LM6211
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SNOSAH2C – FEBRUARY 2006 – REVISED MARCH 2013
LM6211 Low Noise, RRO Operational Amplifier with CMOS Input and 24V Operation
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FEATURES
1
(Typical 24V Supply Unless Otherwise Noted)
2
•
•
•
•
•
•
•
Supply Voltage Range 5V to 24V
Input Referred Voltage Noise 5.5 nV/√Hz
Unity Gain Bandwidth 20 MHz
1/f Corner Frequency 400 Hz
Slew Rate 5.6 V/μs
Supply Current 1.05 mA
Low Input Capacitance 5.5 pF
•
•
•
Temperature Range -40°C to 125°C
Total Harmonic Distortion 0.01% @ 1 kHz,
600Ω
Output Short Circuit Current 25 mA
APPLICATIONS
•
•
•
•
PLL Loop Filters
Low Noise Active Filters
Strain Gauge Amplifiers
Low Noise Microphone Amplifiers
DESCRIPTION
The LM6211 is a wide bandwidth, low noise op amp with a wide supply voltage range and a low input bias
current. The LM6211 operates with a single supply voltage of 5V to 24V, is unity gain stable, has a groundsensing CMOS input stage, and offers rail-to-rail output swing.
The LM6211 is designed to provide optimal performance in high voltage, low noise systems. The LM6211 has a
unity gain bandwidth of 20 MHz and an input referred voltage noise density of 5.5 nV/√Hz at 10 kHz. The
LM6211 achieves these specifications with a low supply current of only 1 mA. The LM6211 has a low input bias
current of 2.3 pA, an output short circuit current of 25 mA and a slew rate of 5.6 V/us. The LM6211 also features
a low common-mode input capacitance of 5.5 pF which makes it ideal for use in wide bandwidth and high gain
circuits. The LM6211 is well suited for low noise applications that require an op amp with very low input bias
currents and a large output voltage swing, like active loop-filters for wide-band PLLs. A low total harmonic
distortion, 0.01% at 1 kHz with loads as high as 600Ω, also makes the LM6211 ideal for high fidelity audio and
microphone amplifiers.
The LM6211 is available in the small SOT-23 package, allowing the user to implement ultra-small and cost
effective board layouts.
Typical Application
1000
CHARGE
PUMP
OUTPUT
VCO
+
INPUT
VS_PLL
2
VOLTAGE NOISE (nV/ Hz)
VS = 5V, 24V
100
10
1
1
10
100
1k
10k
100k
FREQUENCY (Hz)
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2006–2013, Texas Instruments Incorporated
LM6211
SNOSAH2C – FEBRUARY 2006 – REVISED MARCH 2013
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Absolute Maximum Ratings (1) (2)
ESD Tolerance (3)
Human Body Model
2000V
Machine Model
200V
VIN Differential
±0.3V
Supply Voltage (VS = V+ – V−)
25V
+
V +0.3V, V −0.3V
Voltage at Input/Output pins
−65°C to +150°C
Storage Temperature Range
Junction Temperature (4)
+150°C
Soldering Information
(1)
(2)
(3)
(4)
−
Infrared or Convection (20 sec)
235°C
Wave Soldering Lead Temp. (10 sec)
260°C
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For ensured specifications and the test
conditions, see the Electrical Characteristics Tables.
If Military/Aerospace specified devices are required, please contact the TI Sales Office/ Distributors for availability and specifications.
Human Body Model is 1.5 kΩ in series with 100 pF. Machine Model is 0Ω in series with 200 pF.
The maximum power dissipation is a function of TJ(MAX), θJA, and TA. The maximum allowable power dissipation at any ambient
temperature is PD = (TJ(MAX) - TA)/θJA . All numbers apply for packages soldered directly onto a PC board.
Operating Ratings (1)
−40°C to +125°C
Temperature Range
Supply Voltage (VS = V+ – V−)
5V to 24V
Package Thermal Resistance (θJA (2))
(1)
(2)
5-Pin SOT-23
178°C/W
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for
which the device is intended to be functional, but specific performance is not ensured. For ensured specifications and the test
conditions, see the Electrical Characteristics Tables.
The maximum power dissipation is a function of TJ(MAX), θJA, and TA. The maximum allowable power dissipation at any ambient
temperature is PD = (TJ(MAX) - TA)/θJA . All numbers apply for packages soldered directly onto a PC board.
5V Electrical Characteristics (1)
Unless otherwise specified, all limits are ensured for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2. Boldface limits apply at
the temperature extremes.
Symbol
Parameter
Conditions
Min (2)
Typ (3)
Max (2)
Units
0.1
±2.5
±2.8
mV
VOS
Input Offset Voltage
VCM = 0.5V
TC VOS
Input Offset Average Drift
VCM = 0.5V (4)
IB
Input Bias Current
VCM = 0.5V (5) (6)
IOS
Input Offset Current
VCM = 0.5V
CMRR
Common Mode Rejection Ratio
0 V ≤ VCM ≤ 3V
0.4 V ≤ VCM ≤ 2.3 V
83
70
98
PSRR
Power Supply Rejection Ratio
V+ = 5V to 24V, VCM = 0.5V
85
78
98
V = 4.5V to 25V, VCM = 0.5V
80
95
CMRR ≥ 65 dB
CMRR ≥ 60 dB
0
0
+
CMVR
(1)
(2)
(3)
(4)
(5)
(6)
2
Input Common-Mode Voltage
Range
μV/C
2
0.5
5
10
0.1
pA
nA
pA
dB
dB
3.3
2.4
V
Electrical table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device.
Limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlations using the
Statistical Quality Control (SQC) method.
Typical values represent the most likely parametric norm at the time of characterization.
Offset voltage average drift is determined by dividing the change in VOS at the temperature extremes into the total temperature change.
Positive current corresponds to current flowing into the device.
Input bias current is ensured by design.
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5V Electrical Characteristics(1) (continued)
Unless otherwise specified, all limits are ensured for TA = 25°C, V+ = 5V, V− = 0V, VCM = VO = V+/2. Boldface limits apply at
the temperature extremes.
Symbol
AVOL
VO
Large Signal Voltage Gain
Output Swing High
Output Swing Low
IOUT
Output Short Circuit Current
IS
Slew Rate
GBW
Gain Bandwidth Product
en
Input-Referred Voltage Noise
in
VO = 0.35V to 4.65, RL = 2 kΩ to V /2
82
80
110
VO = 0.25V to 4.75, RL = 10 kΩ to V+/2
85
82
110
+
THD
Total Harmonic Distortion
Max (2)
50
150
165
RL = 10 kΩ to V+/2
20
85
90
RL = 2 kΩ to V+/2
39
150
170
RL = 10 kΩ to V+/2
13
85
90
Sourcing to V+/2
VID = 100 mV (7)
13
10
16
Sinking to V+/2
VID = −100 mV (7)
20
10
30
0.96
(8)
1.10
1.25
mA
17
MHz
f = 1 kHz
6.0
AV = 2, RL = 600Ω to V /2
mA
V/μs
5.5
+
mV from
rail
5.5
f = 10 kHz
f = 1 kHz
Units
dB
RL = 2 kΩ to V+/2
AV = +1, 10% to 90%
Input-Referred Current Noise
(8)
Typ (3)
Conditions
Supply Current
SR
(7)
Min (2)
Parameter
nV/√Hz
0.01
pA/√Hz
0.01
%
The device is short circuit protected and can source or sink its limit currents continuously. However, care should be taken such that
when the output is driving short circuit currents, the inputs do not see more than ±0.3V differential voltage.
Slew rate is the average of the rising and falling slew rates.
24V Electrical Characteristics (1)
Unless otherwise specified, all limits are ensured for TA = 25°C, V+ = 24V, V− = 0V, VCM = VO = V+/2. Boldface limits apply at
the temperature extremes.
Symbol
Parameter
Conditions
Min (2)
Typ (3)
Max (2)
Units
0.25
±2.7
±3.0
mV
VOS
Input Offset Voltage
VCM = 0.5V
TC VOS
Input Offset Average Drift
VCM = 0.5V (4)
IB
Input Bias Current
VCM = 0.5V (5)
IOS
Input Offset Current
VCM = 0.5V
CMRR
Common Mode Rejection Ratio
0 ≤ VCM ≤ 21V
0.4 ≤ VCM ≤ 20V
85
70
105
PSRR
Power Supply Rejection Ratio
V+ = 5V to 24V, VCM = 0.5V
85
78
98
V+ = 4.5V to 25V, VCM = 0.5V
80
98
CMRR ≥ 65 dB
CMRR ≥ 60 dB
0
0
CMVR
(1)
(2)
(3)
(4)
(5)
(6)
Input Common-Mode Voltage
Range
μV/C
±2
(6)
2
25
10
0.1
pA
nA
pA
dB
dB
21.5
20.5
V
Electrical table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very
limited self-heating of the device.
Limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlations using the
Statistical Quality Control (SQC) method.
Typical values represent the most likely parametric norm at the time of characterization.
Offset voltage average drift is determined by dividing the change in VOS at the temperature extremes into the total temperature change.
Positive current corresponds to current flowing into the device.
Input bias current is ensured by design.
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24V Electrical Characteristics(1) (continued)
Unless otherwise specified, all limits are ensured for TA = 25°C, V+ = 24V, V− = 0V, VCM = VO = V+/2. Boldface limits apply at
the temperature extremes.
Symbol
AVOL
VO
Large Signal Voltage Gain
Output Swing High
Output Swing Low
IOUT
Min (2)
Typ (3)
VO = 1.5V to 22.5V, RL = 2 kΩ to V /2
82
77
120
VO = 1V to 23V, RL = 10 kΩ to V+/2
85
82
120
Parameter
Output Short Circuit Current
IS
Supply Current
SR
Slew Rate
GBW
Gain Bandwidth Product
en
Input-Referred Voltage Noise
Conditions
+
Max (2)
dB
RL = 2 kΩ to V+/2
212
400
520
RL = 10 kΩ to V+/2
48
150
165
RL = 2 kΩ to V+/2
150
350
420
RL = 10 kΩ to V+/2
38
150
170
Sourcing to V+/2
VID = 100 mV (7)
20
15
25
Sinking to V+/2
VID = −100 mV
30
20
38
(7)
1.05
Units
mV from
rail
mA
1.25
1.40
mA
AV = +1, VO = 18 VPP
10% to 90% (8)
5.6
V/μs
20
MHz
f = 10 kHz
5.5
f = 1 kHz
6.0
nV/√Hz
in
Input-Referred Current Noise
f = 1 kHz
0.01
pA/√Hz
THD
Total Harmonic Distortion
AV = 2, RL = 2 kΩ to V+/2
0.01
%
(7)
(8)
The device is short circuit protected and can source or sink its limit currents continuously. However, care should be taken such that
when the output is driving short circuit currents, the inputs do not see more than ±0.3V differential voltage.
