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LPC661
Low Power CMOS Operational Amplifier
General Description
The LPC661 CMOS operational amplifier is ideal for operation from a single supply. It features a wide range of operating supply voltage from +5V to +15V, rail-to-rail output swing
and an input common-mode range that includes ground.
Performance limitations that have plagued CMOS amplifiers
in the past are not a problem with this design. Input VOS,
drift, and broadband noise as well as voltage gain (into 100
kΩ and 5 kΩ) are all equal to or better than widely accepted
bipolar equivalents, while the supply current requirement is
typically 55 µA.
This chip is built with National’s advanced Double-Poly
Silicon-Gate CMOS process.
See the LPC660 datasheet for a Quad CMOS operational
amplifier or the LPC662 data sheet for a Dual CMOS operational amplifier with these same features.
Features
(Typical unless otherwise noted)
n Rail-to-rail output swing
n
n
n
n
n
n
n
n
n
n
Low supply current 55 µA
Specified for 100 kΩ and 5 kΩ loads
High voltage gain 120 dB
Low input offset voltage 3 mV
Low offset voltage drift 1.3 µV/˚C
Ultra low input bias current 2 fA
Input common-mode range includes GND
Operating range from +5V to +15V
Low distortion 0.01% at 1 kHz
Slew rate 0.11 V/µs
Applications
n
n
n
n
n
n
n
High-impedance buffer
Precision current-to-voltage converter
Long-term integrator
High-impedance preamplifier
Active filter
Sample-and-Hold circuit
Peak detector
Application Circuits
10 Hz Bandpass Filter
1 Hz Low-Pass Filter
(Maximally Flat, Dual Supply Only)
DS011227-21
fO = 10 Hz
Q = 2.1
Gain = 18.9 dB
© 2000 National Semiconductor Corporation
DS011227-23
DS011227
www.national.com
LPC661 Low Power CMOS Operational Amplifier
August 2000
LPC661
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V+ − V−)
Differential Input Voltage
Output Short Circuit to V+
Output Short Circuit to V−
Storage Temperature Range
Lead Temperature
(Soldering, 10 sec.)
Junction Temperature (Note 3)
Power Dissipation
ESD Rating
(C=100 pF, R=1.5 kΩ)
Current at Input Pin
± 18 mA
(V+) +0.3V, (V−) −0.3V
35 mA
Current at Output Pin
Voltage Input/Output Pin
Current at Power Supply Pin
Operating Ratings (Note 1)
16V
± Supply Voltage
(Notes 2, 9)
(Note 2)
−65˚C to +150˚C
Supply Voltage
Junction Temperature Range
LPC661AM
LPC661AI
LPC661I
Power Dissipation
Thermal Resistance (θJA) (Note 8)
8-Pin DIP
8-Pin SO
260˚C
150˚C
(Note 3)
4.75V ≤ V+ ≤ 15.5V
−55˚C ≤ TJ ≤ +125˚C
−40˚C ≤ TJ ≤ +85˚C
−40˚C ≤ TJ ≤ +85˚C
(Note 7)
101˚C/W
165˚C/W
1000V
± 5 mA
DC Electrical Characteristics
The following specifications apply for V+ = 5V, V− = 0V, VCM = 1.5V, VO = 2.5V, and RL = 1M unless otherwise noted. Boldface limits apply at the temperature extremes; all other limits TJ = 25˚C.
