LMC6484IM National Semiconductor, LMC6484IM Datasheet - Page 13

• Manufacturer Part Number: LMC6484IM
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMC6484IM

• No. Of Amplifiers: 4
• Slew Rate: 1.3V/µs
• No. Of Pins: 14
• Amplifier Type: Operational
• Mounting Type: Surface Mount
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Number Of Channels: 4
• Voltage Gain Db: 116.47 dB
• Common Mode Rejection Ratio (min): 65 dB
• Input Offset Voltage: 3 mV at 5 V
• Operating Supply Voltage: 5 V, 9 V, 12 V, 15 V
• Supply Current: 2.8 mA at 5 V
• Maximum Operating Temperature: + 85 C
• Package / Case: SOIC-14 Narrow
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
of the ultra-low input current of the LMC6484, typically less
than 20 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 LMC6484’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 in 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
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 LMC6484’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 10 for
typical connections of guard rings for standard op-amp
configurations.
FIGURE 9. Example of Guard Ring in P.C. Board
Layout
(Continued)
12
, which is nor-
11
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would
13
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
See Figure 11 .
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 10. Typical Connections of Guard Rings
of
using
Non-Inverting Amplifier
FIGURE 11. Air Wiring
Inverting Amplifier
point-to-point
Follower
up-in-the-air
DS011714-23
DS011714-21
www.national.com
DS011714-22
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wiring.