SM73308
SM73308 Low Offset, Low Noise, RRO Operational Amplifier
Literature Number: SNOSB90A
SM73308
June 1, 2011
Low Offset, Low Noise, RRO Operational Amplifier
General Description
The SM73308 is a Single low noise precision operational am-
plifier intended for use in a wide range of applications. Other
important characteristics include: an extended operating tem-
perature range of −40°C to 125°C, the tiny SC70-5 package,
and low input bias current.
The extended temperature range of −40°C to 125°C allows
the SM73308 to accommodate a broad range of applications.
The SM73308 expands National Semiconductor’s Silicon
Dust™ amplifier portfolio offering enhancements in size,
speed, and power savings. The SM73308 is guaranteed to
operate over the voltage range of 2.7V to 5.0V and has rail-
to-rail output.
The SM73308 is designed for precision, low noise, low volt-
age, and miniature systems. This amplifier provides rail-to-rail
output swing into heavy loads. The maximum input offset is
850 μV at room temperature and the input common mode
voltage range includes ground.
The SM73308 is offered in the tiny SC70-5 package.
Features
(Unless otherwise noted, typical values at VS = 2.7V)
■Renewable Energy Grade
■Guaranteed 2.7V and 5V specifications
■Maximum VOS 850μV (limit)
■Voltage noise
—f = 100 Hz 12.5nV/√Hz
—f = 10 kHz 7.5nV/√Hz
■Rail-to-Rail output swing
—RL = 600Ω 100mV from rail
—RL = 2kΩ50mV from rail
■Open loop gain with RL = 2kΩ100dB
■VCM 0 to V+ −0.9V
■Supply current 550µA
■Gain bandwidth product 3.5MHz
■Temperature range −40°C to 125°C
Applications
■Transducer amplifier
■Instrumentation amplifier
■Precision current sensing
■Data acquisition systems
■Active filters and buffers
■Sample and hold
■Portable/battery powered electronics
■Automotive
Connection Diagram
SC70-5
30155567
Top View
Instrumentation Amplifier
30155536
Silicon Dust™ is a trademark of National Semiconductor Corporation.
© 2011 National Semiconductor Corporation 301555 www.national.com
SM73308 Low Offset, Low Noise, RRO Operational Amplifier
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 2)
Machine Model 200V
Human Body Model 2000V
Differential Input Voltage ± Supply Voltage
Voltage at Input Pins (V+) + 0.3V, (V–) – 0.3V
Current at Input Pins ±10 mA
Supply Voltage (V+–V −)5.75V
Output Short Circuit to V+(Note 3)
Output Short Circuit to V−(Note 4)
Mounting Temperture
Infrared or Convection (20 sec) 235°C
Wave Soldering Lead Temp
(10 sec) 260°C
Storage Temperature Range −65°C to 150°C
Junction Temperature (Note 5) 150°C
Operating Ratings (Note 1)
Supply Voltage 2.7V to 5.5V
Temperature Range −40°C to 125°C
Thermal Resistance (θJA)
440 °C/W
2.7V DC Electrical Characteristics (Note 11)
Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 2.7V, V − = 0V, VCM = V+/2, VO = V+/2 and
RL > 1MΩ. Boldface limits apply at the temperature extremes.
Symbol Parameter Condition Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
VOS Input Offset Voltage 0.3 0.85
1.0 mV
TCVOS Input Offset Voltage Average Drift −0.45 µV/°C
IBInput Bias Current (Note 8)VCM = 1V −0.1 100
250 pA
IOS Input Offset Current (Note 8) 0.004 100 pA
ISSupply Current 550 900
910 µA
CMRR Common Mode Rejection Ratio 0.5 ≤ VCM ≤ 1.2V 74
72
80 dB
PSSR Power Supply Rejection Ratio 2.7V ≤ V+ ≤ 5V 82
76
90 dB
VCM
Input Common-Mode Voltage
Range For CMRR ≥ 50dB 0 1.8 V
AV
Large Signal Voltage Gain
(Note 9)
RL = 600Ω to 1.35V,
VO = 0.2V to 2.5V
92
80
100
dB
RL = 2kΩ to 1.35V,
VO = 0.2V to 2.5V
98
86
100
VOOutput Swing
RL = 600Ω to 1.35V
VIN = ± 100mV
0.11
0.14
0.084 to
2.62
2.59
2.56
V
RL = 2kΩ to 1.35V
VIN = ± 100mV
0.05
0.06
0.026 to
2.68
2.65
2.64
IOOutput Short Circuit Current
Sourcing, VO = 0V
VIN = 100mV
18
11
24
mA
Sinking, VO = 2.7V
VIN = −100mV
18
11
22
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SM73308
2.7V AC Electrical Characteristics (Note 11)
Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
SR Slew Rate (Note 10)AV = +1, RL = 10 kΩ 1.4 V/µs
GBW Gain-Bandwidth Product 3.5 MHz
ΦmPhase Margin 79 Deg
GmGain Margin −15 dB
en
Input-Referred Voltage Noise
(Flatband) f = 10kHz 7.5 nV/
enInput-Referred Voltage Noise (l/f) f = 100Hz 12.5 nV/
inInput-Referred Current Noise f = 1kHz 0.001 pA/
THD Total Harmonic Distortion f = 1kHz, AV = +1
RL = 600Ω, VIN = 1 VPP
0.007 %
5.0V DC Electrical Characteristics (Note 11)
Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and
RL > 1MΩ. Boldface limits apply at the temperature extremes.