Symbol
VOS
TCVOS
Parameter
Conditions
Typ
Input Offset Voltage
LPC661AM
LPC661AI
LPC661I
Limit
Limit
Limit
(Note 4)
(Note 4)
(Note 4)
1
Input Offset Voltage
3
3
6
3.5
3.3
6.3
1.3
Units
(Limit)
mV
µV/˚C
Average Drift
IB
Input Bias Current
0.002
20
pA
100
IOS
Input Offset Current
0.001
4
20
Input Resistance
CMRR
Common Mode
+PSRR
83
Rejection Ratio
V = 15V
5V ≤ V+ ≤ 15V
70
63
dB
68
68
61
min
70
70
63
dB
68
68
61
min
94
84
84
74
dB
82
83
73
min
−0.4
−0.1
−0.1
−0.1
V
0
0
0
max
V+ − 2.3
V+ − 2.3
V+ − 2.3
V
V+ − 2.5
min
83
0V ≤ V− ≤ −10V
Rejection Ratio
VCM
Input Common Mode
V+ = 5V and 15V
Voltage Range
for CMRR ≥ 50 dB
V+ − 1.9
+
V − 2.6
AV
Large Signal
Sourcing
Voltage Gain
RL = 100 kΩ (Note 5)
1000
Sinking
400
300
V/mV
300
200
min
180
180
90
V/mV
70
120
70
min
1000
200
200
100
V/mV
150
160
80
min
250
100
100
50
V/mV
35
60
40
min
RL = 5 kΩ (Note 5)
RL = 5 kΩ (Note 5)
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2
V − 2.5
400
500
Sinking
+
250
RL = 100 kΩ (Note 5)
Sourcing
max
70
+
Positive Power Supply
Negative Power Supply
2
Tera Ω
Rejection Ratio
−PSRR
2
>1
0V ≤ VCM ≤ 12.0V
max
pA
100
RIN
4
(Continued)
The following specifications apply for V+ = 5V, V− = 0V, VCM = 1.5V, VO = 2.5V, and RL = 1M unless otherwise noted. Boldface limits apply at the temperature extremes; all other limits TJ = 25˚C.
Symbol
VO
Parameter
Output Swing
Conditions
LPC661AM
LPC661AI
LPC661I
Limit
Limit
Limit
(Note 4)
(Note 4)
(Note 4)
4.970
4.970
4.940
V
4.950
4.950
4.910
min
0.004
0.030
0.030
0.060
V
0.050
0.050
0.090
max
4.940
4.850
4.850
4.750
V
4.750
4.750
4.650
min
0.040
0.150
0.150
0.250
V
0.250
0.250
0.350
max
14.970
14.920
14.920
14.880
V
14.880
14.880
14.820
min
0.030
0.030
0.060
V
0.050
0.050
0.090
max
14.680
14.680
14.580
V
14.600
14.600
14.480
min
0.220
0.220
0.320
V
0.300
0.300
0.400
max
16
16
13
mA
12
14
11
min
16
16
13
mA
12
14
11
min
19
28
23
mA
19
25
20
min
19
28
23
mA
19
24
19
min
60
60
70
µA
70
70
85
max
75
75
90
µA
85
85
105
max
Typ
+
V = 5V
4.987
RL = 100 kΩ to 2.5V
V+ = 5V
RL = 5 kΩ to 2.5V
V+ = 15V
RL = 100 kΩ to 7.5V
0.007
V+ = 15V
14.840
RL = 5 kΩ to 7.5V
0.110
IO
Output Current
Sourcing, VO = 0V
22
Sinking, VO = 5V
21
+
V = 5V
IO
Output Current
Sourcing, VO = 0V
40
Sinking, VO = 13V
39
+
V = 15V
(Note 9)
IS
Supply Current
V+ = 5V, VO = 1.5V
55
V+ = 15V, VO = 1.5V
58
Units
(Limit)
AC Electrical Characteristics
The following specifications apply for V+ = 5V, V− = 0V, VCM = 1.5V, VO = 2.5V, and RL = 1M unless otherwise noted. Boldface limits apply at the temperature extremes; all other limits TJ = 25˚C.
LPC661AM LPC661AI
Symbol
SR
Parameter
Slew Rate
Conditions
Typ
(Note 6)
0.11
LPC661I
Units
(Limit)
Limit
Limit
Limit
(Note 4)
(Note 4)
(Note 4)
0.07
0.07
0.05
V/µs
0.04
0.05
0.03
min
GBW
Gain-Bandwidth Product
350
kHz
φm
Phase Margin
50
Deg
GM
Gain Margin
en
Input Referred Voltage Noise
17
dB
F = 1 kHz
42
nV/√Hz
0.0002
pA/√Hz
in
Input Referred Current Noise
F = 1 kHz
T.H.D.
