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Analog Electronics WorkshopInput/Output Limitations
Rev 0.2
March 13, 2013
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What’s Wrong?
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Common Mode Voltage Definition
+
+
–
+
–
GNDGND
pV nV
GND
+
-
oV
2
VVV
npcm
+
+
– 2
VidGND
+
-dmido AVV
+
–
GND
+
–
2
Vid
cmV
+
-idV
DifferentialAmplifier
dmAGain alDifferenti
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+
-V+
V-
10V
Vcm
Input and Output Swing
Vcm Range
Vout
Output Swing
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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
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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
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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
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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
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Vcm Lab •Measurement
9
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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
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NI myDAQ Exercise-Vcm
• Launch Scope• Scope Settings
– Scale V/Div = 500mV– Time/Div = 200us– Trigger Type = Edge
• Run
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NI myDAQ Exercise-Vcm
• Launch FGEN• FGEN Settings
– Triangle Wave– Frequency=1kHz– Amplitude=4Vpp– Signal Route=AO(1)
• Run
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Further Reading
1
Understanding Operational Amplifier Limitations
and Long-Term Stability
By Marek LisSr Application Engineer
Texas Instruments -Tucson
a1
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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
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Output Swing Lab
• Simulation • Measurement
15
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TINA Exercise
V+
V-
V1 2.5
V2 2.5
V+
V-
Vout+
Vin
R1 34.8kR2 1k
-
++
U1 OPA277_TG
opa277.TSC
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TINA Exercise
• Vin Settings • Triangular Wave Settings
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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
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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
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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+)
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NI myDAQ ExercisesOutput Swing
• Stop Scope• Settings
– Source=AI(1)– Scale V/Div = 500mV– Time/Div = 200us– Trigger Type = Edge
• Run
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NI myDAQ ExerciseOutput Swing
• Stop FGEN• Settings
– Triangle Wave– Frequency=1kHz– Amplitude=160mVpp
• Signal Route=AO(0)• Run
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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
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I/O Limits Homework
24
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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
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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
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Revisions
• 0.2– Added homework– Added Lis / Kay presentation on limits.