Slew rate is the average of the rising and falling slew rates.
Connection Diagram
VOUT
V
-
5
1
2
+
IN+
+
V
3
4
IN-
Figure 1. 5-Pin SOT-23 - Top View
4
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Typical Performance Characteristics
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
Supply Current
vs.
Supply Voltage
VOS
vs.
Supply Voltage
0.8
1.4
1.2
125°C
0.6
125°C
0.4
25°C
1.1
25°C
0.2
1
0.9
VOS (mV)
SUPPLY CURRENT (mA)
1.3
-40°C
0.8
0
-40°C
-0.2
0.7
-0.4
0.6
-0.6
0.5
5
7
11 13 15 17 19 21 23 25
9
5
7
11 13 15 17 19 21 23 25
9
VS (V)
VS (V)
Figure 2.
Figure 3.
VOS
vs.
VCM
VOS
vs.
VCM
1
1
VS = 5V
0.8
125°C
0.8
0.6
125°C
0.6
0.2
25°C
VOS (mV)
VOS (mV)
0.4
0
-40°C
-0.2
-0.4
25°C
0.4
0.2
-40°C
0
-0.2
-0.6
-0.4
-0.8
VS = 24V
-0.6
-1
0
0.5
1
1.5
2
2.5
3
3.5
0
2
4
6
Figure 4.
Figure 5.
Input Bias Current
vs.
VCM
Input Bias Current
vs.
VCM
1
2.5
VS = 5V
VS = 5V
2
0.5
1.5
-40°C
1
IBIAS (nA)
0
IBIAS (pA)
8 10 12 14 16 18 20 22
VCM (V)
VCM (V)
-0.5
-1
0.5
0
-0.5
-1
25°C
-1.5
-1.5
125°C
-2
-2
0
0.5
1
1.5
2
2.5
3
3.5
4
VCM (V)
-2.5
0
0.5
1
1.5
2
2.5
3
3.5
4
VCM (V)
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
Input Bias Current
vs.
VCM
Input Bias Current
vs.
VCM
4
2
VS = 24V
VS = 24V
2
-40°C
0
IBIAS (nA)
IBIAS (pA)
0
-2
25°C
-4
125°C
-2
-4
-6
-8
0
2
4
6
8
-6
10 12 14 16 18 20 22
0
4
6
8
10 12 14 16 18 20 22
VCM (V)
Figure 8.
Figure 9.
Sourcing Current
vs.
Supply Voltage
Sinking Current
vs.
Supply Voltage
30
50
25°C
45
25
-40°C
40
-40°C
35
20
ISINK (mA)
ISOURCE (mA)
2
VCM (V)
125°C
15
10
30
25°C
25
125°C
20
15
10
5
5
0
0
4
6
8
10 12 14 16 18 20 22 24
4
10 12 14 16 18 20 22 24
VS (V)
Figure 11.
Positive Output Swing
vs.
Supply Voltage
Negative Output Swing
vs.
Supply Voltage
60
RL = 10 k:
RL = 10 k:
60
50
125°C
50
VOUT FROM RAIL (mV)
VOUT FROM RAIL (mV)
8
Figure 10.
70
25°C
40
30
-40°C
20
25°C
125°C
40
30
20
-40°C
10
10
0
0
4
6
8
10 12 14 16 18 20 22 24
VS (V)
Figure 12.
6
6
VS (V)
4
6
8
10 12 14 16 18 20 22 24
VS (V)
Figure 13.
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Typical Performance Characteristics (continued)
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
Positive Output Swing
vs.
Supply Voltage
Negative Output Swing
vs.
Supply Voltage
350
250
RL = 2 k:
RL = 2 k:
VOUT FROM RAIL (mV)
VOUT FROM RAIL (mV)
300
250
125°C
25°C
200
150
100
125°C
25°C
150
100
-40°C
50
-40°C
50
200
0
0
4
6
8
4
10 12 14 16 18 20 22 24
6
8
10 12 14 16 18 20 22 24
VS (V)
VS (V)
Figure 14.
Figure 15.
Sourcing Current
vs.
Output Voltage
Sinking Current
vs.
Output Voltage
40
20
VS = 5V
VS = 5V
18
35
-40°C
16
-40°C
30
25°C
ISINK (mA)
ISOURCE (mA)
14
12
125°C
10
8
25
25°C
20
125°C
15
6
10
4
5
2
0
0
0
1
2
3
4
5
0
2
3
4
VOUT (V)
Figure 16.
Figure 17.
Sourcing Current
vs.
Output Voltage
Sinking Current
vs.
Output Voltage
30
5
50
VS = 24V
VS = 24V
45
-40°C
25
25°C
40
-40°C
35
20
15
ISINK (mA)
ISOURCE (mA)
1
VOUT (V)
125°C
10
30
25°C
25
125°C
20
15
10
5
5
0
0
0
4
8
12
16
20
24
0
4
8
12
VOUT (V)
VOUT (V)
Figure 18.