Total Harmonic Distortion
F = 1 kHz, AV = −10
0.01
RL = 100 kΩ, VO = 8 VPP
%
+
V = 15V
3
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LPC661
DC Electrical Characteristics
LPC661
AC Electrical Characteristics
(Continued)
Note 1: 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 do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The
guaranteed specifications apply only for the test conditions listed.
Note 2: Applies to both single supply and split supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed junction temperature of 150˚C. Output currents in excess of ± 30 mA over long term may adversely affect reliability.
Note 3: 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.
Note 4: Limits are guaranteed by testing or correlation.
Note 5: V+ = 15V, VCM = 7.5V and RL connected to 7.5V. For sourcing tests, 7.5V ≤ VO ≤ 11.5V. For sinking tests, 2.5V ≤ VO ≤ 7.5V.
Note 6: V+ = 15V. Connected as Voltage Follower with 10V step input. Number specified is the slower of the positive and negative slew rates.
Note 7: For operating at elevated temperatures the device must be derated based on the thermal resistance θJA with PD = (TJ–TA)/θJA.
Note 8: All numbers apply for packages soldered directly into a PC board.
Note 9: Do not connect output to V+ when V+ is greater than 13V or reliability may be adversely affected.
Typical Performance Characteristics
VS = ± 7.5V, TA = 25˚C unless otherwise specified
Supply Current
vs Supply Voltage
Input Bias Current
vs Temperature
DS011227-26
Common-Mode Voltage Range
vs Temperature
DS011227-27
Output Characteristics
Current Sinking
DS011227-28
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DS011227-29
4
VS = ± 7.5V, TA = 25˚C unless otherwise specified (Continued)
Output Characteristics
Current Sourcing
Input Voltage Noise
vs Frequency
DS011227-30
CMRR vs Frequency
DS011227-31
CMRR vs Temperature
DS011227-32
Power Supply Rejection
Ratio vs Frequency
DS011227-33
Open-Loop Voltage Gain
vs Temperature
DS011227-35
DS011227-34
5
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LPC661
Typical Performance Characteristics
LPC661
Typical Performance Characteristics
VS = ± 7.5V, TA = 25˚C unless otherwise specified (Continued)
Open-Loop
Frequency Response
Gain and Phase Responses
vs Load Capacitance
DS011227-36
DS011227-37
Gain and Phase Responses
vs Temperature
Gain Error
(VOSvs VOUT)
DS011227-38
Non-Inverting Slew Rate
vs Temperature
DS011227-39
Inverting Slew Rate
vs Temperature
DS011227-40
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DS011227-41
6
VS = ± 7.5V, TA = 25˚C unless otherwise specified (Continued)
Large-Signal Pulse
Non-Inverting Response
(AV = +1)
Non-Inverting Small
Signal Pulse Response
(AV = +1)
DS011227-42
Inverting Large-Signal
Pulse Response
DS011227-43
Inverting Small-Signal
Pulse Response
DS011227-44
DS011227-45
7
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LPC661
Typical Performance Characteristics
LPC661
Typical Performance Characteristics
VS = ± 7.5V, TA = 25˚C unless otherwise specified (Continued)
Stability vs Capacitive Load
Stability vs Capacitive Load
DS011227-4
DS011227-5
Note: Avoid resistive loads of less than 500Ω, as they may cause
instability.
Application Hints
ing load resistance of 5 kΩ or less, the gain will be reduced
as indicated in the Electrical Characteristics. The op amp
can drive load resistance as low as 500Ω without instability.
AMPLIFIER TOPOLOGY
The topology chosen for the LPC661 is unconventional
(compared to general-purpose op amps) in that the traditional unity-gain buffer output stage is not used; instead, the
output is taken directly from the output of the integrator, to allow rail-to-rail output swing. Since the buffer traditionally delivers the power to the load, while maintaining high op amp
gain and stability, and must withstand shorts to either rail,
these tasks now fall to the integrator.