Symbol Parameter Condition Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
VOS Input Offset Voltage 0.25 0.85
1.0 mV
TCVOS Input Offset Voltage Average Drift −0.35 µV/°C
IBInput Bias Current (Note 8)VCM = 1V −0.23 100
250 pA
IOS Input Offset Current (Note 8) 0.017 100 pA
ISSupply Current 600 950
960 µA
CMRR Common Mode Rejection Ratio 0.5 ≤ VCM ≤ 3.5V 80
79
90 dB
PSRR Power Supply Rejection Ratio 2.7V ≤ V+ ≤ 5V 82
76
90 dB
VCM
Input Common-Mode Voltage
Range For CMRR ≥ 50dB 0 4.1 V
AV
Large Signal Voltage Gain
(Note 9)
RL = 600Ω to 2.5V,
VO = 0.2V to 4.8V
92
89
100
dB
RL = 2kΩ to 2.5V,
VO = 0.2V to 4.8V
98
95
100
VOOutput Swing
RL = 600Ω to 2.5V
VIN = ± 100mV
0.15
0.23
0.112 to
4.9
4.85
4.77
V
RL = 2kΩ to 2.5V
VIN = ± 100mV
0.06
0.07
0.035 to
4.97
4.94
4.93
IO
Output Short Circuit Current (Note
8, Note 12)
Sourcing, VO = 0V
VIN = 100mV
35
35
75
mA
Sinking, VO = 2.7V
VIN = −100mV
35
35
66
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SM73308
5.0V AC Electrical Characteristics (Note 11)
Unless otherwise specified, all limits are guaranteed for TA = 25°C. V+ = 5.0V, V − = 0V, VCM = V+/2, VO = V+/2 and RL > 1MΩ.
Boldface limits apply at the temperature extremes.
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
SR Slew Rate (Note 10)AV = +1, RL = 10 kΩ 1.4 V/µs
GBW Gain-Bandwidth Product 3.5 MHz
ΦmPhase Margin 79 Deg
GmGain Margin −15 dB
en
Input-Referred Voltage Noise
(Flatband) f = 10kHz 6.5 nV/
enInput-Referred Voltage Noise (l/f) f = 100Hz 12 nV/
inInput-Referred Current Noise f = 1kHz 0.001 pA/
THD Total Harmonic Distortion f = 1kHz, AV = +1
RL = 600Ω, VIN = 1 VPP
0.007 %
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 specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics.
Note 2: Human Body Model is 1.5 kΩ in series with 100 pF. Machine Model is 0Ω in series with 20 pF.
Note 3: Shorting output to V+ will adversely affect reliability.
Note 4: Shorting output to V− will adversely affect reliability.
Note 5: 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)–T A) / θJA. All numbers apply for packages soldered directly into a PC board.
Note 6: Typical values represent the most likely parametric norm.
Note 7: All limits are guaranteed by testing or statistical analysis.
Note 8: Limits guaranteed by design.
Note 9: RL is connected to mid-supply. The output voltage is set at 200mV from the rails. VO = GND + 0.2V and VO = V+ −0.2V
Note 10: The number specified is the slower of positive and negative slew rates.
Note 11: 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 such that TJ = TA.
Note 12: Continuous operation of the device with an output short circuit current larger than 35mA may cause permanent damage to the device.