Figure 19.
16
20
24
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Typical Performance Characteristics (continued)
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
180
160
160
160
140
140
120
120
100
100
PHASE
120
RL = 2k:, 10 k:, 10M:
100
80
80
60
60
40
GAIN
40
GAIN (dB)
180
160
PHASE (°)
180
140
GAIN (dB)
Open Loop Gain and Phase with Capacitive Load
180
0
-40
100
1k
10k 100k
10M
1M
GAIN
60
CL = 100 pF
40
40
20
20
0
-20
-20
-40
100M
-40
100
80
CL = 50 pF
60
0
RL = 2k:, 10 k:, 10M:
-20
120
CL = 20 pF
80
20
20
140
PHASE
PHASE (°)
Open Loop Gain and Phase with Resistive Load
0
CL = 20 pF, 50 pF, 100 pF
100
1k
10k 100k
1M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 20.
Figure 21.
Input Referred Voltage Noise
vs.
Frequency
THD+N
vs.
Frequency
-20
10M
-40
100M
0.1
1000
0.01
100
RL = 600:, VS = 5V
THD+N (%)
VOLTAGE NOISE (nV/ Hz)
VS = 5V, 24V
RL = 600:, VS = 24V
RL = 100 k:, VS = 5V
0.001
10
RL = 100 k:, VS = 24V
0.0001
10
1
1
1k
100
10
10k
100k
100
FREQUENCY (Hz)
Figure 23.
THD+N
vs.
Output Amplitude
THD+N
vs.
Output Amplitude
100k
1
VS = 5V
Vs = 24V
0.1
THD+N (%)
0.1
THD+N (%)
10k
Figure 22.
1
0.01
RL = 600:
0.01
RL = 600:
0.001
0.001
RL = 100 k:
RL = 100 k:
0.0001
0.001
0.01
0.1
1
10
0.0001
0.001
0.01
0.1
1
10
100
OUTPUT AMPLITUDE (V)
OUTPUT AMPLITUDE (V)
Figure 24.
8
1k
FREQUENCY (Hz)
Figure 25.
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Typical Performance Characteristics (continued)
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
Slew Rate
vs.
Supply Voltage
Overshoot and Undershoot
vs.
Capacitive Load
6
OVERSHOOT AND UNDERSHOOT (%)
70
SLEW RATE (V/Ps)
5.8
5.6
FALLING EDGE
5.4
5.2
RISING EDGE
5
4.8
4.6
4.4
5
7
9
OVERSHOOT %
60
50
UNDERSHOOT %
40
30
11 13 15 17 19 21 23 25
15
25
35
45
55
VS (V)
CAPACITIVE LOAD (pF)
Figure 26.
Figure 27.
Small Signal Transient Response
Large Signal Transient Response
6
0.015
VS = 24V
CL = 10 pF
0.01
4
2
VOUT (V)
VOUT (V)
0.005
0
0
-0.005
-2
-0.01
-4
-0.015
-6
0
1
2
3
4
5
6
7
8
9
10
VS = 24V
CL = 10 pF
0
1
2
3
TIME (Ps)
4
5
6
7
Figure 29.
Phase Margin
vs.
Capacitive Load (Stability)
Phase Margin
vs.
Capacitive Load (Stability)
VS = 24V
50
50
RL = 2 k:
PHASE MARGIN (°)
PHASE MARGIN (°)
10
60
VS = 5V
40
RL = 10 k:
30
RL = 10 M:
10
0
10
9
Figure 28.
60
20
8
TIME (Ps)
RL = 2 k:
40
RL = 10 k:
30
RL = 10 M:
20
10
100
1000
0
10
100
CAPACITIVE LOAD (pF)
CAPACITIVE LOAD (pF)
Figure 30.
Figure 31.
1000
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Typical Performance Characteristics (continued)
Unless otherwise specified, TA = 25°C, VS = 24V, V+ = VS, V− = 0 V, VCM = VS/2.
Closed Loop Output Impedance
vs.
Frequency
PSRR
vs.
Frequency
100
0
-20
10
VS = 5V, -PSRR
PSRR (dB)
ZOUT (:)
-40
1
0.1
VS = 5V
VS = 24V, -PSRR
-60
-80
0.01
-100
VS = 24V
0.001
10
100
1k
10k
100k
1M
-120
10
10M
VS = 5V, +PSRR
100
1k
10k
VS = 24V, +PSRR
100k
1M
10M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 32.
Figure 33.
CMRR
vs.
Frequency
0
-20
CMRR (dB)
-40
-60
-80
VS = 5V
-100
VS = 24V
-120
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 34.
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APPLICATION NOTES
ADVANTAGES OF THE LM6211
High Supply Voltage, Low Power Operation
The LM6211 has performance ensured at supply voltages of 5V and 24V. The LM6211 is ensured to be
operational at all supply voltages between 5V and 24V. In this large range of operation, the LM6211 draws a
fairly constant supply current of 1 mA, while providing a wide bandwidth of 20 MHz. The wide operating range
makes the LM6211 a versatile choice for a variety of applications ranging from portable instrumentation to
industrial control systems.
