Download Output Swing Lab

Analog Electronics Workshop
Input/Output Limitations
Rev 0.2
March 13, 2013
What’s Wrong?
Common Mode Voltage Definition
Vcm 
Vp  Vn
2
+
+
Vo
+
–
Vn
Differential Gain  A dm
GND
+
+
Vid
-
GND
Differential
Amplifier
GND
Vid
2
+
–
Vp
+
–
+
–
Vcm
+
–
GND
Vid
2
+
Vo  Vid  Adm
GND
Input and Output Swing
Vcm Range
Vcm
10V
-
Output Swing
V+
Vout
+
V-
Vcm – Two Examples
Vcm = 0V
Vcm = 5V
R1 2k
RF 10k
R1 2k
RF 10k
V3 5
VEE 18
VEE 18
+
Vout
+
+
VCC 18
Vout
+
V3 5
VCC 18
Input Stage
Output Stage
+Vsupply
Vgs=0.9V
IS1
VIN-
Q2
VSAT
Vgs=0.9V
Q1
VIN+
Q3
Vsat=0.1V
Q4
Q2
to second
stage
VCE =0.6V
Q1
-Vsupply
VBE
VOUT
Translating the Data Sheet
Parameter
Conditions
Min
Typ
Max
Unit
(V+)-1.5
V
Input Voltage Range
(V-) – 0.1V
Common-Mode Voltage Range VCM
Output
Voltage Output Vout
RL = 10kΩ
20
50
-2.6V< Vcm < 1.0V
-2.95< Vout < 2.95V
-2.6V< Vcm < 1.0V
mV
Vcm or Output Swing Problem?
2.00
T
V+
V1 2.5
Vin
V-
V2 2.5
-
V-
+
+
Vin
Vout
-2.00
999.94m
+
U1 OPA735
V+
Vcm=Vin!
Vout
-1.99
0.00
500.00u
• Remember
• -2.95V<Vout<2.95V
• -2.6V<Vcm<1.0V
• What is the common-mode voltage?
• This is a Vcm violation!
1.00m
Time (s)
1.50m
2.00m
•Measurement
9
NI myDAQ Exercise-Vcm
• Populate U1 with OPA735
• Set J2 to 1-2 position
R6 DNP
R5 0
V-
++
+
7
3
AO(1)
J2
4
2
V+
6
U1-OPA735
U1Out
AO(0)
1
3
2
AI(0+)
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
a1
Understanding Operational Amplifier Limitations
and Long-Term Stability
By Marek Lis
Sr Application Engineer
Texas Instruments -Tucson
1
NI myDAQ Exercise-Vcm
TINA Results
Lab Results
1.50
T
Vin
-1.50
999.90m
Vout
-1.50
0.00
500.00u
1.00m
Time (s)
1.50m
2.00m
• Simulation
• Measurement
15
TINA Exercise
V+
R2 1k
R1 34.8k
+
V1 2.5
Vin
V2 2.5
V-
V-
+
Vout
+
U1 OPA277_TG
V+
TINA Exercise
• Vin Settings
• Triangular Wave
Settings
TINA Exercise
• Analysis->Transient
T
• View->Separate
Curves
80.00m
Vin
-80.00m
1.53
Vout
-2.32
0.00
500.00u
1.00m
Time (s)
1.50m
2.00m
Vcm or Output Swing Problem?
R1 34.8k
+
R2 1k
Vin
3
Vcm=0V!
++
7
2
4
V6
Vout
U2 OPA277
V+
Output Swing Violation!
• 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!
NI myDAQ Exercise-Output
Swing
• Populate U2 with OPA277
• Inverting Configuration
R7 34.8k
+
2
3
4
V-
AO(0)
-
++
7
R8 1k
V+
AI(1+)
6
U2-OPA277
NI myDAQ Exercises
Output Swing
• Stop Scope
• Settings
–
–
–
–
Source=AI(1)
Scale V/Div = 500mV
Time/Div = 200us
Trigger Type = Edge
• Run
NI myDAQ Exercise
Output Swing
• Stop FGEN
• Settings
– Triangle Wave
– Frequency=1kHz
– Amplitude=160mVpp
• Signal Route=AO(0)
• Run
NI myDAQ Exercise
Output Swing
TINA Results
T
Lab Results
80.00m
Vin
-80.00m
1.53
Vout
-2.32
0.00
500.00u
1.00m
Time (s)
1.50m
2.00m
24
1. For the circuit below: The output reads -260mV. The offset is 30uV typical.
Why is the output so large?
V1 2.5
+
VF1
-260.570012mV
+
U1 OPA140
V2 2.5
What causes the problem here?
1. For the circuit below: The output reads 171mV. The offset is 30uV typical.
Why is the output so large?
-
Vout2 171.4mV
VF2 171.4mV
+
+
U2 OPA140
V3 5
An Invalid Op Amp Configuration
(Output outside of the linear range)
Revisions
• 0.2
– Added homework
– Added Lis / Kay presentation on limits.