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PDF LM308 Data sheet ( Hoja de datos )
| Número de pieza | LM308 | |
| Descripción | Operational Amplifiers | |
| Fabricantes | National Semiconductor | |
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May 1989
LM108A LM208A LM308A Operational Amplifiers
General Description
The LM108 LM108A series are precision operational ampli-
fiers having specifications about a factor of ten better than
FET amplifiers over their operating temperature range In
addition to low input currents these devices have extremely
low offset voltage making it possible to eliminate offset ad-
justments in most cases and obtain performance ap-
proaching chopper stabilized amplifiers
The devices operate with supply voltages from g2V to
g18V and have sufficient supply rejection to use unregulat-
ed supplies Although the circuit is interchangeable with and
uses the same compensation as the LM101A an alternate
compensation scheme can be used to make it particularly
insensitive to power supply noise and to make supply by-
pass capacitors unnecessary
The low current error of the LM108A series makes possible
many designs that are not practical with conventional ampli-
fiers In fact it operates from 10 MX source resistances
introducing less error than devices like the 709 with 10 kX
sources Integrators with drifts less than 500 mV sec and
analog time delays in excess of one hour can be made us-
ing capacitors no larger than 1 mF
The LM208A is identical to the LM108A except that the
LM208A has its performance guaranteed over a b25 C to
a85 C temperature range instead of b55 C to a125 C
The LM308A devices have slightly-relaxed specifications
and performances over a 0 C to a70 C temperature range
Features
Y Offset voltage guaranteed less than 0 5 mV
Y Maximum input bias current of 3 0 nA over temperature
Y Offset current less than 400 pA over temperature
Y Supply current of only 300 mA even in saturation
Y Guaranteed 5 mV C drift
Compensation Circuits
Standard Compensation Circuit
Alternate Frequency Compensation
Cf
t
R1 CO
R1aR2
CO e 30 pF
TL H 7759 – 1
Bandwidth and slew rate are proportional to 1 Cf
Improves rejection of power supply
noise by a factor of ten
Bandwidth and slew rate are proportional to 1 Cs
Feedforward Compensation
TL H 7759 – 2
C1995 National Semiconductor Corporation TL H 7759
TL H 7759 – 3
RRD-B30M115 Printed in U S A
1 page  
Application Hints
A very low drift amplifier poses some uncommon application
and testing problems Many sources of error can cause the
apparent circuit drift to be much higher than would be pre-
dicted
Thermocouple effects caused by temperature gradient
across dissimilar metals are perhaps the worst offenders
Only a few degrees gradient can cause hundreds of micro-
volts of error The two places this shows up generally are
the package-to-printed circuit board interface and tempera-
ture gradients across resistors Keeping package leads
short and the two input leads close together helps greatly
Resistor choice as well as physical placement is important
for minimizing thermocouple effects Carbon oxide film and
some metal film resistors can cause large thermocouple er-
rors Wirewound resistors of evanohm or manganin are best
since they only generate about 2 mV C referenced to cop-
per Of course keeping the resistor ends at the same tem-
perature is important Generally shielding a low drift stage
electrically and thermally will yield good results
Schematic Diagram
Resistors can cause other errors besides gradient generat-
ed voltages If the gain setting resistors do not track with
temperature a gain error will result For example a gain of
1000 amplifier with a constant 10 mV input will have a 10V
output If the resistors mistrack by 0 5% over the operating
temperature range the error at the output is 50 mV Re-
ferred to input this is a 50 mV error All of the gain fixing
resistor should be the same material
Testing low drift amplifiers is also difficult Standard drift
testing technique such as heating the device in an oven and
having the leads available through a connector thermo-
probe or the soldering iron method do not work Thermal
gradients cause much greater errors than the amplifier drift
Coupling microvolt signal through connectors is especially
bad since the temperature difference across the connector
can be 50 C or more The device under test along with the
gain setting resistor should be isothermal
TL H 7759 – 6
5
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| PDF Descargar | [ Datasheet LM308.PDF ] | |
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