Low Input Referred Noise
The LM6211 has very low flatband input referred voltage noise, 5.5 nV/√Hz. The 1/f corner frequency, also very
low, is about 400 Hz. The CMOS input stage allows for an extremely low input current (2 pA) and a very low
input referred current noise (0.01 pA/√Hz). This allows the LM6211 to maintain signal fidelity and makes it ideal
for audio, wireless or sensor based applications.
Low Input Bias Current and High Input Impedance
The LM6211 has a CMOS input stage, which allows it to have very high input impedance, very small input bias
currents (2 pA) and extremely low input referred current noise (0.01 pA/√Hz). This level of performance is
essential for op amps used in sensor applications, which deal with extremely low currents of the order of a few
nanoamperes. In this case, the op amp is being driven by a sensor, which typically has a source impedance of
tens of MΩ. This makes it essential for the op amp to have a much higher impedance.
Low Input Capacitance
The LM6211 has a comparatively small input capacitance for a high voltage CMOS design. Low input
capacitance is very beneficial in terms of driving large feedback resistors, required for higher closed loop gain.
Usually, high voltage CMOS input stages have a large input capacitance, which when used in a typical gain
configuration, interacts with the feedback resistance to create an extra pole. The extra pole causes gain-peaking
and can compromise the stability of the op amp. The LM6211 can, however, be used with larger resistors due to
its smaller input capacitance, and hence provide more gain without compromising stability. This also makes the
LM6211 ideal for wideband transimpedance amplifiers, which require a wide bandwidth, low input referred noise
and low input capacitance.
RRO, Ground Sensing and Current Limiting
The LM6211 has a rail-to-rail output stage, which provides the maximum possible output dynamic range. This is
especially important for applications requiring a large output swing, like wideband PLL synthesizers which need
an active loop filter to drive a wide frequency range VCO. The input common mode range includes the negative
supply rail which allows direct sensing at ground in a single supply operation. The LM6211 also has a short
circuit protection circuit which limits the output current to about 25 mA sourcing and 38 mA sinking, and allows
the LM6211 to drive short circuit loads indefinitely. However, while driving short circuit loads care should be
taken to prevent the inputs from seeing more than ±0.3V differential voltage, which is the absolute maximum
differential input voltage.
Small Size
The small footprint of the LM6211 package saves space on printed circuit boards, and enables the design of
smaller and more compact electronic products. Long traces between the signal source and the op amp make the
signal path susceptible to noise. By using a physically smaller package, the LM6211 can be placed closer to the
signal source, reducing noise pickup and enhancing signal integrity
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STABILITY OF OP AMP CIRCUITS
Stability and Capacitive Loading
GAIN
The LM6211 is designed to be unity gain stable for moderate capacitive loads, around 100 pF. That is, if
connected in a unity gain buffer configuration, the LM6211 will resist oscillation unless the capacitive load is
higher than about 100 pF. For higher capacitive loads, the phase margin of the op amp reduces significantly and
it tends to oscillate. This is because an op amp cannot be designed to be stable for high capacitive loads without
either sacrificing bandwidth or supplying higher current. Hence, for driving higher capacitive loads, the LM6211
needs to be externally compensated.
STABLE
ROC ± 20 dB/decade
UNSTABLE
ROC = 40 dB/decade
0
FREQUENCY (Hz)
Figure 35. Gain vs. Frequency for an Op Amp
An op amp, ideally, has a dominant pole close to DC, which causes its gain to decay at the rate of 20 dB/decade
with respect to frequency. If this rate of decay, also known as the rate of closure (ROC), remains at 20
dB/decade at the unity gain bandwidth of the op amp, the op amp is stable. If, however, a large capacitance is
added to the output of the op amp, it combines with the output impedance of the op amp to create another pole
in its frequency response before its unity gain frequency (Figure 35). This increases the ROC to 40 dB/decade
and causes instability.
In such a case a number of techniques can be used to restore stability to the circuit. The idea behind all these
schemes is to modify the frequency response such that it can be restored to a ROC of 20 dB/decade, which
ensures stability.
In the Loop Compensation
Figure 36 illustrates a compensation technique, known as ‘in the loop’ compensation, that employs an RC
feedback circuit within the feedback loop to stabilize a non-inverting amplifier configuration. A small series
resistance, RS, is used to isolate the amplifier output from the load capacitance, CL, and a small capacitance, CF,
is inserted across the feedback resistor to bypass CL at higher frequencies.
VIN
+
ROUT
-
RS
CL
RL
CF
RF
RIN
Figure 36. In the Loop Compensation
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The values for RS and CF are decided by ensuring that the zero attributed to CF lies at the same frequency as the
pole attributed to CL. This ensures that the effect of the second pole on the transfer function is compensated for
by the presence of the zero, and that the ROC is maintained at 20 dB/decade. For the circuit shown in Figure 36
the values of RS and CF are given by Equation 1. Table 1 shows different values of RS and CF that need to be
used for maintaining stability with different values of CL, as well as the phase margins to be expected. RF and RIN
are assumed to be 10 kΩ, RL is taken as 2 kΩ, while ROUT is taken to be 60Ω.
RS = ROUTRIN
RF
§ RF + 2RIN
©
2
RF
§
¨
¨
©
CF = ¨¨
CLROUT
(1)
Table 1.
