LMC6044AIM National Semiconductor, LMC6044AIM Datasheet - Page 10

• Manufacturer Part Number: LMC6044AIM
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMC6044AIM

• No. Of Amplifiers: 4
• Slew Rate: 0.02V/µs
• No. Of Pins: 8
• Mounting Type: Surface Mount
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Package / Case: 8-NSOIC
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Hints
of the ultra-low bias current of the LMC6044, typically less
than 2 fA, it is essential to have an excellent layout. Fortu-
nately, the techniques of obtaining low leakages are quite
simple. First, the user must not ignore the surface leakage of
the PC board, even though it may sometimes appear accept-
ably low, because under conditions of high humidity or dust
or contamination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LMC6044’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs, as in Fig-
ure 4. 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 amplifer 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
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 100 times degradation from the LMC6044’s
actual performance. However, if a guard ring is held within 5
mV of the inputs, then even a resistance of 10
cause only 0.05 pA of leakage current. See Figure 5 for
typical connections of guard rings for standard op-amp con-
figurations.
FIGURE 4. Example of Guard Ring
in P.C. Board Layout
(Continued)
12
Ω, which is nor-
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11
Ω would
10
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
insulator. 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
See Figure 6.
FIGURE 5. Typical Connections of Guard Rings
of
using
Non-Inverting Amplifier
Inverting Amplifier
point-to-point
Follower
up-in-the-air
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01113810
wiring.