Download What Does “Rail-to-Rail” Operation Really Mean?

What Does
“Rail-to-Rail” Operation
Really Mean?
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
1
Agenda
O
What does “Rail-to-Rail” output
operation mean
Amplifier Output Stage
O
Distortion
O
O
O
Techniques to Compare Rail-to-Rail Op
Amps
Microchip Op Amps and References
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
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Rail-to-Rail Operation
O
Rail means supply voltage (VDD and VSS)
Rail-to-Rail output of amplifier
O
In reality:
O
O
O
Output can not reach the rails
The difference from the rail is called
Headroom
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
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Amplifier Output Stage
VIN+
VDD
VOH
VOL
VOUT
O
Push-Pull output
O
Headroom
O
Transistor junction drop
VIN—
VSS
Headroom
O
When VOUT → VDD :
VDD – VOUT = VOH
© 2004 Microchip Technology Incorporated. All Rights Reserved.
O
When VOUT → VSS :
VOUT – VSS = VOL
What does “Rail-to-Rail Operation” really mean?
4
Amplifier Output Stage
O
Headroom increases with increasing
output current
Output Voltage Headroom
(mV typ.)
1,000
T A = 25°C
100
VOH
VOL
10
1
10µ
1. E - 0 5
1. E - 0 4
1. E - 0 3
100µ
1m
Output Current Magnitude (A)
© 2004 Microchip Technology Incorporated. All Rights Reserved.
1. E - 0 2
10m
What does “Rail-to-Rail Operation” really mean?
5
Output Distortion
Voltage
O
O
time
Signal distortion (clipping)
Approaching the rail distorts
the output
5.5
RL = 10kΩ
TA = 25°C
VOUT (V typ.)
5.0
VDD = 5V
4.5
VOUT
4.0
3.5
Time
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
6
Amplifier Non-Linearity
O
Non-linearity when VOUT → the rails
Input Offset Voltage
(µV typ.)
200
150
RL = 10kΩ
VDD = 5.5 V
TA = 25°C
100
50
0
-50
O
-100
-150
-200
-1.0
VCM = VSS
0.0
1.0
2.0
3.0
4.0
5.0
Non-linear
Regions
6.0
Output Voltage (V)
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
7
Amplifier Linearity
O
Open Loop Gain (AVOL) measurement range
O
provides the clue to determine the linear region
O
AVOL = ∆VOS/∆VOUT
O
Input Offset Voltage
(µV typ.)
200
150
100
50
0
-50
-100
VDD = 5.5 V
VCM = VSS
-150
two point measurement:
VOS → VOUT = 300mV
VOS → VOUT = VDD - 300mV
© 2004 Microchip Technology Incorporated. All Rights Reserved.
TA = 25°C
RL = 10kΩ
-200
-1.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Output Voltage (V)
(300mV for MCP6001)
What does “Rail-to-Rail Operation” really mean?
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Amplifier Linearity
O
What causes the distortion?
O
O
transistors no longer in linear operation
Linear Region
O
O
O
Specified in measurement conditions
Recommended headroom for MCP6001
O 300mV with 10kΩ load to VDD/2
sink/source current dependent
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
9
Rail-to-Rail Output Amplifiers
Comparison Techniques
O
O
O
Compare headroom vs. sink/source
current
Compare the AVOL measurement range
Do bench verification
O
Compare expected vs. measured output
voltage
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
10
Microchip Op Amps
Part number
MCP6271
MCP6281
MCP6291
MCP6S21
MCP6241
MCP6001
MCP6140
MCP6041
MCP6021
TC1034
MCP606
I/O
Range
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R I/O
R/R O
VDD (V)
Range
2.0-5.5
2.2-5.5
2.4-5.5
2.5-5.5
1.8-5.5
1.8-5.5
1.4-5.4
1.4-5.5
2.5-5.5
1.8-5.5
2.5-5.5
IQ (µA)
max
240
570
1300
1300
50
100
1
1
1400
8µA
25µA
MCP616
MCP601
R/R O
R/R O
2.3-5.6
2.7-5.5
25µA
325µA
R/R I/O → Rail-to-Rail Input/Output
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
11
Conclusion
O
O
O
O
Amplifier output can not reach the rails
Headroom is dependent on sink/source
current
Approaching the rails introduces distortion
Bench test device to determine linear
region
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
12
References
O
Application Notes
O
O
O
O
O
O
O
O
O
O
ADN009, What Does “Rail-to-Rail” Operation Really Mean?
ADN003, Select the Right Operational Amplifier for your Filtering
Circuits
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AN722, Operational Amplifier Topologies and DC Specifications
AN723, Operational Amplifier AC Specifications and Applications
AN695, Interfacing Pressure Sensors to Microchip’s Analog
Peripherals
AN737, Using Digital Potentiometers to Design Low Pass
Adjustable Filters
AN246, Driving the Analog Inputs of a SAR A/D Converter
AN688, Layout Tips for 12-Bit A/D Converter Application
AN884, Driving Capacitive Loads With Op Amps
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
13
WebSeminar:
April 21, 2004
What Does
“Rail-to-Rail” Operation
Really Mean?
© 2004 Microchip Technology Incorporated. All Rights Reserved.
What does “Rail-to-Rail Operation” really mean?
14