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SM73308
Connection Diagram
SC70-5
30155567
Top View
Ordering Information
Package Part Number Package Marking Transport Media NSC Drawing
SC70-5
SM73308MG
S08
1k Units Tape and Reel
MAA05ASM73308MGX 3k Units Tape and Reel
SM73308MGE 250 Units Tape and Reel
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SM73308
Typical Performance Characteristics
VOS vs. VCM Over Temperature
30155527
VOS vs. VCM Over Temperature
30155526
Output Swing vs. VS
30155525
Output Swing vs. VS
30155524
Output Swing vs. VS
30155523
IS vs. VS Over Temperature
30155530
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SM73308
Input Voltage Noise vs. Frequency
30155508
Input Bias Current Over Temperature
30155535
Input Bias Current Over Temperature
30155534
Input Bias Current Over Temperature
30155533
THD+N vs. Frequency
30155507
THD+N vs. VOUT
30155566
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SM73308
Slew Rate vs. Supply Voltage
30155501
Open Loop Frequency Response Over Temperature
30155502
Open Loop Frequency Response
30155503
Open Loop Frequency Response
30155504
Open Loop Gain & Phase with Cap. Loading
30155505
Open Loop Gain & Phase with Cap. Loading
30155506
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SM73308
Non-Inverting Small Signal Pulse Response
30155517
Non-Inverting Large Signal Pulse Response
30155511
Non-Inverting Small Signal Pulse Response
30155516
Non-Inverting Large Signal Pulse Response
30155510
Non-Inverting Small Signal Pulse Response
30155515
Non-Inverting Large Signal Pulse Response
30155509
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SM73308
Inverting Small Signal Pulse Response
30155519
Inverting Large Signal Pulse Response
30155514
Inverting Small Signal Pulse Response
30155520
Inverting Large Signal Pulse Response
30155513
Inverting Small Signal Pulse Response
30155518
Inverting Large Signal Pulse Response
30155512
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SM73308
Stability vs. VCM
30155521
Stability vs. VCM
30155522
PSRR vs. Frequency
30155568
CMRR vs. Frequency
30155565
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SM73308
Application Note
SM73308
The SM73308 is a precision amplifier with very low noise and
ultra low offset voltage. SM73308's extended temperature
range of −40°C to 125°C enables the user to design a variety
of applications including automotive.
The SM73308 has a maximum offset voltage of 1mV over the
extended temperature range. This makes the SM73308 ideal
for applications where precision is important.
INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires
close attention to the input impedance of the amplifier, gain
of the overall signal on the inputs, and the gain on each input
since we are only interested in the difference of the two inputs
and the common signal is considered noise. A classic solution
is an instrumentation amplifier. Instrumentation amplifiers
have a finite, accurate, and stable gain. Also they have ex-
tremely high input impedances and very low output
impedances. Finally they have an extremely high CMRR so
that the amplifier can only respond to the differential signal. A
typical instrumentation amplifier is shown in Figure 1.
30155536
FIGURE 1. Instrumentation Amplifier
There are two stages in this amplifier. The last stage, output
stage, is a differential amplifier. In an ideal case the two am-
plifiers of the first stage, input stage, would be set up as
buffers to isolate the inputs. However they cannot be con-
nected as followers because of real amplifier's mismatch.
That is why there is a balancing resistor between the two. The
product of the two stages of gain will give the gain of the in-
strumentation amplifier. Ideally, the CMRR should be infinite.
However the output stage has a small non-zero common
mode gain which results from resistor mismatch.
In the input stage of the circuit, current is the same across all
resistors. This is due to the high input impedance and low
input bias current of the SM73308. With the node equations
we have:
(1)
By Ohm’s Law:
(2)
However:
(3)
So we have:
(4)
Now looking at the output of the instrumentation amplifier:
(5)
Substituting from Equation 4:
(6)
This shows the gain of the instrumentation amplifier to be:
−K(2a+1)
Typical values for this circuit can be obtained by setting: a =
12 and K= 4. This results in an overall gain of −100.
Figure 2 shows typical CMRR characteristics of this Instru-
mentation amplifier over frequency. Three SM73308 ampli-
fiers are used along with 1% resistors to minimize resistor
mismatch. Resistors used to build the circuit are: R1 =
21.6kΩ, R11 = 1.8kΩ, R2 = 2.5kΩ with K = 40 and a = 12. This
results in an overall gain of −1000, −K(2a+1) = −1000.
30155573
FIGURE 2. CMRR vs. Frequency
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SM73308
ACTIVE FILTER
Active filters are circuits with amplifiers, resistors, and capac-
itors. The use of amplifiers instead of inductors, which are
used in passive filters, enhances the circuit performance
while reducing the size and complexity of the filter.
The simplest active filters are designed using an inverting op
amp configuration where at least one reactive element has
been added to the configuration. This means that the op amp
will provide "frequency-dependent" amplification, since reac-
tive elements are frequency dependent devices.
LOW PASS FILTER
The following shows a very simple low pass filter.
