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

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 05% 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 methoddo 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

Schematic Diagram

TLH7759 – 6

5