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參數(shù)資料
| 型號(hào)： | LM208AJ-8 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 運(yùn)算放大器 |
| 英文描述： | CAC 5C 5#16S PIN RECP BOX |
| 中文描述： | OP-AMP, 1000 uV OFFSET-MAX, 1 MHz BAND WIDTH, CDIP8 |
| 封裝： | CERAMIC, DIP-8 |
| 文件頁數(shù)： | 5/8頁 |
| 文件大?。?/td> | 158K |
| 代理商： | LM208AJ-8 |
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
m
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.
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
m
V 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 methodDdo 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
TL/H/7759–6
5
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