30155547
FIGURE 3. Lowpass Filter
The transfer function can be expressed as follows:
By KCL:
(7)
Simplifying this further results in:
(8)
or
(9)
Now, substituting ω=2πf, so that the calculations are in f(Hz)
and not ω(rad/s), and setting the DC gain HO = −R2/R1 and
H = VO/Vi
(10)
Set: fo = 1/(2πR1C)
(11)
Low pass filters are known as lossy integrators because they
only behave as an integrator at higher frequencies. Just by
looking at the transfer function one can predict the general
form of the bode plot. When the f/fO ratio is small, the capacitor
is in effect an open circuit and the amplifier behaves at a set
DC gain. Starting at fO, −3dB corner, the capacitor will have
the dominant impedance and hence the circuit will behave as
an integrator and the signal will be attenuated and eventually
cut. The bode plot for this filter is shown in the following pic-
ture:
30155553
FIGURE 4. Lowpass Filter Transfer Function
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SM73308
HIGH PASS FILTER
In a similar approach, one can derive the transfer function of
a high pass filter. A typical first order high pass filter is shown
below:
30155554
FIGURE 5. Highpass FIlter
Writing the KCL for this circuit :
(V1 denotes the voltage between C and R1)
(12)
(13)
Solving these two equations to find the transfer function and
using:
(high frequency gain) and
Which results:
(14)
Looking at the transfer function, it is clear that when f/fO is
small, the capacitor is open and hence no signal is getting in
to the amplifier. As the frequency increases the amplifier
starts operating. At f = fO the capacitor behaves like a short
circuit and the amplifier will have a constant, high frequency,
gain of HO. Figure 6 shows the transfer function of this high
pass filter:
30155558
FIGURE 6. Highpass Filter Transfer Function
BAND PASS FILTER
30155560
FIGURE 7. Bandpass Filter
Combining a low pass filter and a high pass filter will generate
a band pass filter. In this network the input impedance forms
the high pass filter while the feedback impedance forms the
low pass filter. Choosing the corner frequencies so that f1 <
f2, then all the frequencies in between, f1 ≤ f ≤ f2, will pass
through the filter while frequencies below f1 and above f2 will
be cut off.
The transfer function can be easily calculated using the same
methodology as before.
(15)
Where
The transfer function is presented in the following figure.
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SM73308
30155562
FIGURE 8. Bandpass filter Transfer Function
STATE VARIABLE ACTIVE FILTER
State variable active filters are circuits that can simultane-
ously represent high pass, band pass, and low pass filters.
The state variable active filter uses three separate amplifiers
to achieve this task. A typical state variable active filter is
shown in Figure 9. The first amplifier in the circuit is connected
as a gain stage. The second and third amplifiers are connect-
ed as integrators, which means they behave as low pass
filters. The feedback path from the output of the third amplifier
to the first amplifier enables this low frequency signal to be
fed back with a finite and fairly low closed loop gain. This is
while the high frequency signal on the input is still gained up
by the open loop gain of the 1st amplifier. This makes the first
amplifier a high pass filter. The high pass signal is then fed
into a low pass filter. The outcome is a band pass signal,
meaning the second amplifier is a band pass filter. This signal
is then fed into the third amplifiers input and so, the third am-
plifier behaves as a simple low pass filter.
30155574
FIGURE 9. State Variable Active Filter
The transfer function of each filter needs to be calculated. The
derivations will be more trivial if each stage of the filter is
shown on its own.
The three components are:
30155580
30155581
For A1 the relationship between input and output is:
This relationship depends on the output of all the filters. The
input-output relationship for A2 can be expressed as:
And finally this relationship for A3 is as follows:
Re-arranging these equations, one can find the relationship
between VO and VIN (transfer function of the lowpass filter),
VO1 and VIN (transfer function of the highpass filter), and
VO2 and VIN (transfer function of the bandpass filter) These
relationships are as follows:
Lowpass Filter
Highpass Filter
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SM73308
Bandpass Filter
The center frequency and Quality Factor for all of these filters
is the same. The values can be calculated in the following
manner:
A design example is shown here:
Designing a bandpass filter with center frequency of 10kHz
and Quality Factor of 5.5
To do this, first consider the Quality Factor. It is best to pick
convenient values for the capacitors. C2 = C3 = 1000pF. Also,
choose R1 = R4 = 30kΩ. Now values of R5 and R6 need to be
calculated. With the chosen values for the capacitors and re-
sistors, Q reduces to:
or
R5 = 10R6
R6 = 1.5kΩ
R5 = 15kΩ
Also, for f = 10kHz, the center frequency is ωc = 2πf =
62.8kHz.
Using the expressions above, the appropriate resistor values
will be R2 = R3 = 16kΩ.
The following graphs show the transfer function of each of the
filters. The DC gain of this circuit is:
30155590
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SM73308