As a result of these demands, the integrator is a compound
affair with an embedded gain stage that is doubly fed forward
(via Cf and Cff) by a dedicated unity-gain compensation
driver. In addition, the output portion of the integrator is a
push-pull configuration for delivering heavy loads. While
sinking current the whole amplifier path consists of three
gain stages with one stage fed forward, whereas while
sourcing the path contains four gain stages with two fed
forward.
COMPENSATING INPUT CAPACITANCE
Refer to the LMC660 or LMC662 datasheets to determine
whether or not a feedback capacitor will be necessary for
compensation and what the value of that capacitor would be.
CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LPC661 may oscillate when
its applied load appears capacitive. The threshold of oscillation varies both with load and circuit gain. The configuration
most sensitive to oscillation is a unity-gain follower. See the
Typical Performance Characteristics.
The load capacitance interacts with the op amp’s output resistance to create an additional pole. If this pole frequency is
sufficiently low, it will degrade the op amp’s phase margin so
that the amplifier is no longer stable at low gains. The addition of a small resistor (50Ω to 100Ω) in series with the op
amp’s output, and a capacitor (5 pF to 10 pF) from inverting
input to output pins, returns the phase margin to a safe value
without interfering with lower-frequency circuit operation.
Thus, larger values of capacitance can be tolerated without
oscillation. Note that in all cases, the output will ring heavily
when the load capacitance is near the threshold for
oscillation.
DS011227-6
FIGURE 1. LPC661 Circuit Topology
The large signal voltage gain while sourcing is comparable
to traditional bipolar op amps, for load resistance of at least
5 kΩ. The gain while sinking is higher than most CMOS op
amps, due to the additional gain stage; however, when drivwww.national.com
8
LPC661
Application Hints
(Continued)
DS011227-24
FIGURE 3. Compensating for Large
Capacitive Loads with A Pull Up Resistor
DS011227-7
FIGURE 2. Rx, Cx Improve Capacitive Load Tolerance
PRINTED-CIRCUIT-BOARD LAYOUT
FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000 pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low bias current of the LPC661, typically less
than 0.04 pA, it is essential to have an excellent layout. Fortunately, the techniques for obtaining low leakages are quite
simple. First, the user must not ignore the surface leakage of
the PC board, even though it may sometimes appear acceptably low, because under conditions of high humidity or dust
or contamination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LPC661’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs. See Figure
4. To have a significant effect, guard rings should be placed
on both the top and bottom of the PC board. This PC foil
must then be connected to a voltage which is at the same
voltage as the amplifier inputs, since no leakage current can
flow between two points at the same potential. For example,
a PC board trace-to-pad resistance of 1012Ω, which is normally considered a very large resistance, could leak 5 pA if
the trace were a 5V bus adjacent to the pad of an input. This
would cause a 100 times degradation from the LPC660’s actual performance. However, if a guard ring is held within
5 mV of the inputs, then even a resistance of 1011Ω would
cause only 0.05 pA of leakage current, or perhaps a minor
(2:1) degradation of the amplifier’s performance. See Figures 5, 6, 7 for typical connections of guard rings for standard op-amp configurations. If both inputs are active and at
high impedance, the guard can be tied to ground and still
provide some protection; see Figure 8.
Capacitive load driving capability is enhanced by using a pull
up resistor to V+ (Figure 3). Typically a pull up resistor conducting 50 µA or more will significantly improve capacitive
load responses. The value of the pull up resistor must be determined based on the current sinking capability of the amplifier with respect to the desired output swing. Open loop gain
of the amplifier can also be affected by the pull up resistor
(see Electrical Characteristics).