CL (pF)
RS (Ω)
CF (pF)
Phase Margin (°)
250
60
4.5
39.8
300
60
5.4
49.5
500
60
9
53.1
Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.
The closed loop bandwidth of the circuit is now limited by RS and CF.
Compensation by External Resistor
In some applications it is essential to drive a capacitive load without sacrificing bandwidth. In such a case, in the
loop compensation is not viable. A simpler scheme for compensation is shown in Figure 37. A resistor, RISO, is
placed in series between the load capacitance and the output. T110his introduces a zero in the circuit transfer
function, which counteracts the effect of the pole formed by the load capacitance, and ensures stability.
Figure 37. Compensation By Isolation Resistor
The value of RISO to be used should be decided depending on the size of CL and the level of performance
desired. Values ranging from 5Ω to 50Ω are usually sufficient to ensure stability. A larger value of RISO will result
in a system with lesser ringing and overshoot, but will also limit the output swing and the short circuit current of
the circuit.
Stability and Input Capacitance
In certain applications, for example I-V conversion, transimpedance photodiode amplification and buffering the
output of current-output DAC, capacitive loading at the input of the op amp can endanger stability. The
capacitance of the source driving the op amp, the op amp input capacitance and the parasitic/wiring capacitance
contribute to the loading of the input. This capacitance, CIN, interacts with the feedback network to introduce a
peaking in the closed loop gain of the circuit, and hence causes instability.
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CF
R2
R1
+
VIN
CIN
+
-
+
-
VOUT
Figure 38. Compensating for Input Capacitance
This peaking can be eliminated by adding a feedback capacitance, CF, as shown in Figure 38. This introduces a
zero in the feedback network, and hence a pole in the closed loop response, and thus maintains stability. An
optimal value of CF is given by Equation 2. A simpler approach is to select CF = (R1/R2)CIN for a 90° phase
margin. This approach, however, limits the bandwidth excessively.
Typical Applications
ACTIVE LOOP FILTER FOR PLLs
A typical phase locked loop, or PLL, functions by creating a negative feedback loop in terms of the phase of a
signal. A simple PLL consists of three main components: a phase detector, a loop filter and a voltage controlled
oscillator (VCO). The phase detector compares the phase of the output of the PLL with that of a reference signal,
and feeds the error signal into the loop filter, thus performing negative feedback. The loop filter performs the
important function of averaging (or low-pass filtering) the error and providing the VCO with a DC voltage, which
allows the VCO to modify its frequency such that the error is minimized. The performance of the loop filter affects
a number of specifications of the PLL, like its frequency range, locking time and phase noise.
Since a loop filter is a very noise sensitive application, it is usually suggested that only passive components be
used in its design. Any active devices, like discrete transistors or op amps, would add significantly to the noise of
the circuit and would hence worsen the in-band phase noise of the PLL. But newer and faster PLLs, like TI’s
LMX2430, have a power supply voltage of less than 3V, which limits the phase-detector output of the PLL. If a
passive loop filter is used with such circuits, then the DC voltage that can be provided to the VCO is limited to
couple of volts. This limits the range of frequencies for which the VCO, and hence the PLL, is functional. In
certain applications requiring a wider operating range of frequencies for the PLL, like set-top boxes or base
stations, this level of performance is not adequate and requires active amplification, hence the need for active
loop filters.
An active loop filter typically consists of an op amp, which provides the gain, accompanied by a three or four pole
RC filter. The non-inverting input of the op amp is biased to a fixed value, usually the mid-supply of the PLL,
while a feedback network provides the gain as well as one, or two, poles for low pass filtering. Figure 39
illustrates a typical active loop filter.
CHARGE
PUMP
OUTPUT
VCO
+
INPUT
VS_PLL
2
Figure 39. A Typical Active Loop Filter
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Certain performance characteristics are essential for an op amp if it is to be used in a PLL loop filter. Low input
referred voltage and current noise are essential, as they directly affect the noise of the filter and hence the phase
noise of the PLL. Low input bias current is also important, as bias current affects the level of ‘reference spurs’,
artifacts in the frequency spectrum of the PLL caused by mismatch or leakage at the output of the phase
detector. A large input and output swing is beneficial in terms of increasing the flexibility in biasing the op amp.
The op amp can then be biased such that the output range of the PLL is mapped efficiently onto the input range
of the VCO.
With a CMOS input, ultra low input bias currents (2 pA) and low input referred voltage noise (5.5 nV/√Hz), the
LM6211 is an ideal op amp for using in a PLL active loop filter. The LM6211 has a ground sensing input stage, a
rail-to-rail output stage, and an operating supply range of 5V - 24V, which makes it a versatile choice for the
design of a wide variety of active loop filters.
Figure 41 shows the LM6211 used with the LMX2430 to create an RF frequency synthesizer. The LMX2430
detects the PLL output, compares it with its internal reference clock and outputs the phase error in terms of
current spikes. The LM6211 is used to create a loop filter which averages the error and provides a DC voltage to
the VCO. The VCO generates a sine wave at a frequency determined by the DC voltage at its input. This circuit
can provide output signal frequencies as high as 2 GHz, much higher than a comparative passive loop filter.
Compared to a similar passive loop filter, the LM6211 doesn’t add significantly to the phase noise of the PLL,
except at the edge of the loop bandwidth, as shown in Figure 40. A peaking of loop gain is expected, since the
loop filter is deliberately designed to have a wide bandwidth and a low phase margin so as to minimize locking
time.
