LM108A National Semiconductor, LM108A Datasheet - Page 5

• Manufacturer Part Number: LM108A
• Description: Operational Amplifiers
• Manufacturer: National Semiconductor

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 V 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
5
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
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
V error All of the gain fixing
TL H 7759– 6