9
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LPC661
Application Hints
(Continued)
DS011227-8
FIGURE 4. Example of Guard Ring in P.C. Board Layout, Using the LPC660
DS011227-9
FIGURE 5. Inverting Amplifier
Guard Ring Connections
DS011227-12
FIGURE 8. Howland Current Pump
Guard Ring Connections
The designer should be aware that when it is inappropriate
to lay out a PC board for the sake of just a few circuits, there
is another technique which is even better than a guard ring
on a PC board: Don’t insert the amplifier’s input pin into the
board at all, but bend it up in the air and use only air as an insulator. Air is an excellent insulator. In this case you may
have to forego some of the advantages of PC board construction, but the advantages are sometimes well worth the
effort of using point-to-point up-in-the-air wiring. See Figure
9.
DS011227-10
FIGURE 6. Non-Inverting Amplifier
Guard Ring Connections
DS011227-11
FIGURE 7. Follower
Guard Ring Connections
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10
LPC661
Application Hints
(Continued)
DS011227-13
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
DS011227-14
FIGURE 9. Air Wiring
FIGURE 10. Simple Input Bias Current Test Circuit
BIAS CURRENT TESTING
The test method of Figure 10 is appropriate for bench-testing
bias current with reasonable accuracy. To understand its operation, first close switch S2 momentarily. When S2 is
opened, then
A suitable capacitor for C2 would be a 5 pF or 10 pF silver
mica, NPO ceramic, or air-dielectric. When determining the
magnitude of I−, the leakage of the capacitor and socket
must be taken into account. Switch S2 should be left shorted
most of the time, or else the dielectric absorption of the capacitor C2 could cause errors.
Similarly, if S1 is shorted momentarily (while leaving S2
shorted)
where Cx is the stray capacitance at the + input.
Typical Single-Supply Applications
(V+ = 5.0 VDC)
Photodiode Current-toVoltage Converter
Micropower Current Source
DS011227-16
DS011227-15
Note: A 5V bias on the photodiode can cut its capacitance by a factor of 2
or 3, leading to improved response and lower noise. However, this bias on
the photodiode will cause photodiode leakage (also known as its dark
current).
(Upper limit of output range dictated by input common-mode range; lower
limit dictated by minimum current requirement of LM385.)
11
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LPC661
Typical Single-Supply Applications
(V+ = 5.0 VDC) (Continued)
Low-Leakage Sample-and-Hold
DS011227-17
Sine-Wave Oscillator
DS011227-18
Oscillator frequency is determined by R1, R2, C1, and C2:
fOSC = 1/2πRC
where R = R1 = R2 and C = C1 = C2.
This circuit, as shown, oscillates at 2.0 kHz with a
peak-to-peak output swing of 4.5V
1 Hz Square-Wave Oscillator
Power Amplifier
DS011227-20
DS011227-19
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12
LPC661
Typical Single-Supply Applications
(V+ = 5.0 VDC) (Continued)
10 Hz Bandpass Filter
10 Hz High-Pass Filter (2 dB Dip)
DS011227-22
DS011227-21
fO = 10 Hz
Q = 2.1
Gain = 18.9 dB
fc = 10 Hz
d = 0.895
Gain = 1
1 Hz Low-Pass Filter
(Maximally Flat, Dual Supply Only)
DS011227-23
Connection Diagram
8-Pin DIP/SO
DS011227-1
Ordering Information
Package
Temperature Range
Military
−55˚C to +125˚C
8-Pin
Molded DIP
NSC
Drawing
Transport
Media
M08A
Tape and Reel
N08E
Rail
−40˚C to +85˚C
LPC661AIM
Small Outline
8-Pin
Industrial
LPC661IM
LPC661AMN
LPC661AIN
Rail
LPC661IN
13
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LPC661
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Pin Small Outline Molded Package (M)
Order Number LPC661AIM or LPC661IM
NS Package Number M08A
8-Pin Molded Dual-In-Line Package (N)
Order Number LPC661AIN, LPC661IN or LPC661AMN
NS Package Number N08E
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14
LPC661 Low Power CMOS Operational Amplifier
Notes
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
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Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
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English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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Response Group
Tel: 65-2544466
Fax: 65-2504466
Email: ap.support@nsc.com
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Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.