ADDED PHASE NOISE (dB)
4.0
ACTIVE LOOP FILTER
WITH LM6211
3.0
2.0
1.0
0.0
PASSIVE LOOP FILTER
-1.0
1k
10k
100k
1M
OFFSET FREQUENCY (Hz)
Figure 40. Effect of LM6211 on Phase Noise of PLL
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C27
100 pF
18
GND
OSC_EN
D0_RF
OSC_OUT/FL0_IF
OSC_IN
8
IF_PLL I/O's
GND
13
12
5
4
VCC 13
14
15
C6
100 pF
C41
0.1 PF
C1
C2
C4
R3
+
R2
LM6211
VS_PLL
R46
100 k:
C14
100 pF
C8
0.01 PF
C9
100 pF
10
V586ME04
11
LMX2430
L2
12
V
6
R50
10:
9
8
C5
100 pF
VS_OP AMP
RFOUT
7
14
VS_PLL
C13
0.01 PF
C11
100 pF
+
15
C10
0.01 PF
2
1
CLK
FIN_RF
D0_IF
VS
7
FINB_RF
CE
16
FL0_RF
6
VS
10
5
FIN_IF
F0LD
4
LE
9
3
DATA
VS
2
GND
17
VS_RF
C38
3
19
20
C40
100 pF
R47
RF_OUT
R6
18:
C39
VS_PLL
1
C7
100 pF
R7
18:
11
C26
0.01 PF
R5
18:
RF_PLL PROGRAMMING INPUTS
16
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R41
C3
C36
R45
100 k:
C37
0.1 PF
Figure 41. LM6211 in the Active Loop Filter for LMX2430
ADC INPUT DRIVER
A typical application for a high performance op amp is as an ADC driver, which delivers the analog signal
obtained from sensors and actuators to ADCs for conversion to the digital domain and further processing.
Important requirements in this application are a slew rate high enough to drive the ADC input and low input
referred voltage and current noise. If an op amp is used with an ADC, it is critical that the op amp noise does not
affect the dynamic range of the ADC. The LM6211, with low input referred voltage and current noise, provides a
great solution for this application. For example, the LM6211 can be used to drive an ADS121021, a 12-bit ADC
from TI. If it provides a gain of 10 to a maximum input signal amplitude of 100 mV, for a bandwidth as wide as
100 kHz, the average noise seen at the input of the ADC is only 44.6 µVrms. Hence the dynamic range of the
ADC, measured in Effective Number of Bits or ENOB, is only reduced by 0.3 bits, despite amplifying the input
signal by a gain of 10. Low input bias currents and high input impedance also help as they prevent the loading of
the sensor and allow the measurement system to function over a large range.
Figure 42 shows a circuit for monitoring fluid pressure in a hydraulic system, in which the LM6211 is used to
sense the error voltage from the pressure sensor. Two LM6211 amplifiers are used to make a difference
amplifier which senses the error signal, amplifies it by a gain of 100, and delivers it to the ADC input. The ADC
converts the error voltage into a pressure reading to be displayed and drives the DAC, which changes the
voltage driving the resistance bridge sensor. This is used to control the gain of the pressure measurement circuit,
such that the range of the sensor can be modified to obtain the best resolution possible.
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4.096V
3
VOUT
1
C1
6 SYNC
5 SCLK
4 DOUT
120 pF
2
+5V
4
5
1,2
6
B1
0.1 PF
1 PF
7,8
+5V
3
5
+
4
-
180:
1
A2
2
100 k:
0.2 PF
2.048V
VREF
1
470 pF
2
8
7 SCLK
6 DOUT
+IN
C2
2.02 k:
AV = 100
+5V
3
5 CS
-IN
4
+5V
3
1
A1
4
+
470 pF
0.2 PF
100 k:
5
-
180:
2
PRESSURE SENSOR
0.2 mV/Volt/PSI
A1, A2 = LM6211
B1 = LM4140ACM-2.0
C1 = DAC081S101
C2 = ADC121S625
Figure 42. Hydraulic Pressure Monitoring System
DAC OUTPUT AMPLIFIER
Op amps are often used to improve a DAC's output driving capability. High performance op amps are required as
I-V converters at the outputs of high resolution current output DACs. Since most DACs operate with a single
supply of 5V, a rail-to-rail output swing is essential for this application. A low offset voltage is also necessary to
prevent offset errors in the waveform generated. Also, the output impedance of DACs is quite high, more than a
few kΩ in some cases, so it is also advisable for the op amp to have a low input bias current. An op amp with a
high input impedance also prevents the loading of the DAC, and hence, avoids gain errors. The op amp should
also have a slew rate which is fast enough to not affect the settling time of the DAC output.
The LM6211, with a CMOS input stage, ultra low input bias current, a wide bandwidth (20 MHz) and a rail-to-rail
output swing for a supply voltage of 24V is an ideal op amp for such an application. Figure 43 shows a typical
circuit for this application. The op amp is usually expected to add another time constant to the system, which
worsens the settling time, but the wide bandwidth of the LM6211 (20 MHz) allows the system performance to
improve without any significant degradation of the settling time.
