LMC6462BIM National Semiconductor, LMC6462BIM Datasheet - Page 13

• Manufacturer Part Number: LMC6462BIM
• Description: IC, OP-AMP, 50kHZ, 0.015V/µs, SOIC-8
• Manufacturer: National Semiconductor

Specifications of LMC6462BIM

• Op Amp Type: Low Power
• No. Of Amplifiers: 2
• Bandwidth: 50kHz
• Slew Rate: 0.015V/µs
• Supply Voltage Range: 3V To 15.5V
• Amplifier Case Style: SOIC
• No. Of Pins: 8
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
and many more characteristics as listed on the macromodel
disk.
Contact the National Semiconductor Customer Response
Center to obtain an operational amplifier Spice model library
disk.
7.0 PRINTED-CIRCUIT-BOARD LAYOUT FOR
HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000 pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low input current of the LMC6462/4, typically 150
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 leakage of the PC
board, even though it may sometimes appear acceptably
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 LMC6462’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs, as in Fig-
ure 11. 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 30 times degradation from the LMC6462/4’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 12 for
typical connections of guard rings for standard op-amp con-
figurations.
FIGURE 11. Example of Guard Ring in P.C. Board
Layout
12
(Continued)
Ω, which is nor-
11
Ω would
01205115
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
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 of using point-to-point up-in-the-air wiring. See Figure
13.
FIGURE 12. Typical Connections of Guard Rings
Non-Inverting Amplifier
Inverting Amplifier
Follower
01205118
www.national.com
01205116
01205117