.32

1.53

Vcm or Output Swing Problem?

• For OPA277• -2.0V<Vout<1.3V (RL=10k)• -0.5V<Vcm<0.5V • What is the common-mode voltage?

• We can’t violate Vcm in this configuration!

Vcm=0V!

Output Swing Violation!

V+

V-

Vout+

Vin

R1 34.8kR2 1k

-

+ +3

2

6

74

U2 OPA277

NI myDAQ Exercise-Output Swing

• Populate U2 with OPA277• Inverting Configuration

V+

V-

R7 34.8k

-

+ +3

2

6

74

U2-OPA277

R8 1k

+

AO(0)

AI(1+)

NI myDAQ ExercisesOutput Swing

• Stop Scope• Settings

– Source=AI(1)– Scale V/Div = 500mV– Time/Div = 200us– Trigger Type = Edge

• Run

NI myDAQ ExerciseOutput Swing

• Stop FGEN• Settings

– Triangle Wave– Frequency=1kHz– Amplitude=160mVpp

• Signal Route=AO(0)• Run

NI myDAQ ExerciseOutput Swing

TINA Results Lab Results

T

Time (s)

0.00 500.00u 1.00m 1.50m 2.00m

Vin

-80.00m

80.00m

Vout

-2.32

1.53

I/O Limits Homework

24

1. For the circuit below: The output reads -260mV. The offset is 30uV typical. Why is the output so large?

-

+ +

U1 OPA140

V1 2.5

V2 2.5

VF1

What causes the problem here?

-260.570012mV

1. For the circuit below: The output reads 171mV. The offset is 30uV typical. Why is the output so large?

V3 5

Vout2-

+ +

U2 OPA140

VF2

V1 5

Vout1-

+ +

U1 OPA140

Vos

RF 1kRIN 1k

V2 -2.5

A Valid Op Amp Configuration (Input and Output within the linear range)

An Invalid Op Amp Configuration (Output outside of the linear range)

26.7uV2.5V 171.4mV

171.4mV

Revisions

• 0.2– Added homework– Added Lis / Kay presentation on limits.