VS
10 PF
VS
3
SYNC 6
SCLK 5
-
0.1 PF
LM6211
DAC081S101
DOUT 4
1
+
2
Figure 43. DAC Driver Circuit
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AUDIO PREAMPLIFIER
With low input referred voltage noise, low supply voltage and low supply current, and low harmonic distortion, the
LM6211 is ideal for audio applications. Its wide unity gain bandwidth allows it to provide large gain over a wide
frequency range and it can be used to design a preamplifier to drive a load of as low as 600Ω with less than
0.001% distortion. Two amplifier circuits are shown in Figure 44 and Figure 45. Figure 44 is an inverting
amplifier, with a 10 kΩ feedback resistor, R2, and a 1 kΩ input resistor, R1, and hence provides a gain of −10.
Figure 45 is a non-inverting amplifier, using the same values for R1 and R2, and provides a gain of 11. In either of
these circuits, the coupling capacitor CC1 decides the lower frequency at which the circuit starts providing gain,
while the feedback capacitor CF decides the frequency at which the gain starts dropping off. Figure 46 shows the
frequency response of the circuit in Figure 44 with different values of CF.
CF
CC1
R1
1 k:
R2
10 k:
+
VIN
CC2
-
-
+
RB1
V
+
+
-
VOUT
RB2
R2
= -10
R1
AV = -
Figure 44. Inverting Audio Amplifier
+
V
CC2
RB1
+
VIN
-
RB2
R1
1 k:
CC1
+
-
+
R2
10 k:
-
VOUT
CF
AV = 1 +
R2
R1
= 11
Figure 45. Non-Inverting Audio Preamplifier
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25
CF = 10 pF
20
15
CF = 1 nF
GAIN (dB)
10
CF = 100 pF
5
0
-5
-10
-15
-20
1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 46. Frequency Response of the Non-Inverting Preamplifier
TRANSIMPEDANCE AMPLIFIER
A transimpedance amplifier converts a small input current into a voltage. This current is usually generated by a
photodiode. The transimpedance gain, measured as the ratio of the output voltage to the input current, is
expected to be large and wide-band. Since the circuit deals with currents in the range of a few nA, low noise
performance is essential. The LM6211, being a CMOS input op amp, provides a wide bandwidth and low noise
performance while drawing very low input bias current, and is hence ideal for transimpedance applications.
A transimpedance amplifier is designed on the basis of the current source driving the input. A photodiode is a
very common capacitive current source, which requires transimpedance gain for transforming its miniscule
current into easily detectable voltages. The photodiode and amplifier’s gain are selected with respect to the
speed and accuracy required of the circuit. A faster circuit would require a photodiode with lesser capacitance
and a faster amplifier. A more sensitive circuit would require a sensitive photodiode and a high gain. A typical
transimpedance amplifier is shown in Figure 47. The output voltage of the amplifier is given by the equation
VOUT = −IINRF. Since the output swing of the amplifier is limited, RF should be selected such that all possible
values of IIN can be detected.
The LM6211 has a large gain-bandwidth product (20 MHz), which enables high gains at wide bandwidths. A railto-rail output swing at 24V supply allows detection and amplification of a wide range of input currents. A CMOS
input stage with negligible input current noise and low input voltage noise allows the LM6211 to provide high
fidelity amplification for wide bandwidths. These properties make the LM6211 ideal for systems requiring wideband transimpedance amplification.
CF
RF
IIN
CCM
CD
VB
+
+
VOUT
CIN = CD + CCM
VOUT
= - RF
IIN
Figure 47. Photodiode Transimpedance Amplifier
The following parameters are used to design a transimpedance amplifier: the amplifier gain-bandwidth product,
A0; the amplifier input capacitance, CCM; the photodiode capacitance, CD; the transimpedance gain required, RF;
and the amplifier output swing. Once a feasible RF is selected using the amplifier output swing, these numbers
can be used to design an amplifier with the desired transimpedance gain and a maximally flat frequency
response. The input common-mode capacitance with respect to VCM for the LM6211 is give in Figure 48.
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20
VS = 5V
CIN (pF)
15
10
VS = 24V
5
0
0
2
4 6
8 10 12 14 16 18 20 22 24
VCM (V)
Figure 48. Input Common-Mode Capacitance vs. VCM
An essential component for obtaining a maximally flat response is the feedback capacitor, CF. The capacitance
seen at the input of the amplifier, CIN, combined with the feedback resistor, RF, generates a phase lag which
causes gain-peaking and can destabilize the circuit. CIN is usually just the sum of CD and CCM. The feedback
capacitor CF creates a pole, fP in the noise gain of the circuit, which neutralizes the zero in the noise gain, fZ,
created by the combination of RF and CIN. If properly positioned, the noise gain pole created by CF can ensure
that the slope of the gain remains at 20 dB/decade till the unity gain frequency of the amplifier is reached, thus
ensuring stability. As shown in Figure 50, fP is positioned such that it coincides with the point where the noise
gain intersects the op amp’s open loop gain. In this case, fP is also the overall 3 dB frequency of the
transimpedance amplifier. The value of CF needed to make it so is given by Equation 2. A larger value of CF
causes excessive reduction of bandwidth, while a smaller value fails to prevent gain peaking and maintain
stability.
CF =
1 + 1 + 4SRFCINA0
2SRFA0
(2)
Calculating CF from Equation 2 can sometimes return unreasonably small values (
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