參數(shù)資料
型號(hào)： LM6482
廠(chǎng)商： National Semiconductor Corporation
英文描述： CMOS Dual Rail-To-Rail Input and Output Operational Amplifier
中文描述： CMOS雙軌至軌輸入和輸出運(yùn)算放大器
文件頁(yè)數(shù)： 14/22頁(yè)
文件大?。?/td> 1001K
代理商： LM6482
Application Information
(Continued)
6.0 Printed-Circuit-Board Layout for High-Impedance
Work
It is generally recognized that any circuit which must oper-
rate with less than 1000 pA of leakage current requires spe-
cial layout of the PC board. When one wishes to take advan-
tage of the ultra-low input current of the LMC6482, typically
less than 20 fA, it is essential to have an excellent layout.
Fortunately, the techniques of obtaining low leakages are
quite simple. First, the user must not ignore the surface leak-
age of the PC board, even through it may sometimes appear
acceptably low, because under conditions of high humidity or
dust or contamination, the surface leakage will be appre-
ciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LM6482’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs, as in Fig-
ure 9 To have a significant effect, guard rings should be
placed on both the top and bottom of the PC board. This PC
foil must then be connected to a voltage which is at the same
voltage as the amplifier inputs, since no leakage current can
flow between two points at the same potential. For example,
a PC board trace-to-pad resistance of 10
, which is nor-
mally considered a very large resistance, could leak 5 pA if
the trace were a 5V bus adjacent to the pad of the input. This
would cause a 250 times degradation from the LMC6482’s
actual performance. However, if a guard ring is held within 5
mV of the inputs, then even a resistance of 10
11
would
cause only 0.05 pA of leakage current. See Figure 10 for
typical connections of guard rings for standard op-amp
configurations.
The designer should be aware that when it is inappropriate
to lay out a PC board for the sake of just a few circuits, there
is another technique which is even better than a guard ring
on a PC board: Don’t insert the amplifier’s input pin into the
board at all, but bend it up in the air and use only air as an in-
sulator. Air is an excellent insulator. In this case you may
have to forego some of the advantages of PC board con-
struction, but the advantages are sometimes well worth the
effort
of
using
point-to-point
See Figure 11
up-in-the-air
wiring.
DS011713-20
FIGURE 9. Example of Guard Ring in P.C. Board
Layout
DS011713-21
Inverting Amplifier
DS011713-22
Non-Inverting Amplifier
DS011713-23
Follower
FIGURE 10. Typical Connections of Guard Rings
DS011713-24
(Input pins are lifted out of PC board and soldered directly to components.
FIGURE 11. Air Wiring
www.national.com
14
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